Table of Contents
©John M. Randall/The Nature Conservancy
No naturally occurring hybrids are reported in the literature. There may be a number of ornamental hybrids, as Hickman  indicates that most plants reported as French broom may be hybrids.LIFE FORM:
French broom is 1 of 4 nonnative invasive broom species that occur in North America. Scotch broom (Cytisus scoparius), Portuguese broom (C. striatus), and Spanish broom (Spartium junceum) occur in similar habitats and have some similar morphological and ecological characteristics. Common gorse (Ulex europaeus) is another leguminous shrub that occurs in similar habitats, but is morphologically distinct from the brooms.
The following lists include vegetation types in which French broom is known or thought to be potentially invasive, based on reported occurrence and biological tolerances to site conditions. Precise distribution information is limited; therefore, these lists may not be exhaustive.ECOSYSTEMS :
In California, French broom occurs in annual grasslands, oak woodlands, coastal scrub, chaparral, conifer, and relatively open mixed evergreen forests [11,68,73]. At sites in California where nonnative annual grasses typically codominate with native perennial grasses and forbs, there has been substantial French broom invasion into the otherwise intact open grassland. At more than half of these sites, Scotch broom is also invading . On the edge of Mt. Tamalpais State Park, French broom occurs on an exposed, west-facing slope dominated by nonnative grasses with patches of coyote bush (Baccharis pilularis) and several nonnative forbs and shrubs including bigleaf periwinkle (Vinca major), silverleaf cotoneaster (Cotoneaster pannosus), narrowleaf plantain (Plantago lanceolata), evergreen blackberry (Rubus laciniatus), and poison hemlock (Conium maculatum). Here French broom and Scotch broom grow interleaved, with Scotch broom individuals somewhat more sparse (about 15% cover) and spread out than French broom (about 30% cover). At China Camp State Park, French broom and Scotch broom grow in patches and as scattered individuals, with a total cover of about 35% and 15%, respectively. Other common species included coyote bush, toyon (Heteromeles arbutifolia), poison-oak (Toxicodendron diversilobum), nonnative musk thistle (Carduus nutans), Uruguayan pampas grass (Cortaderia selloana), and other nonnative grasses . On a coastal grassland site in Marin County, dense cover of French broom occurs in a matrix currently dominated by nonnative annual grasses including oats (Avena spp.), sixweeks grasses (Vulpia spp.), and bromes (Bromus spp.), and thought to have formerly been composed of purple needlegrass (Nasella pulchra), California oatgrass (Danthonia californica), red fescue (Festuca rubra), and blue wildrye (Elymus glaucus). Scotch broom was also present in small numbers (<5% of aboveground plants) in some areas .
French broom also occurs in the understory of bristlecone fir (Abies bracteata) forests . Scotch broom and French broom are also persistent problems along roadsides in redwood (Sequoia sempervirens) ecosystems of California, and occur in redwood forests after clear-cutting, persisting until the canopy closes .
French broom invades disturbed areas in maritime chaparral stands around Monterey Bay, where several native species of concern occur. These include Hooker's manzanita (Arctostaphylos hookeri ssp. hookeri), Pajaro manzanita (A. pajaroensis), Monterey ceanothus (Ceanothus cuneatus var. rigidus), Monterey spineflower (Chorizanthe pungens var. pungens), Eastwood's goldenbush (Ericameria fasciculata), and Yadon's piperia (Piperia yadonii) .
There was no information in the literature on plant communities in which French broom occurs in either Oregon or Washington.
©Mandy Tu/The Nature Conservancy
The following description of French broom provides a summary of the range of characteristics described in reviews [11,22,54] and florae . It provides characteristics that may be relevant to fire ecology, and is not meant for identification. Hickman  provides a key for identification of French broom.
French broom is an upright, evergreen shrub, commonly less than 10 feet (3 m) tall, occasionally to 16 feet (5 m) tall. Stems are erect, dense, and green and densely covered with silky, silvery hairs. French broom is typically leafy (as compared with Scotch broom, which has few leaves), with compound, deciduous leaves with leaflets 0.4 to 0.8 inch (10-20 mm) long and petioles 5 mm long. French broom inflorescences are dense clusters occurring in racemes on axillary short-shoots. They are composed of 4 to 10 pea-like flowers, 0.2 to 0.3 inch (5-7 mm) long on axillary pedicels 1 to 3 mm long. The fruit is a legume, 0.5 to 1 inch (15-25 mm) long and 5 mm wide with 3 to 8 seeds per pod. French broom seeds are round tooval and have elaiosomes.
