Hedera helix



INTRODUCTORY


Photo by Chuck Bargeron, University of Georgia, Bugwood.org

AUTHORSHIP AND CITATION:
Waggy, Melissa A. 2010. Hedera helix. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/ [].

FEIS ABBREVIATION:
HEDHEL

NRCS PLANT CODE [179]:
HEHE

COMMON NAMES:
English ivy
common ivy
ivy

TAXONOMY:
The scientific name of English ivy is Hedera helix L. (Araliaceae) [20,60,72,92,103,122,138]. Surveys conducted in 2006 of invasive populations of English ivy in British Columbia, Washington, and Oregon suggest that most populations in those areas may be H. hibernica rather than H. helix [66]. The North American range of H. hibernica was previously thought to be limited to North Carolina and South Carolina [179]. This change, however, has not been acknowledged by North American taxonomic sources as of this writing (2010) [67,72,179]. H. hibernica is not listed in regional floras for that area [60,138] and will not be further considered in this review.

Two subspecies of English ivy are recognized in Europe [70]:

H. helix L. subsp. canariensis (Willd.) Cout.
H. helix L. subsp. helix

English ivy has numerous cultivars [30,31,89,186]. The American Ivy League indicates there are 400 distinct cultivars developed in North America and other countries that are distinguished primarily by their morphology, growth rate, and hardiness to adverse growing conditions (e.g., full sun, cold temperatures) [158].

SYNONYMS:
None

LIFE FORM:
Vine-liana
Shrub

DISTRIBUTION AND OCCURRENCE

SPECIES: Hedera helix
GENERAL DISTRIBUTION:
Nonnative range: English ivy is nonnative to North America. In the eastern United States, it occurs from Massachusetts south throughout the mid-Atlantic and southeastern states to Florida and across the south-central states as far east as Texas. It occurs in some of the Great Lakes states including Michigan, Indiana, and Illinois [108,179], and in Ontario, Canada [108]. One invasive plant publication indicated it also occurred in Wisconsin [25]. In the West, English ivy occurs in all Pacific Coast states and British Columbia, in addition to Idaho, Utah, and Arizona [108,179]. It also occurs in Hawaii [186].

Based on regional floras [30,46,60,92,103,104,122,186,196], invasive plant publications [25,125,193], and websites [17,45,74,96,101,106,113,149,163,176,177], English ivy is most invasive in the Pacific Northwest. Based on a regional invasive species map, English ivy is not widespread in the Southeast [100], but it may be locally abundant in some mid-Atlantic states including Virginia [115,117], West Virginia, and in some southern states including North Carolina, South Carolina, Alabama, and Arkansas [100]. Several sources indicate that English ivy is not as problematic in the Northeast as in the Pacific Northwest, mid-Atlantic, and Southeast. A review of floristic surveys from the eastern United States found that English ivy occurred in temperate forests in this area but was not common [88]. Local floras indicate that English ivy occasionally escapes cultivation in the Northeast [46,92], and English ivy is not listed on a Northeastern invasive species website [96]. It may, however, be locally abundant in parts of Washington, DC [164,170]. NatureServe provides a map of English ivy's North American distribution.

Brought to North America by colonial settlers [99,125,146], English ivy was first documented in North America in Virginia around 1800 [191]. Since then English ivy has been widely cultivated as an ornamental [90,104,138,184,192], although its cultivation in Hawaii did not begin until the early 1900 [186]. In many places where it is planted, English ivy escapes cultivation to varying degrees [46,92,103,104,122,195] and invades and persists in wildlands [30,60,138,186,190,195].

English Ivy has been introduced to South Africa, India, Australia, New Zealand, Brazil, and Mexico [97,190].

Native range: English ivy is native to Eurasia [46,52,103,104,186,192,196], occurring from the Caucasus Mountains [184,186] to Norway [52,145] and south to Iran [97] and northern Africa [97,184]. It tends to be less abundant north of the Alps [145].

HABITAT TYPES AND PLANT COMMUNITIES:
In North America, English ivy frequently occurs in upland ([125,137,170]) and riparian [43,119,157,169,183] deciduous forest communities of variable species assemblages. It occasionally occurs in conifer forests or savannas [134,140,156], in conifer-deciduous mixed forests [66,173,194], and in thickets and shrublands [114]. In North America, English ivy is widespread in urban and disturbed forests and is commonly associated with other nonnative species [107,114,116,117,162,170,183,194]. Forests containing English ivy may have a diverse assemblage of plant species but may become less diverse as English ivy spreads (see Potential successional stage and Impacts).

Pacific Northwest and California: In the Pacific Northwest, English ivy occurs in deciduous or conifer-deciduous mixed forests with bigleaf maple (Acer macrophyllum) [66], red alder (Alnus rubra), Oregon ash (Fraxinus latifolia), black cottonwood (Populus balsamifera subsp. trichocarpa), and Oregon white oak (Quercus garryana) [66,111]. It is considered a threat to nearly all forest types in the Pacific Northwest coastal area below 3,000 feet (900 m) [4]. In Oregon, it occurred in a dense conifer-deciduous mixed forest with Douglas-fir (Pseudotsuga menziesii), western redcedar (Thuja plicata), and bigleaf maple ([66],Waggy 2000 personal observation [185]). It is particularly common in forests near populated areas ([4], Waggy 2000 personal observation [185]). In California, it occurs in redwood (Sequoia sempervirens) forests [134,140] and riparian forests dominated by Fremont cottonwood (Populus fremontii), white alder (Alnus rhombifolia), and willow (Salix spp.) [157].

Southwest: Available information at the time of this writing (2010) indicated that English ivy may not be common in native plant communities in the Southwest. Floras from Texas [30] and Utah [192] indicate English ivy typically occurs as an ornamental around homesites, but it may occasionally escape to wooded areas [30].

Mid-Atlantic and southeastern states: In this portion of North America, English ivy occurs with many species of deciduous trees and does not appear to be closely associated with any specific forest type. Canopy dominants commonly occurring with English ivy include maple (Acer spp.), hickory (Carya spp.), American beech (Fagus grandifolia), ash (Fraxinus spp.), oak (Quercus spp.), sweetgum (Liquidambar styraciflua), yellow-poplar (Liriodendron tulipifera), sycamore (Platanus occidentalis), and American elm (Ulmus americana) [12,43,107,114,116,117,119,156,162,194]. In the southeastern United States, English ivy occurs in pine (Pinus spp.) savanna, tropical hardwood, and baldcypress (Taxodium distichum) communities [156]. In Florida [196] and Georgia [198], English ivy occasionally occurs in disturbed hammocks [196].

In the mid-Atlantic and southeastern United States, English ivy occasionally occurs in plant communities that have been disturbed or may not contain native assemblages. On one site in North Carolina, English ivy occurred in a transitional loblolly pine (Pinus taeda) forest that was likely planted [194]. On a disturbed terrace in South Carolina, English ivy occurred in a forest that contained a mix of deciduous and conifer trees that included river birch (Betula nigra), water oak (Quercus nigra), and Virginia pine (Pinus virginiana) [173]. In Rock Creek Park in Washington, DC, English ivy occurs in an Allegheny blackberry (Rubus allegheniensis) shrubland codominated by nonnative vines including porcelainberry (Ampelopsis brevipedunculata).

Northeast, Great Lakes area, and Canada: At the time of this writing (2010), information pertaining to common plant associates of English ivy in these areas of North America was lacking.

European range: In Europe, English ivy occurs in plant communities similar to those described for North America. It occurs in deciduous forests dominated by maple (Acer spp.), birch (Betula spp.), European beech (Fagus sylvatica), European ash (Fraxinus excelsior), oak (Quercus spp.), littleleaf (Tilia cordata), and wych elm (Ulmus glabra) [22,27,63,65,68,73,75,79,144,170]. English ivy was the dominant groundlayer species in a forest that contained young European ash and sycamore maple (Acer pseudoplatanus) and a mix of old trees including European ash, wych elm, European beech, Norway pine (Abies excelsa), European larch (Larix europaea), spruce (Picea sp.), Scots pine (Pinus sylvestris), European aspen (Populus tremula), and English yew (Taxus baccata) [73]. In France, English ivy was common in an oak and European hornbeam (Carpinus betulus) forest [168]. One study from Great Britain indicated that English ivy was more common in beech than ash forests [36], and a study from France indicated it favored European ash and English oak (Quercus robur) forest over other forest types [136].