Little mention of French broom root systems appears in the literature, although Pitcairn  notes that plants have a deep, branching taproot. As a legume, French broom forms a symbiotic association with nitrogen-fixing bacteria located in nodules on its roots. Seedlings of French broom have well developed root nodules, even at the 1st leaf stage .
Research on stem photosynthesis has been conducted and results reported on Spanish broom and Scotch broom [7,48]. Spanish and Scotch broom rely heavily on stem photosynthesis during the hot summer months, and are well adapted to open, high sunlight environments. However, drought stress severely inhibits photosynthesis of brooms . Nilsen and others  also studied stem photosynthesis in French broom and, while they did not provide data for this species, they stated that it has minimal stem photosynthetic capacity but does well in similar environments (see Site Characteristics).
Growth form and stand structure: Dense broom infestations produce large amounts of dry matter, which can create a serious fire hazard. This is particularly true for gorse and French broom (, and references therein).RAUNKIAER  LIFE FORM:
Breeding system: French broom is monoecious and does not show appreciable levels of selfing .
Pollination: Both native and nonnative insects pollinate French broom. Parker and others [51,53] demonstrated pollinator limitation in French broom, underscoring the potential importance of pollinators to its fecundity and spread. Patterns of fecundity were not, however, strongly predicted by differences in pollen limitation between species (French and Scotch broom) or between sites . Because nonnative honeybees are often themost common pollinators of brooms [52,72], potential negative impacts of beekeeping on broom management have been suggested .
Seed production: French broom becomes reproductive at 2 to 3 years of age, or on reaching a height of about 1.5 to 2 feet (45-60 cm) [1,11]. A medium-sized shrub can produce over 8,000 seeds a year (Bossard, unpublished data, cited by ). Adams and Simmons  found an average of 7,400 pods per bush, with an average of 4.8 seeds per pod, and 5% of the pods damaged by insects in a dense infestation of French broom in dry sclerophyll vegetation in Victoria, Australia.
Seed dispersal: French broom pods burst open explosively, ejecting seeds up to 13 feet (4 m). Seeds are further dispersed by ants, birds, and other animals and in river water, rain wash, and mud, and by vehicles and machinery [1,11].
Seed banking: Dormancy of French broom seed is enforced by a hard coat that prevents imbibition. A large proportion (60%-82%) of French broom seed is dormant upon dispersal ([1,49], and references therein). The high rate of seed production coupled with high rates of seed dormancy result in a rapid build-up of persistent, soil-stored seed . French broom seeds are known to survive at least 5 years in soil (Bossard, unpublished data cited in ). A large number of dormant French broom seeds in the soil seed bank can lead to high germination rates following soil and vegetation disturbance such as that caused by fire . Even though seed densities tend to decrease with distance away from broom stands, there may be sufficient numbers to support a population expansion after fire ( and references therein).
A review by Bossard  reports French broom seed bank densities range from 465 to 6,733 seeds/m². An average of 10,000 dormant broom seeds/m² occurred in the litter and soil under mature French broom stands in Marin County (Parker and Kersnar 1989, cited by ). Average French broom seed bank densities of 3,774 seeds/m² and 2,563 seeds/m² were recorded at 2 sites in Australia . Seed bank densities seem to increase with soil depth with the age of the broom stand .
French broom seed bank densities may be related to stand age, although no clear relationship was found in French broom stands of varying ages on a coastal grassland site in Marin County, California . Seed density of French and Scotch broom, native perennial grasses, native forbs, nonnative annual grasses, nonnative perennial grasses, and nonnative forbs were compared in areas where the grassland matrix under the broom stands was dominated by nonnative annual grasses. Scotch broom was present only in small numbers (<5% of total aboveground plants), and no distinction was made between seeds of the 2 broom species. Broom stands ranged from 5 to 15 years old, and broom seed bank density ranged between 900 and 10,582 seeds/m². Younger and older stands tended to have fewer seeds than stands in the middle age range, with a general trend of increasing seed numbers as stands age, a stabilization of numbers in middle-aged stands, and a decrease in seed density among the oldest stands . A similar trend was observed in a 4-year study of Scotch broom in Australia . However, variation in seed bank density among sites was large, and there was no clear relationship between stand age and seed bank density. Density of all nonbroom seeds as a group decreased with stand age, although seed densities of individual species were not different between sites of different ages. There were no significant relationships (p=0.734) between stand age and the number of dormant or dead broom seeds in the seed bank. However, there was a trend toward an increasing percentage of deeply dormant seeds with increasing stand age. The large variation in broom seed numbers among sites may be due to differences in productivity and fecundity brought about by differences in availability of soil resources and/or pollinators; or by differences in seed dispersal or predation by granivorous insects among sites .