The following table is a list of North American plant communities where English ivy occurs.

Vegetation classifications from North America where English ivy is a dominant, characteristic, or common species
Vegetation Classification
Location
Dominant plants and other features
Mid-Atlantic states
yellow-poplar/northern spicebush (Lindera benzoin)/Jack-in-the-pulpit (Arisaema triphyllum) forest; coastal plain and Piedmont small-stream floodplain forest historical sites, Virginia Canopy dominants include sweetgum, yellow-poplar, and red maple (Acer rubrum). English ivy occurs in the disturbed and fragmented phase of this association [107,114,116,117,162].
disturbed calcareous forest Petersburg National Battle Field, Virginia This occurs on forested bluff with open canopy of large yellow-poplar, hackberry (Celtis occidentalis), black walnut (Juglans nigra), and American elm. The understory and herb layer is dominated by nonnative species including English ivy [116].
sycamore-green ash
( Fraxinus pennsylvanica) forest
Rock Creek Park, Washington, DC The canopy is dominated by sycamore and boxelder (Acer negundo). English ivy is a frequent associate.
Allegheny blackberry/porcelainberry shrubland Rock Creek Park, Washington, DC Allegheny blackberry, greenbrier (Smilax spp.) and various nonnative species dominate this community. English ivy is a common nonnative species in this community [167].
disturbed seepage swamp forested wetland Colonial National Historical Park, Virginia Red maple dominates the canopy. English ivy is a characteristic species in this association.
successional mixed scrub Colonial National Historical Park, Virginia Species composition and dominance vary greatly from site to site. On some sites English ivy may form a thicket with other nonnative and native vines [114].
successional yellow-poplar forest Fredericksburg and Spotsylvania National Park, Virginia Yellow-poplar, red maple, and sweetgum association. English ivy is common in this association [162].
Southeast
yellow-poplar/northern spicebush/Jack-in-the-pulpit forest coastal plain and Piedmont small-stream floodplain forest Guilford Courthouse National Military Park, North Carolina Canopy dominants include sweetgum, yellow-poplar, and red maple. English ivy occurs in the disturbed and fragmented version of this association.
successional loblolly pine-sweetgum forest Guilford Courthouse National Military Park, North Carolina Loblolly pine, Virginia pine and sweetgum dominate these forests. This is a human-modified community that is easily invaded by nonnative species including English ivy [194].

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

SPECIES: Hedera helix
Juvenile growth phase
Photo by Chuck Bargeron, University of Georgia, Bugwood.org
Adult growth phase
Photo by James H. Miller, Bugwood.org

GENERAL BOTANICAL CHARACTERISTICS:
Botanical description: This description covers characteristics that may be relevant to fire ecology and is not meant for identification. Keys for identification are available (e.g., [30,46,60,103,122]).

Aboveground: English ivy is a woody [184], evergreen [30,46,52,60,99,184], trailing or climbing liana [30,92] or shrub [129]. In Europe, English ivy occasionally grows as a tree [97,136].

English ivy has 2 distinct growth phases, the vegetative phase (juvenile) and the sexual reproductive phase (adult) [153].

Comparison of juvenile and adult growth phase of English ivy [153]
Characteristic Juvenile Adult
Growth habit plagiotrophic orthotrophic
Flowers absent present
Leaf arrangement alternate 2/5th spiral
Leaf production 1 leaf/week 2 leaves/week
Shoot growth vigorous slight
Leaf shape lobed entire
Rooting ability good weak
Aerial roots present absent

Individual English ivy plants may have both juvenile and adult stems. The juvenile phase typically forms the ground cover [31,99]. Juvenile English ivy begins to climb when vertical structure is available (e.g., trees, shrubs, buildings, utility poles), and vertical stems transition to the adult phase [97].

As a ground cover, English ivy grows from 6 to 8 inches (10-20 cm) tall [31,99]. Once stems begin climbing, they typically reach 90 feet (30 m) in height [25,31,46,65,99,152,184] but occasionally may climb higher, reaching the tops of 300-foot (90 m) conifers [146]. English ivy climbs with the aid of root-like structures [19,25,31,46,84,152] that exude an adhesive substance [99]. One publication from England indicated that English ivy attaches to substrates by numerous small roots [52]. Branches are typically slender on low-growing plants [99], but climbing and older trailing branches may be 4 [31,135] to 12 inches (10-30 cm) in diameter [25,65,99,146,152], with furrowed bark [31]. Leaves are typically 4 inches (10 cm) long × 2.5 to 5 inches (6.4-13 cm) wide and are 3 to 5 lobed in the juvenile phase and broadly lanceolate and unlobed in the adult phase. Flowers are clustered in umbels on adult stems [99]. There are 8 [182] to 20 [97] flowers/umbel; umbels grow in clusters of 3 [97] to 6 [14]. English ivy fruit is a berry [30,60,104] about 5 [186] to 9 mm long [60,122] and 6 [192] to 9 mm in diameter [145], with an average weight of 281.5 mg [59]. Its fruit contains 2 to 5 seeds [30,31,59,60,145,184] that are about 5.7 × 3.7 mm in size [52].

Belowground: Information from an exotic pest website in the Southeast suggests that English ivy does not form an extensive underground root system [149]. An invasive plant publication from California [126] and a publication suggesting landscaping plants for use in chaparral plant communities to reduce fire hazard [123] suggests that English ivy's roots are generally shallow. In Washington, DC, English ivy root depth ranged from 1 to 4.13 inches (3.0 -10.5 cm) below the soil surface [169]. A study from Spain measured rooting depth for several vines including English ivy and found the average rooting depth for all species was statistically similar (P<0.001); root depth for 100 mg of plant dry mass was 3.8 inches (9.6 cm), and for 1.00 g of plant dry mass it was 6.34 inches (16.1 cm). English ivy roots were highly dissected, and the average root diameter 5 mm from the root tip was 0.45 mm [133].

Stand structure: In woodlands, English ivy frequently forms a dense ground cover that may occupy large areas made up of numerous individuals [97]. In North America, English ivy has been described as forming an "ivy desert" [125]. In the Green River Gorge in North Carolina, English ivy completely covered the ground in a deciduous forest [120]. In floodplain forests along the Rhine River in France, English ivy "carpets" the forest floor until it eventually reaches a tree and begins to ascend [135]. In one study in these forests, average English ivy stem density was 24.5/ha on sites that experience brief annual flooding compared to 61.9/ha on sites where flooding had been excluded. Clumping (more than one English ivy vine/host tree) was common, particularly on trees in the canopy and subcanopy of the forest, largely due to the greater abundance of support in these layers [136]. In another study in the Rhine River floodplain in France, English ivy stem density reached 120 stems/ha in a dense Lombardy poplar (Populus nigra) and English oak forest [135].

Longevity: As of this writing (2010), information pertaining to English ivy's longevity in North America was lacking. In forests along the Rhine River in France, the oldest English ivy vines at 1 site were 50 years old, while the oldest vines at another site were at least 66 years old [136]. There is a report of a 433-year-old stem of English ivy that was over 20 inches (60 cm) in diameter, but it is unclear where it occurred (Schenk 1893 cited in [37]).

Raunkiaer [124] life form:
Chamaephyte

SEASONAL DEVELOPMENT:
English ivy is evergreen. Leaf initiation occurs primarily from April to May, although leaves may be produced almost continuously from March to October (Metcalfe and Sack unpublished data cited in [97]). The main period of litterfall occurs in late spring [7]. Leaves are long-lived (3-4 years) but show seasonal changes in leaf chlorophyll content, with concentration dipping in mature leaves in early April with the onset of leaf initiation and recovering back to "normal" levels within 3 to 4 weeks (Metcalfe and Sack unpublished data cited in [97]). When grown in shade, English ivy percent canopy cover increased gradually in the spring and summer, peaked in the fall, and declined slightly during the winter. Development was similar for plants grown in the sun, with peaks occurring in the fall; however, percent canopy cover was much less for plants grown in sun [150].

English ivy's flowering period in North America is variable; reports range from late spring to fall (see Table). As of this writing (2010), little had been reported on its fruiting period in North America. One report from the Pacific Northwest indicated that English ivy flowers in the fall and fruits in the spring [146]; however, a local flora for that area gives a flowering period of May through June [60]. In the southeastern United States, English ivy fruits from October to May [99].

Reported flowering periods for English ivy in North America by geographic area
Area Flowering period
Florida Summer [196]
Illinois June-September [103]
Blue Ridge Province June-July [195]
Northeastern United States and adjacent Canada September [46]
Pacific Northwest May-June [60]
Southeast June-October [99]

Available evidence suggests that in its European range, English ivy generally flowers in the fall, and fruit ripens over the winter from December to early spring.

Reported phenology for English Ivy in parts of its European range
Area Event
The former Czechoslovakia fruit ripens in winter [14]
Italy flowers from mid-September to early November [182]
United Kingdom Most plants begins to flower in the fall [52,97,145]; flowering may extend from late August through late November [97,145]. Fruit generally ripens from March-April of the next year; ripening may extend from December (for early-flowering plants) to May (for late-flowering plants) [145]

REGENERATION PROCESSES:
English ivy fruit.
Photo by Forest & Kim Starr, U.S. Geological Survey, Bugwood.org

English ivy spreads vegetatively, either by rooting from stems and stem fragments that contact the soil [66,126,146,159,160,190] or from fragmented roots [146]. English ivy reproduces sexually by seed [25,52,77,97,126,146], typically in open or disturbed habitat [97]. In some locations, establishment from seed may be infrequent [16,52].

Vegetative regeneration: Several invasive plant publications [25,126,159,160,190] and one study from North America [13] indicate that English ivy sprouts from stem fragments and cut stumps. Stems [66,146] and stem fragments root easily when they are in contact with the soil [160,190], and plants spread from adventitious roots that develop along the stem [126]. Fragmented roots left in the soil may sprout a new stem [146]. In the United Kingdom, English ivy spreads extensively by vegetative regeneration; establishment from seed may be infrequent [52,97].

Pollination and breeding system: Sexual reproduction typically occurs in climbing adult plants that reach sufficient light, but trailing plants may occasionally produce fruit, especially if they are growing in full sunlight [146]. English ivy flowers are bisexual [122,126], protandrous, and cross-pollinated by a wide variety of insects [52,97,182]. Faegri and van der Pijl [38] speculated that plants that flower late in the year, like English ivy, may be completely dependent on flies for pollination because numerous other insects like bumblebees and bees are not present during that time of year.

In the Netherlands, pollen counts collected from various sampling sites (e.g., water trough, roof tiles, moss on a thatched roof) in October determined that one large, profusely flowering English ivy produced several billion pollen grains annually [14].

Seed production: In France, 741 English ivy seeds were collected in a 427.9 foot² (39.75 m²) plot from December 1981 to May 1983 on a site where English ivy was common in the aboveground vegetation [27]. English ivy's seed production may be limited in cooler climates. Based on an analysis by local researchers, Michigan's Invasive Plant Council does not consider English ivy invasive in the Great Lakes region because seed production may be inconsistent [98]. In Europe, cold weather may prevent early fruit crops from ripening so that no viable seed is produced [145]. In climates like those of the Czech Republic and Slovakia, fruits may not produce seed every year [85]. English ivy fruit size or production may be limited for plants growing above 869 feet (265 m) in elevation (Macleod 1983 cited in [145]).

Seed dispersal: English ivy seed is dispersed by birds [25,60,99,146,159]. In North America, birds that distribute English ivy seed included European starling, cedar waxwing, American robin, Steller's jay, mockingbird, and house sparrow [160]. Snow and Snow [145] provide a list of birds that may act as potential dispersers of English ivy seed in the United Kingdom (see Importance to Wildlife and Livestock). After digesting the fruit, birds may regurgitate English ivy seed one at a time [23]. One literature review suggested that English ivy seed is deposited in bird droppings as well as regurgitated [97]; however, a seed germination study found no evidence that English ivy seed is defecated by birds [23]. Researchers in the Netherlands speculated that English ivy has an 80% chance of being dispersed to a forest if there is a seed source 3 feet (1 m) or less away, but at 3,000 feet (1,000 m) away, the probability decreases to nearly 0% [48].

English ivy phenology may vary for different locations and influence the dates when seed becomes available for dispersal [145]. In the United Kingdom, English ivy seeds are dispersed in early winter (November and December), but early flowering plants likely have their fruit taken sooner [97,145]. In Germany, English ivy seed was dispersed in later spring (Kollman 1994 cited in [76]).

Seed banking: Publications from the United Kingdom indicate that English ivy seed is short-lived and does not form a persistent seed bank [52,97]. Because English ivy's flowering and fruiting is limited by shade (see Pollination and breeding system), accumulation of English ivy seed in woodlands may be an "expensive luxury" [171]. Under experimental conditions, English ivy seed planted in various size containers and soil types continued to germinate for 1 year after planting; however, germination and seedling survival generally declined over time, particularly for seed buried 5 months or longer [152].

English ivy's presence in the aboveground vegetation may not indicate its presence in the seed bank. In deciduous forests in northwestern Greece [22] and Denmark [75], English ivy occasionally occurred in the aboveground vegetation but its seed was not found in the soil seed bank. In a deciduous forest in Spain, English ivy was 1 of 4 dominant shrub species in the understory (3% aboveground cover) but did not appear in the seed bank at samplings depths of up to 4 inches (10 cm) [112].

Germination: Information pertaining to English ivy's germination requirements was incomplete as of 2010. English ivy belongs to a family of plants with seeds containing small embryos, and its seed may be dormant [9]. An invasive species publication indicated that English ivy has a hard seed coat that must be scarified before germination occurs, a process that is likely achieved when English ivy seed passes through a bird's digestive system [126]. Conversely, one publication on berry consumption by birds from the United Kingdom indicated that English ivy seeds are "unusually soft" [145]. Results from a study in western France suggest that English ivy seed requires no scarification for germination. English ivy germination was highest when fruit pulp was removed either manually (97%) or by bird ingestion and subsequent regurgitation (99%) compared to seed left in fresh (53%) or dried fruit (0 %). Because germination was similar when pulp was removed manually or by bird ingestion, researchers speculated that ingestion by birds appeared to have no other effect than pulp removal [23]. Pulp removal may facilitate germination in English ivy [31] by shortening its dormancy requirements [23]. Under cultivation, uncleaned English ivy seed did not germinate immediately or when cold stratified; however, 70% of the seeds germinated once the fruit pulp was removed [31]. In the laboratory, average length of seed dormancy in English ivy ranged from 4.2 weeks to 8.5 weeks; however, dormancy was shortened when fruit pulp was removed [23].

Available evidence suggests that English ivy seed germinates fast and has a high germination capacity. Clergeau [23] reported that about 60% to 100% of English ivy seed germinates in 15 days or less. Nearly 100% of the seed germinated in less than 10 days when fruit pulp was removed from the seed by hand or by bird ingestion [23]. In the laboratory, most viable English ivy seed germinated within 5 to 20 days. Percent germination was from 0% to 92% for variable temperature regimes. Low rates of germination were attributed to high rates of seed decay. English ivy seed not fully ripened before falling from the mother plant was prone to decay [85]. One flora from Texas indicated that English ivy seed usually germinates the 2nd year after it is planted [184].

In a riparian forest in France, short periods of flooding favored English ivy germination but not seedling establishment. Six-month-old English ivy seedlings were significantly (P<0.01) more abundant on a site that experienced brief annual flooding than on a site where flooding had been excluded. However, there was no significant difference in the number of 1-year-old seedlings between the 2 sites. Researchers attributed this to high mortality of English ivy seedlings during the 1st year of growth caused by flooding, "competition", and browsing [136].