Germination: French broom is polymorphic in its germination behavior, with about 18%  to 40% (Parker and Kersnar 1989, cited in ) of fresh seed germinating immediately on exposure to suitable conditions, and the remainder dormant. Scotch broom seed buried below about 3 inches (8 cm) does not germinate , and very few broom seeds occur below 4 inches (10 cm) (Parker and Kersnar 1989, cited in ), [3,24]. French broom seed may be stimulated to germinate by rain, scarification during soil disturbance, or fire.
In field studies in Australia, Adams and Simmons  observed a flush of French broom seedlings after autumn rains in April 1988. Average seedling counts were 790 seedlings/m² under dense mature stands without disturbance.
Several researchers have observed a flush of French broom seedlings following fire [12,13,20,49], and laboratory studies indicate that heat scarification stimulates germination of French broom ([1,11,49] and references therein) and Scotch broom [8,60,67,74] seeds. Heat of 122 to 212 °F (50-100 °C) doubled French broom germination rates, and seed mortality ensued at about 257 °F (125 °C) (Parker and Kersnar 1989, cited in ). Cheng (in press) reports that heat treating seeds with temperatures of 149 °F (65 °C) improved germination of seed in some populations but not in others . Seeds that were heat treated with boiling water "to simulate the cracking of the testa by fire" germinated rapidly in light at 68 °F (20 °C), with over 50% of viable seeds germinating within 12 days, and 90% within 14 days .
Hand scarification also stimulates germination of French broom seed .
Seedling establishment/growth: Very little information is available on French broom seedling establishment and growth. There is a high potential for seedling recruitment following fire [1,12,13,20,49] (see Plant Response to Fire).
Regular recruitment can lead to rapid French broom population expansion. At a site in Australia the invasion front moved over 10 feet (3 m) in 12 months .
Asexual regeneration: French broom can sprout from the root crown after cutting. Once seedlings are taller than approximately 8 inches (20 cm), their rate of sprouting after cutting can be over 90%, particularly if cut in the rainy season (Bossard, unpublished data in ). Boyd  also reports sprouting from the stem following top-kill by fire.SITE CHARACTERISTICS:
French broom is common in disturbed places such as riverbanks, road cuts, logged or burned areas, pastures, and road and utility rights-of-way [11,35]. It often occurs on steep and exposed slopes .
In California, French broom occurs on sites with varied soil moisture conditions, but seems to prefer siliceous soils. Unlike other broom species in California, it grows reasonably well on alkaline soils with pH 8. Its ability to fix nitrogen allows growth in low-fertility soils .
French broom seedlings are less tolerant of frost than Scotch broom seedlings, and consequently are less often found at high elevations . Hickman  reports that French broom occurs to 1,600 feet (500 m), and Bossard  reports that it occurs to 2,600 feet (800 m) in California. In the Santa Lucia Ranges, southern Los Padres region, and San Diego region, French broom is reported below 1,700 feet (520 m) .SUCCESSIONAL STATUS:
French broom flowers from March to May in southern California  and on inland sites, and from March to July on the northern coast. Flowers appear just prior to emergence of new leaves. Seeds mature in June and July. French broom seed pods burst in the heat of spring and summer, dispersing seed. Sometimes a 2nd flowering occurs towards the end of summer .
As in other brooms, most photosynthate is moving up in the shrub toward branch tips during flowering, bud break, and seed set. Photosynthate starts moving down toward roots of French broom after seeds are well grown but before seed release (Bossard and others 1995, cited in ).French broom produces new growth each winter and spring .
Several reports indicate a postfire flush of French broom germination from the soil seed bank [12,13,20,49]. Several studies indicate increased germination of French broom ([1,11,49] and references therein) and Scotch broom [8,60,67,74] following heat treatment of seeds, indicating an adaptation for postfire germination.
Fuels/flammability: A review by Bossard  suggests that French broom burns readily and carries fire to the tree canopy layer, increasing both the frequency and intensity of fires in invaded areas. Similarly, Parsons and Cuthbertson  suggest that French broom causes concern in forest areas in Australia because it forms a flammable understory at the forest edge, where fires are most likely to start. Conversely, combustion of live, standing broom is difficult under conditions in which prescribed burns are typically conducted in California (cool, wet, low wind days that provide lower risk of an escaped fire), unless fuel loads are artificially increased. Despite high temperatures and low humidities, researchers in Marin County, California were unable to burn a mature, uncut French broom stand, and a young uncut stand had only spotty combustion .
Fire regimes: There is no information available on fire regimes in the native range in which French broom evolved. However, Scotch broom and Genista florida, a close relative of French broom, were early successional species following fire in their native range in Spain .