Seedling establishment and plant growth: In the United Kingdom, English ivy is considered tolerant of stress and able to establish in a range of environments [52]. A literature review indicated that English ivy seedlings establish on disturbed or open sites [97], although it may not establish well under frequent or severe disturbance [52]. English ivy seedlings may also establish under forest [131,135] or shrub [77] canopy; however, growth rate may be less than for unshaded seedlings. In the greenhouse, English ivy's relative growth rate for potted seedlings was significantly less (P=0.001) for plants in neutral shade (3.5% daylight) than for plants in 35% daylight. English ivy seedlings were moderately tolerant of shade under drought conditions [132]. On an experimental site in England, English ivy seedling density was positively correlated with soil moisture (r²= 0.65) and was highest under shrubs associated with relatively high soil moisture content [77]. Researchers in Spain reported a greater than 90% survival rate for English ivy seedlings after 1 year (Laskurain and others cited in [66]).

Seedlings are shallowly rooted and grow a single, slender, vertical primary root that bears many short, fine branches, some of which develop into horizontal secondary and lateral stems [97]. English ivy has 2 distinct growth phases (see Botanical description); plants in the juvenile phase are adapted to shade [97,136] and remain vegetative, while plants in the adult growth phase occur in the sun and flower. Juvenile plants root from the nodes of the aboveground creeping stem [97] and grow rapidly along the ground [146]. A publication from North America suggests that the juvenile period is long, often lasting 10 years or more [126]. English ivy begins to climb when it reaches a vertical structure [146], taking approximately 30 to 40 years to reach tree canopies, but growth may be faster on flooded sites [136]. Both trailing and climbing vines spread by rooting at the nodes [99]. Juveniles climbing small hosts may live in perpetually lightly-shaded environments, except for periods when deciduous trees lose leaves. Most eventually transition to the adult phase but do not develop large-diameter stems even though they may be a few decades old [135].

Once English ivy begins to ascend a tree, its photosynthetic capacity increases due to increased light, which facilitates a transition to the adult growth phase. In a riparian forest in France, Schnitzler and Heuze [136] observed that the transition took place when vines were about 49 to 70 feet (15-20 m) tall. Under experimental conditions, net photosynthesis was higher in adult than juvenile leaves at low and high light levels [62]. English ivy's net photosynthesis may vary under changing light regimes (see Shade tolerance).

Adult English ivy flowers in bright light [97]. Adult English ivy growing in light shade may produce fruit but in limited quantities [135].

One review from the United Kingdom suggested that English ivy grows most "vigorously" in shaded, moist sites on heavy, fertile soils [97]. However, on Potomac Island in Washington, DC, English ivy's biomass increased with increased light in upland sites. Light, however, was less a factor for growth on newly invaded sites [170]. A study from the Pacific Northwest indicated that English ivy grows throughout the year, although growth may slow or stop during extended drought or periods of intense cold [146]. Growth may slow once English ivy transitions to the adult phase [126]. In experimental plots, English ivy's average total dry shoot weight was higher for plants grown in 60% shade (19.6 g) than for plants grown in full sun (3.3 g) [150]. Researchers from France attributed a reduction in English ivy radial growth to either flood elimination or aging of the plant [65]. Cultivars may have variable growth rates [89].

English ivy does not typically spread to another support if its original host (e.g., tree, shrub) is uprooted or seriously damaged [135,136]. In France, English ivy did not spread to another host tree after the original host tree was uprooted, but it was able to survive another 17 years by developing adventitious roots [135].

SITE CHARACTERISTICS:
Climate: In North America and Europe, English ivy occurs in climates of moderate to high annual precipitation ranging from 20 to 100 inches (see table). In western Oregon, presence of English ivy was negatively correlated (R² = -0.64) with summer precipitation [50].

Reported average precipitation ranges for English ivy
Area Precipitation (inches)
Pacific Northwest 42.1 [105] to 100 [174]
Northeast and mid-Atlantic 40.6 [114] to 55.9 [32,53,116,162,194,197]
Southeast >47 [148] to 62.6 [173]
Europe 20 to 30 [22,65]

In the Pacific Northwest—where English ivy is most abundant (see General Distribution)—the climate is maritime, characterized by moderate, wet winters and cool, dry summers. Two localized examples from the Pacific Northwest indicate that English ivy occurs in areas with a mean January temperature of approximately 40 °F (4 °C) [105,174] and a mean August temperature of 59 °F (15 °C) [174].

In northeastern and mid-Atlantic states, English ivy has been found in areas with average minimum temperature ranging from 44 °F (6.6 °C) [162] to 47 °F (8.6 °C) [114,116] and average maximum temperatures ranging from 68.4 °F (20.2 °C) [116] to 69.8 °F (21.0 °C) [114,162]. English ivy occurs in several National Parks in the east with reported mean January temperatures ranging from 24 °F (-4.3 °C) [116] to 32 °F (0 °C) [114,162,197], suggesting that some North American populations of English ivy may be able to tolerate moderately cold winter temperatures. The North American Plant Conservation Alliance considers English ivy cold hardy [160].

In the Southeast, where English ivy is not as common as in other parts of North America (see General Distribution), climates are humid [148] and characterized by warm summers and mild winters [173]. English ivy occurred in the Chauga River Gorge in South Carolina, where midsummer and midwinter temperatures averaged approximately 97 °F (36 °C) and 6.8 °F (-14 °C), respectively [173].

In Europe, English ivy is classified as a southern-temperate species; these are species that are likely to occur in either temperate or Mediterranean climates [118]. It normally develops and fruits where the average temperature in the warmest month is about 55.4 °F (13 °C) but not in areas where the average temperature for the coldest month is 29 °F (-1.5 °C) or less [97]. English ivy is susceptible to low winter temperatures (Godwin 1975 cited in [52]) but may adapt to low winter temperatures by decreasing its metabolic activities  [42]. Based on its elevational range in the Alps (up to about 4,100 feet (1,250 m)), it has been speculated that English ivy's distribution is determined by its limited tolerance to frost. Its northern and eastern distributional limits coincide closely with the -13 °F (-25 °C) minimum-isotherm. At its latitudinal and longitudinal limits, English ivy retains its juvenile form and may be killed by severe frost at -7.6 °F (-22 °C) or less. The adult form of English ivy may be more cold tolerant than the juvenile form [3].

Low temperatures may prevent English ivy from dominating forests in its native range. In northern areas of its native range, temperatures may be low enough to limit photosynthesis and subsequent growth. English ivy’s net photosynthesis increases with warm temperatures [97]. In climates like those of Romania, English ivy stops photosynthesizing for 2 months/yr (Atanasiu 1965 cited in [170]). Seed production may also be limited in cold climates (see Seed production).

Elevation and aspect: English ivy is not common at high elevations. In California, English ivy occurs from sea level to about 3,300 feet (1,000 m) [104,126]. In western Oregon, English ivy became less frequent with increasing elevation [50]. A flora from Utah indicates English ivy occurs at low elevations in that state [192]. In Virginia and North Carolina, English ivy occurs in several National Parks that occur from 49 feet (15 m) [162] to 869 feet (265 m) [116,194] in elevation. In the United Kingdom, English ivy is infrequent at high elevations [145]. Publications from English ivy’s European native range report elevations for English ivy ranging from 300 feet (80 m) (Macleod 1983 cited in [145]) to 2,020 feet (615 m) [52,79,97]. In the Alps, English ivy seldom surpasses 4,100 feet (1,250 m) in elevation [3].

One report from the United Kingdom indicated that English ivy may be slightly more frequent and abundant on north-facing slopes than on other aspects [52].

General habitat: In North America, English ivy commonly occurs in deciduous forest and occasionally in conifer forest ([194], Waggy 2000 personal observation [185]), particularly in the Pacific Northwest (Waggy 2000 personal observation [185]). English ivy occurs in both riparian [86,93,94,119,128,147,157,170] and upland forests and woodlands ([125,137,170], Waggy 2000 personal observation [185]), on forest edges [154,190], roadsides [154], and rocky sites [83,190]. In California, English ivy occurs in wetlands [35] and in valley grasslands and foothill oak woodlands [137]. In the mid-Atlantic states, it occurs in coastal areas, salt marsh edges, and fields in addition to other habitats listed above [159]. Its occurrence is often associated with natural or anthropogenic disturbance [43,104,127,130,160,169,170,197,198], buildings and gardens [30,93,143,170,184,192,197], and urban forests ([50,86,170,183], Waggy 2000 personal observation [185]). Researchers studied the effects of logging on 10 sites in 9 coastal redwood riparian forests in California. Time since last harvest ranged from 10 years to over 100. English ivy was more common on sites logged relatively recently compared to sites that had been logged decades ago [130].

Moisture: In North America, English ivy tolerates a wide range of moisture regimes, from uplands ([125,170], Waggy 2000 personal observation [185]) to floodplains [58,86,169,170,187]. In the mediterranean climate region of California, English ivy was classified as flood tolerant. In this region, English ivy may withstand flooding for most of one growing season and may produce limited root development when flooded [187]. In another study in California, one English ivy plant survived 37 days of flooding at water up to 0.5 foot (0.2 m) deep [58]. In Washington, DC, "excessive" moisture in the root zone did not appear to limit the spread of English ivy in flat portions of a floodplain [169], but on another site in Washington, DC, English ivy's growth in the moist floodplain was slower than on upland sites [170]. Little has been reported on English ivy's tolerance to drought in North American populations, but in Arizona, English ivy was recommended for landscaping based on its low evapotranspiration rate (Pittenger 1990, 1992 cited in [39]).

A literature review from the United Kingdom states that English ivy is tolerant of all but the most water-logged or very dry soils and is favored by moist soils ranging from fairly dry to slightly damp [52,97], although short periods of flooding may favor English ivy germination. In France, the mean stem density of English ivy was significantly (P<0.05) higher on an unflooded site (61.9/ha) compared to a site that received periodic flooding (29.4/ha) [136]. In a fen in England, English ivy was rare on sites saturated for 227 days/year, uncommon (5% of the total dry weight of the vegetation) on sites saturated for 54 days, and dominated sites (77% of the total dry weight of the vegetation) that were saturated for 13 days [73]. Because the relative growth rate for potted English ivy seedlings was not significantly reduced when water was limited, researchers concluded that English ivy was tolerant of drought. Others have described English ivy as a xerophyte (Mittmeyer 1931 cited in [170]). English ivy persists through months of drought in the understories of deeply shaded evergreen forest in the Mediterranean Basin [131,132]. Researchers speculated that English ivy may be favored in the Mediterranean Basin over other woodland species if climates become drier [132]. English ivy may not be as drought tolerant in all parts of the Mediterranean, and summer drought may influence its distribution in that region of the world (Huntley and Birks 1983 cited in [97]).

Substrate: English ivy occurs on a variety of soil types and textures in North America [146,151] and Europe [133,145,188,189]. In the Willamette Valley in Oregon, English ivy occurred in a conifer-deciduous mixed forest in silt loam [66]. In the United Kingdom, it may be abundant on heavy clay soils in low-lying areas [145]. In France, English ivy grows best in sandy soils (Beekman 1984 cited in [135]). In Spain, English ivy seedlings grew on 2 sites in mixed sand, clay, and silt. One site was nearly 50% clay, while the other site was a near-even mixture of the 3 soil textures [133]. In England [38] and the Czech Republic [63], English ivy occurs on limestone and in England [188] and France [135], it occurs on calcareous soil.

One report indicates that English ivy grows well in both acid and basic soils in North America [126]. A Plant Conservation Alliance [160] fact sheet suggests it prefers slightly acid soil (pH 6). A publication recommending species for highway planting in Nevada suggests that English ivy tolerates alkaline and saline soils [151]. In the United Kingdom, English ivy is tolerant of all but extremely acid soil, seldom occurring at pH below 4.0 [52,97], and may be most frequent in soils with pH above 6.0 [52]. In southern England, English ivy occurred on a site with pH of 5.24 or less [102] and in southern Spain, English ivy seedlings occurred in soils with pH of 6.5 and 7.2 [133].

SUCCESSIONAL STATUS:
Shade tolerance: Several studies [32,49,62,132,136,150], reviews [51,97], and invasive plant publications [99,190] indicate that English ivy grows in a wide range of light conditions, from full shade to full sunlight. In general, the juvenile form of English ivy most often occurs in the shade [62,97], while the adult form occurs in full sun [32,99]. English ivy's tolerance to shade [32,51,99] has been described by one propagation manual as "legendary" [31]. English ivy's tolerance to full sun may vary and depend on the cultivar or site conditions [150,158]. English ivy's ability to adjust its photosynthetic capacity under variable light regimes may explain its tolerance to a wide range of light levels [62].

English ivy's photosynthetic capacity adjusts for variable light levels; to what degree may be determined by the life phase (juvenile or adult) of the plant [10,62,110]. In general, adult leaves have a greater photosynthetic capacity than juvenile leaves, even on the same plant [62,97]. Under experimental conditions, juvenile English ivy's capacity to accumulate light was not as well developed as its adult phase. Juvenile leaves, however, tolerated light and were not damaged by increased light [62]. English Ivy may respond quickly to changes in light level by temporarily increasing or decreasing photosynthetic rates. English ivy growing in the shade may undergo brief periods of photoinhibition in the winter when leaves of deciduous trees are shed. For English ivy growing in constant light, photoinhibition may be facilitated by low temperatures [110]. Grime [51] speculated that for juvenile English ivy growing in the shade, carbohydrate availability may depend more on energy conservation than accumulation efficiency.

In some locations, English ivy may reach its greatest abundance in shade. In France, English ivy reached its highest frequency in a floodplain forest on "dark" plots with less than 2.5% light transmission [142]. In experimental plots, English ivy cover grown in 60% shade ranged from about 10% to 70% during a 1-year period. During that same time, English ivy cover failed to reach 20% during any time of the year for plants grown in full sun [150].

Potential successional stage: English ivy's tolerance to a wide range of light levels suggests it may establish and/or persist throughout most successional stages. Information pertaining to English ivy's successional role in its North American range is limited. In forests that had been previously clearcut in western Oregon, English ivy was not present in seedling or sapling stands but occurred in mature stands (frequency <2%; cover ~2%) with large-diameter trees. Thinning mature stands had little effect on English ivy frequency (<1%) compared to unthinned stands (<1%), however, its cover was less in thinned stands (1.5%) than in unthinned stands (3.5%). English ivy frequency generally increased with increased tree canopy cover [50]. In Washington, DC, English ivy occurred in a deciduous forest that had not been logged for 102 years [43]. In a secondary piedmont forest in Georgia, English ivy persisted for at least 30 years (Carter personal communication cited in [11]). In a forest along the Bronx River Parkway in New York, it persisted for at least 25 years [44]. In another secondary piedmont forest in Washington, DC, English ivy displaced the groundlayer vegetation, previously dominated by Virginia springbeauty (Claytonia virginica), within 10 years of its establishment [170].

Several studies and publications from Europe describe English ivy's successional role in parts of its native range [38,56,76,121,135,168]. One long-term study from the United Kingdom indicates that English ivy's successional role may be highly variable. Researchers observed successional changes over 100 years in 2 secondary woodlands where English ivy occurred. The woodlands established on previously cultivated land that had been abandoned for about 20 years. Site 1 consisted of woodland and meadow plant communities and was first surveyed in 1886. English ivy was first observed in the woodland community on Site 1 in 1903. Over the next 53 years English ivy continued to spread, and by 1998 it formed a "dense carpet" in the woodland. In the meadow portion of Site 1, English ivy established in 1913 but was absent by 1945. While it was present during the 1951 and 1965 surveys, it was once again absent from the meadow in 1998. On Site 2, which was primarily a grassland with a few woody species, English ivy was not observed there until 1957, approximately 60 years after surveying began. While it persisted on Site 2 throughout the remainder of the study (1998), it did not dominate [56].

Other studies from Europe indicate that English ivy occurs in early to midsuccession; however, its abundance at any given stage may vary [38,76,121,135,168]. In England, English ivy is typically uncommon in woods except around edges. It establishes during the early stages of succession and may persist for centuries [38,121]. In a previously cultivated hay meadow in England that had been abandoned for 20 to 30 years, English ivy established during the "building phase" (average plant age 15-50 years) of plant community development [76]. In the Chiltern Hills in England, English ivy occurs in various stages of succession, but it is more frequent in early stages of oak woodland succession and in developing woodland [189]. In an oak forest in France, English ivy's frequency increased by 10% within 19 years [168]. In the United Kingdom, English ivy established in a field about 15 years after cultivation ceased and dominated the ground flora of a secondary woodland within 50 years [56]. Because English ivy seedlings establish and grow in both sun and shade (see Shade tolerance), it seems able to establish during early to midstages of succession.

Studies from Europe suggest that English ivy may persist into late succession. One literature review indicated that populations of English ivy that established in northern Eurasia during a global cooling period that ended about 5,000 years ago have persisted through vegetative reproduction [97]. In England, English ivy occurred in a woodland that had undergone selective logging but still contained 200-year-old trees [144]. Based on vegetation surveys in the Rhine Forest in France, researchers classified English ivy as a species adapted to highly fragmented, mid- to end-successional stages [135].

On sites where English ivy becomes dominant, it may influence succession. English ivy may inhibit regeneration of the understory, resulting in an English ivy- dominated community with few if any woody plants getting tall enough to form a shrub layer [170]. Because English ivy facilitates tree fall (see Impacts) and accelerates forest gaps [136,170], it may influence succession by creating canopy gaps.

FIRE EFFECTS AND MANAGEMENT

SPECIES: Hedera helix
FIRE EFFECTS: Immediate fire effect on plant: As of this writing (2010), information pertaining to the immediate fire effects on English ivy was limited. One literature review indicated English ivy may have low tolerance to fire [97]. Based on its ability to sprout when cut [25,159], English ivy may sprout from fire-damaged stumps; however, its root system may be too shallow to survive surface fire (see Botanical Description). Because English ivy seed is short-lived (see Seed banking), there may be little opportunity for postfire germination from the seed bank. Researchers in Spain found no evidence of rapid postfire establishment of English ivy from the seed bank [181].

Postfire regeneration strategy (adapted from [155]):
Prostrate woody plant, stem growing in organic soil
Surface rhizome and/or a chamaephytic root crown in organic soil or on soil surface
Liana, adventitious buds and/or a sprouting root crown
Initial off-site colonizer (off site, initial community)
Secondary colonizer (on- or off-site seed sources)

Fire adaptations and plant response to fire: Fire adaptations: As of this writing (2010), it was unclear how well English ivy is adapted to fire. Based on inferences made from its known botanical characteristics, English ivy may reproduce after fire through vegetative regeneration or seedling establishment.

Because English ivy sprouts when cut and roots from stem fragments (see Vegetative regeneration), it may do so if aboveground vines are damaged or killed by fire.

English ivy does not form a persistent seed bank (see Seed banking). Researchers in Spain studied the postfire recruitment of several woody species from the soil seed bank. Based on English ivy's postfire germination rate and its response to experimental smoke and heat treatments, researchers inferred that English ivy seed is not protected from the heat of fire (e.g., lacks hardcoated seed) [181]. Conversely, one invasive species publication indicated that English ivy has a hard seedcoat [126] but provided no evidence in support of this claim.

Inferences based on English ivy's morphology and regeneration suggest some ways in which fire could favor its spread. There is potential for English ivy to establish after fire from off-site seed if there are populations of fruiting English ivy nearby. English ivy seed may be dispersed to burned sites by birds; however, the farther away the source the less likely dispersal is to occur, especially if the source is greater than 3,000 feet (1,000 m) away (see Seed dispersal). A literature review from Great Britain indicated that English ivy seedlings establish on disturbed or open sites [97], suggesting that fire could create conditions favorable for its establishment. However, it may not establish well on sites that experience frequent fire (see Plant response to fire and Fire regimes). English ivy seedlings grow more rapidly on open sites than in shade (see Shade tolerance), suggesting that canopy openings resulting from fire could facilitate rapid growth of English ivy seedlings if they establish on burned sites.

Plant response to fire: Based on its abilities to regenerate vegetatively, adjust to variable light levels, establish on open disturbed sites, and disperse seed over a large area, English ivy may respond favorably to fire. However, the limited available evidence suggests otherwise.

One paleoecological study suggests English ivy may not be favored by fire. Researchers in Switzerland reconstructed historic fire records based on a charcoal and pollen analysis of 2 lakes. Peaks in charcoal particle abundance—presumed to be associated with fire events—were significantly correlated (P=0.05) with repeated declines in the abundance of English ivy pollen [172].

In southern Switzerland, researchers studied historic fire records to evaluate the effects of increasing fire frequency on vegetation. English ivy frequency was greatest on sites with average fire-return intervals of >100 years, declined for sites with average fire-return intervals from 25 to 100 years, and was nearly absent from sites with average fire-return intervals of <25 years. English ivy's frequency generally increased with increasing time since the last fire [28], suggesting that fire exclusion may favor its spread.

English ivy frequency in forests in southern Switzerland with varied fire frequency [28]
Average fire frequency (years)
7-25
25-50
50-100
>100
Years since last fire 0-3 4-15 16-100 0-3 4-15 16-100 0-3 4-15 16-100 >100
English ivy frequency (%) 0 1-10 0 1-10 0 11-20 1-10 1-10 11-20 31-40
Number of samples 10 70 48 10 11 36 21 11 36 11

FUELS AND FIRE REGIMES: Fuels: As of this writing (2010), information pertaining to English ivy's fuel characteristics was limited to anecdotal evidence and inference. Given English ivy's abundance near populated areas, further research on its fuel characteristics may be warranted [4].

Because English ivy is evergreen and has a relatively high water content (230 g of water/100 g dry leaf mass (65-70% wet mass)), it may not readily ignite and may burn slowly. Planting English ivy has been recommended to reduce fire risk in seasonally dry areas such as in Utah [80,97], and in chaparral-urban interfaces in California [123].

While dense populations of English ivy clearly affect the structure of surface and crown fuels (see Stand structure), their impact on fire behavior has not been documented. Researchers in the northeastern United States speculated that English ivy may contribute to ladder fuels [29]. A state forester in Delaware also suggested English ivy contributes to ladder fuels and considered it a serious fire hazard near urban communities [166]. Conversely, an ecologist in Portland, Oregon, speculated that English ivy does not contribute to ladder fuels because of its high moisture content. One researcher in that area attempted to burn English ivy that was growing on cliffs with various grasses. The grasses burned but the English ivy did not, presumably because it was "too green" (personal communication [40]). It has been speculated that English ivy may increase fuel loading and continuity by growing up and over supporting vines, shrubs, and trees and by killing the vegetation beneath it [29].

Fire regimes: In its nonnative range in North America, English ivy occurs in plant communities with variable fire regimes, but at the time of this writing (2010), no information was available on how it responds to or influences fire regimes in these communities. Land managers in the Pacific Northwest speculated that English ivy may influence riparian fire regimes; however, the magnitude and direction of its effects on fuel characteristics and fire regimes is unknown. In moist forests where English ivy occurs, extreme fire weather may be a more important driving force of fire intensity and severity than fuel characteristics [1]; therefore, even if English ivy causes marked changes in fuel characteristics, it may have little or no influence on local fire regimes [4].

Studies from Europe indicate that English ivy occurs in communities with variable fire frequency. On the Iberian Peninsula in Spain, English ivy is associated with plant communities occurring in warm, dry Mediterranean climates that are subject to high fire frequency and montane vegetation in subhumid climates where fires are rare [181]. In southern Switzerland, English ivy occurred in forest types that had mean fire-return intervals ranging from 7 to more than 100 years, but its frequency was generally lower in areas with greater fire frequency (see Plant response to fire).

See the Fire Regime Table for further information on fire regimes of vegetation communities in which English ivy may occur.

FIRE MANAGEMENT CONSIDERATIONS:
Preventing postfire establishment and spread: English ivy may establish after fire by seed on sites where it did not previously occur (see Plant response to fire). Preventing its establishment in burned areas is the most effective and least costly management method of control. This may be accomplished through early detection and eradication, careful monitoring and follow-up, and limiting dispersal of its seed into burned areas. General recommendations for preventing postfire establishment and spread of invasive plants include: For more detailed information on these topics, see the following publications: [6,15,47,178].

Use of prescribed fire as a control agent: As of this writing (2010), no information was available on the use of prescribed fire to control English ivy. Several invasive species publications indicated that repeat burning with a blowtorch at regular intervals has had some success in controlling English ivy; however, no examples using this method were described [25,125,190]. Although blowtorching may not directly kill English ivy, it may deplete its energy reserves by causing it to continually sprout, which may eventually kill it [125]. Reichard [125] cautioned that this approach requires considerable care. This approach also seems infeasible for large populations of English ivy.

Altered fuel characteristics: Researchers in the northeastern United States speculate that English ivy may alter fuel characteristics by acting as ladder fuel (see Fuels) or by killing the trees and shrubs on which it climbs [29].

MANAGEMENT CONSIDERATIONS

SPECIES: Hedera helix
FEDERAL LEGAL STATUS:
None

OTHER STATUS:
Information on state-level noxious weed status of plants in the United States is available at Plants Database.

IMPORTANCE TO WILDLIFE AND LIVESTOCK:
Several invasive species publications indicate that English ivy is of little value to wildlife native to North America [25,125,146,190]. Although some native birds eat the berries (see Seed dispersal), English ivy fruit is preferred primarily by European starlings [146,190]. In the Pacific Northwest, sites dominated by English ivy have lower diversity of mammals, birds, and amphibians compared to uninvaded sites, and appear to be good habitat only for rats [146]. In California, English ivy is unused by most herbivores except nonnative rats [34]. In South Carolina, the leaves of English ivy were important forage for white-tailed deer during the summer [55].

In various parts of its European range, English ivy fruit is eaten by numerous native birds including blackcap, European robin, thrushes, and European starling [14,145]. Wood pigeons are the only know predator of English ivy seed [145]. English ivy foliage is subject to extremely low rates of herbivory in Great Britain [52]. In Denmark, farm animals including cattle, domestic sheep, geese, and peafowl eat the juvenile form of English ivy [14].

Palatability and/or nutritional value: English ivy berries are mildly toxic if overconsumed [8,78,99,145,160]. If ingested, English ivy leaves and berries may cause gastrointestinal upset, diarrhea, hyperactivity, breathing difficulty, coma, fever, dilated pupils, muscular weakness, and lack of coordination [160]. English ivy contains allergens that may affect humans [70].

In Britain, English ivy has one of the most nutritious fruits available to birds. Its fruit contains an especially high fat content [145].

Cover value: In Great Britain, English ivy provides excellent cover for some early-nesting birds [52].

OTHER USES:
English ivy is sold as an ornamental plant by nurseries in the United States [32,57,159]. It is widely planted in landscapes because of its evergreen foliage and dependability as a year-round ground cover [32,87,150,159]. English ivy is planted for erosion control in parts of the United States [126] and Serbia [152]. The America Ivy League was formed to promote the propagation and use of English ivy in landscaping and indoor gardening [165]. It is occasionally recommended for landscaping to reduce fire hazard in seasonally dry areas (see Fuels).

Historically, English ivy was used as a topical agent for its antifungal and antimicrobial properties [70].

IMPACTS AND CONTROL:
Impacts: Reports on English ivy's impacts within its North American range are variable. English ivy threatens native plant communities and wildlands in Oregon [106,113], California [17,35], Washington D.C. [169], Kentucky [74], Georgia [45], and Alabama [2]. It is a potential threat in the upper Great Lakes areas [25], Missouri [101], and Tennessee [163]. English ivy is a particularly serious threat to native plant communities in the coastal Pacific Northwest states [106] and was placed on Oregon's list of quarantine species in 2010 [113]. A 1988 publication indicated that English ivy was not widespread in the southeastern United States [19]; however, a more recent review (2007) indicates that English ivy is rapidly invading forests in this area [12]. English ivy impacts may be less in the northeastern United States [33,176] and Canada [20,193].

NatureServe [107] has given English ivy a ranking of medium for its ecological impacts; its impacts to community structure are of greatest concern. The Plant Conservation Alliance [160] considers English ivy a "vigorous" vine that may impact all strata of a forest. In general, English ivy primarily impacts ecological communities by displacing native ground flora, weakening and/or killing host trees and providing opportunity for invasion by other nonnative species [160].

In locations where it is most invasive, English ivy may form near monocultures in the understory [106,160] and suppress growth of ground flora [4,18,24,26,106,146,170]. On Potomac Island in Washington, DC, English ivy suppressed herbs and may have suppressed woody species on upland sites. Because upland sites are not subject to flooding, Thomas [170] speculated that English ivy's impacts may be greater on upland than riparian sites. Thomas further speculated that English ivy's ability to photosynthesize year-round may improve its capacity to suppress the growth of other plants that photosynthesize seasonally [170]. As it spreads, English ivy may eventually displace [140] or inhibit the regeneration of native species [125,190]. Increased shade produced by English ivy may make it difficult for native species to establish in the understory [26]. Because English ivy displaces native plants, wildlife that utilize native plants for forage or cover may also be impacted.

Trees hosting English ivy may be susceptible to windfall during storms [97,125,136,146,160] especially if they are weak [97] or when they are supporting several English ivy stems [136]. Reichard [126] speculated that the additional weight of water or ice on the evergreen leaves of English ivy may increase storm damage to trees. Invasive plant publications suggested that English ivy decreases "vigor" in host trees [99,146], and a study from Oklahoma suggests that English ivy may inhibit development of top and root mass of host trees, particularly maples [141]. Anecdotal information suggests that as English ivy climbs, it covers and kills supporting tree branches by blocking sunlight. The host tree may eventually die from steady weakening [160,170]. American elm trees may be particularly susceptible to weakening by English ivy. In a riparian forest in Washington, DC, 13% of fallen American elm trees had supported English ivy, whereas only 9% of all the other fallen trees species supported English ivy [170].

In a North Carolina riparian forest, English ivy was associated with several other exotic species, and its occurrence was negatively correlated with native species richness (r²= -0.42). Researchers speculate that only the most "aggressive species" were able to coexist with English ivy and that English ivy's presence may promote invasion by other nonnative species because it spreads fast and displaces most native species [183].

Several other ecological impacts of English ivy invasion have been described in the literature, although most have not been well documented. One report from the Pacific Northwest suggested that English ivy may decrease water quality and increase erosion. Researchers have identified English ivy as a host for bacterial leaf scorch (Xylella fastidiosa), a plant pathogen that harms native trees including elms, oaks, and maples [95]. There is some concern that leaf litter from English ivy increases soil nitrogen, which may negatively impact native plant species that grow best in low nutrient conditions (Tremolieres and others 1988 cited in [126]). Based on stream surveys in California, North Dakota, and South Dakota, microinvertebrate frequency was reduced on sites where English ivy occurred in the riparian vegetation compared to sites where it did not occur; however, the difference was not significant [127].

Invasion by English ivy may have societal impacts as well. Trees susceptible to windfall may create a hazard if near roads, walkways, homes, or other developed areas [160]. Loss of shade trees, increased erosion, decreased water quality, and a loss of forest production due to the invasion of English ivy may be costly for public agencies as well as private land owners [146]. In Mediterranean Italy, English ivy growing on old buildings was detrimental to the preservation of an archaeological site [21].

Control: Control of English ivy has received little attention or research. Past research has focused on establishing new cultivars rather than controlling or eliminating the plant [126]. Complicating matters, English ivy continues to be sold at nurseries for landscaping [32,57,150,159], and the American Ivy Society promotes its use in gardens [165].

Fire: For information on the use of prescribed fire to control this species, see Fire Management Considerations.

Prevention: It is commonly argued that the most cost-efficient and effective method of managing invasive species is to prevent their establishment and spread by maintaining "healthy" native communities [91,139] (e.g., avoid road building in wildlands [175]) and by monitoring several times each year [69]. Managing to maintain the integrity of the native plant community and mitigate the factors enhancing ecosystem invasibility is likely to be more effective than managing solely to control the invader [61].

English ivy's escape from cultivation may be slowed or prevented if native species are substituted in landscaping projects. In an attempt to slow English ivy's spread in Oregon, officials have placed English ivy on the list of quarantined species, making it illegal to propagate, transport, purchase, or sell English ivy in that state [113]. It has been suggested that the best way to prevent English ivy invasion is to avoid growing it near forests [25]; however, since its seeds are dispersed by birds (see Seed dispersal), this may not prevent its invasion entirely. One study from the Netherlands suggests that the frequency of English ivy may decrease with increasing size of "woodlot" perimeter [180], so limiting forest fragmentation may reduce English ivy invasion. Thomas [169] suggested that anthropogenic ground disturbance that alters topographic relief may promote invasion by English ivy and other nonnative species and recommended that original topography be restored to sites to preclude or slow English ivy's spread.

Weed prevention and control can be incorporated into many types of management plans, including those for logging and site preparation, grazing allotments, recreation management, research projects, road building and maintenance, and fire management [178]. See the Guide to noxious weed prevention practices [178] for specific guidelines in preventing the spread of weed seeds and propagules under different management conditions.

Cultural control: See Integrated management.

Physical or mechanical control: Several invasive species publications recommend hand removal to control English ivy. Vines may be cut and then pulled down from trees and off the forest floor [25,125,159,190]. Alternatively, English ivy may be pulled up from its roots; however, this method may disturb soil and promote erosion or compaction of the soil [13]. Soil disturbance may facilitate reinvasion by English ivy and/or the establishment of other invasive plants [25,126]. It may be necessary to follow hand removal with additional types of treatments (see Integrated management). Soll [146] cautions that hand removal of English ivy may be costly. In the Pacific Northwest, 2002 cost estimates ranged from $2,000 to $8,000 per acre when paying minimum wage [146].

Researchers in the United Kingdom suggested early thinning of English ivy to help prevent monocultures from forming [56].

Biological control: There are no biological control agents for English ivy. Because English ivy is an important landscape plant and has strong support from the horticultural community, it is extremely unlikely that one will be developed [146]. A study from Oregon evaluated the use of domestic goat browsing to control English ivy in a mixed-deciduous forest where English ivy formed a near monoculture in the groundlayer vegetation. English ivy's average cover declined significantly (P=0.0002) in plots that were browsed by domestic goats compared to unbrowsed plots. Average cover of English ivy was reduced to 23% on sites browsed for 1 year and to 4% on plots browsed for 2 years [66]. In the Netherlands, English ivy invaded a forest and began to climb trees soon after domestic sheep browsing was discontinued [14].

Chemical control: Information pertaining to the chemical control of English ivy is inconsistent. An invasive species report indicated that at best, chemicals offer incomplete control of English ivy [146]. English ivy may be tolerant of preemergent herbicides (Derr 1993 cited in [126]), and its waxy leaves make effective application of postemergent herbicide difficult [190] even when a surfactant is added [126]. Researchers in Portland, Oregon, suggest the under some circumstances, herbicides may provide safe and effective control of English ivy, even during the winter. English ivy's response to chemical control may be influenced by the type of herbicide used, herbicide concentration, and application timing. Herbicide may be most effective when used as a part of an integrated management plan.

Researchers evaluating various chemicals for English ivy control have obtained variable results [13,97,109,161]. For information on using herbicides to control English ivy, see these publications [13,25,109,146,161].

Integrated management: If hand removal is used, follow-up with other types of treatments may improve control. Sprouts from the stumps of cut vines may be treated with herbicide [25,159] or cut repeatedly until sprouting stops [159]. A follow-up planting with native species may help prevent other undesirable plants from becoming established [13,125].

In a Southeastern hardwood forest infested with English ivy, researchers compared the effectiveness of herbicide versus hand-pulling on the establishment of native plants from seed after treatment. In one plot, vines were sprayed with glyphosate (30%) after having their leaves removed. In a 2nd plot, vines were pulled manually from their roots; in a 3rd plot, vines were untreated. Plots were then split; one half was seeded with native seeds, while the other half received no seed additions. Although both treatments reduced English ivy compared to untreated plots, hand-pulling resulted in more native seedlings, increased species richness, and higher species diversity than did spraying. Researchers speculated that ground disturbance from hand-pulling may have facilitated native seedling establishment [13].

APPENDIX: FIRE REGIME TABLE

SPECIES: Hedera helix
The following table provides fire regime information that may be relevant to English ivy habitats. Follow the links in the table to documents that provide more detailed information on these fire regimes.

Fire regime information on vegetation communities in which English ivy may occur. This information is taken from the LANDFIRE Rapid Assessment Vegetation Models [82], which were developed by local experts using available literature, local data, and/or expert opinion. This table summarizes fire regime characteristics for each plant community listed. The PDF file linked from each plant community name describes the model and synthesizes the knowledge available on vegetation composition, structure, and dynamics in that community. Cells are blank where information is not available in the Rapid Assessment Vegetation Model.
Pacific Northwest California Northeast
Southern Appalachians Southeast
Pacific Northwest
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
(years)
Minimum interval
(years)
Maximum interval
(years)
Northwest Woodland
Oregon white oak Replacement 3% 275    
Mixed 19% 50    
Surface or low 78% 12.5    
Northwest Forested
Douglas-fir (Willamette Valley foothills) Replacement 18% 150 100 400
Mixed 29% 90 40 150
Surface or low 53% 50 20 80
Douglas-fir-western hemlock (dry mesic) Replacement 25% 300 250 500
Mixed 75% 100 50 150
Douglas-fir-western hemlock (wet mesic) Replacement 71% 400    
Mixed 29% >1,000    
Mixed conifer (southwestern Oregon) Replacement 4% 400    
Mixed 29% 50    
Surface or low 67% 22    
California
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
(years)
Minimum interval
(years)
Maximum interval
(years)
California Woodland
California oak woodlands Replacement 8% 120    
Mixed 2% 500    
Surface or low 91% 10    
California Forested
California mixed evergreen Replacement 10% 140 65 700
Mixed 58% 25 10 33
Surface or low 32% 45 7  
Coast redwood Replacement 2% ≥1,000    
Surface or low 98% 20    
Northeast
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
(years)
Minimum interval
(years)
Maximum interval
(years)
Northeast Forested
Northern hardwoods (Northeast) Replacement 39% >1,000    
Mixed 61% 650    
Appalachian oak forest (dry-mesic) Replacement 2% 625 500 >1,000
Mixed 6% 250 200 500
Surface or low 92% 15 7 26
Beech-maple Replacement 100% >1,000    
Southern Appalachians
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
(years)
Minimum interval
(years)
Maximum interval
(years)
Southern Appalachians Forested
Bottomland hardwood forest Replacement 25% 435 200 >1,000
Mixed 24% 455 150 500
Surface or low 51% 210 50 250
Mixed mesophytic hardwood Replacement 11% 665    
Mixed 10% 715    
Surface or low 79% 90    
Appalachian oak-hickory-pine Replacement 3% 180 30 500
Mixed 8% 65 15 150
Surface or low 89% 6 3 10
Appalachian oak forest (dry-mesic) Replacement 6% 220    
Mixed 15% 90    
Surface or low 79% 17    
Southeast
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
(years)
Minimum interval
(years)
Maximum interval
(years)
Southeast Forested
Coastal Plain pine-oak-hickory Replacement 4% 200    
Mixed 7% 100      
Surface or low 89% 8    
Maritime forest Replacement 18% 40   500
Mixed 2% 310 100 500
Surface or low 80% 9 3 50
Loess bluff and plain forest Replacement 7% 476    
Mixed 9% 385    
Surface or low 85% 39    
Southern floodplain Replacement 7% 900    
Surface or low 93% 63    
*Fire Severities
Replacement: Any fire that causes greater than 75% top removal of a vegetation-fuel type, resulting in general replacement of existing vegetation; may or may not cause a lethal effect on the plants.
Mixed: Any fire burning more than 5% of an area that does not qualify as a replacement, surface, or low-severity fire; includes mosaic and other fires that are intermediate in effects.
Surface or low: Any fire that causes less than 25% upper layer replacement and/or removal in a vegetation-fuel class but burns 5% or more of the area [56,82].

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