It is unclear how the presence of French broom might affect fire regimes in invaded communities. In general, in ecosystems where French broom replaces plants similar to itself (in terms of fuel characteristics), French broom may alter fire intensity or slightly modify an existing fire regime. However, if French broom invasion introduces novel fuel properties to the invaded ecosystem, it has the potential to alter fire behavior and potentially alter the fire regime (sensu [14,19]). No experimental information is available regarding fire regimes changed by French broom.
French broom occurs in a variety of ecosystems in North America that represent a range of historic fire regimes. In many areas where French broom occurs, historic fire regimes have been dramatically altered due to fire exclusion and due to massive disturbances associated with human settlement. The historic fire regimes of native communities in which French broom occurs range from high frequency fires in grasslands, to high frequency, low-severity surface fires in open ponderosa pine forests; and moderate frequency, high severity fires in California chaparral (see Fire Regime table, below). French broom did not occur in these communities at a time when presettlement fire regimes were functioning, but has established since fire exclusion and habitat alteration began. It is unclear how presettlement fire regimes might affect French broom populations.
Herbaceous communities dominated by nonnative annual grasses and forbs of Mediterranean origin occur throughout the Coast Ranges and foothills of the Cascade Range and the Sierra Nevada. A review by Keeley  indicates that much of the nonnative annual grassland in the coastal ranges of central and southern California derives from a fire-induced type conversion of shrublands. The herbaceous communities that have long dominated these landscapes were largely created by anthropogenic burning by Native Americans, and were further maintained by intensive land use with fire and livestock grazing by European-Americans. In recent decades, however, grazing has been eliminated in some areas and anthropogenic fires reduced such that woody vegetation is reestablishing. Along with native shrubs, nonnative shrubs such as French broom, Scotch broom, and gorse colonize these sites. Nonnative shrub colonization of grasslands may decrease fire frequency but increase fuel loads and alter fire behavior ( and references therein).
It is also unclear how the use of fire to control broom in invaded communities might impact native species. Plants are adapted to particular fire regimes, or combinations of fire frequency, intensity, extent, and season. The frequency, intensity, and season of burning prescribed to control nonnative species must be carefully chosen to avoid damaging native species. Prescribed fire may have undesirable effects if introduced into an ecosystem that has undergone shifts in species composition, structure, and fuel characteristics outside a natural range of variability of these attributes . When the natural fire regime is altered, even highly fire-adapted plant communities may be vulnerable to competition from nonnative species .
According to Swezy , fire is an effective management tool for French broom, but is used primarily in mixed evergreen forest and grassland communities in California, where repeated annual burning for broom control "appears to have no unwanted side effects." Prescribed fire is used less frequently in chaparral communities where frequent burning or burning outside the natural fire season may have adverse effects on native communities. It has been suggested that prescribed fire may have the added benefit of promoting native species in California grasslands ; however, the long-term effects of repeated burning on the grassland seed bank are not known .
The following list provides fire return intervals for plant communities and ecosystems where French broom is or may be important. It may not be inclusive. If you are interested in plant communities or ecosystems that are not listed, see the complete FEIS Fire Regime Table.
|Community or Ecosystem||Dominant Species||Fire Return Interval Range (years)|
|California chaparral||Adenostoma and/or Arctostaphylos spp.||< 35 to < 100|
|California montane chaparral||Ceanothus and/or Arctostaphylos spp.||50-100|
|California steppe||Festuca-Danthonia spp.||<35 |
|western juniper||Juniperus occidentalis||20-70|
|Rocky Mountain juniper||Juniperus scopulorum||< 35 |
|Jeffrey pine||Pinus jeffreyi||5-30|
|Pacific ponderosa pine*||Pinus ponderosa var. ponderosa||1-47 |
|quaking aspen (west of the Great Plains)||Populus tremuloides||7-120 [5,32,44]|
|coastal Douglas-fir*||Pseudotsuga menziesii var. menziesii||40-240 [5,46,61]|
|California mixed evergreen||Pseudotsuga menziesii var. menziesii-Lithocarpus densiflorus-Arbutus menziesii||< 35|
|California oakwoods||Quercus spp.||< 35 |
|coast live oak||Quercus agrifolia||2-75 |
|canyon live oak||Quercus chrysolepis||<35 to 200|
|blue oak-foothills pine||Quercus douglasii-P. sabiniana||<35|
|Oregon white oak||Quercus garryana||< 35 |
|California black oak||Quercus kelloggii||5-30 |
|redwood||Sequoia sempervirens||5-200 [5,27,71]|
|western redcedar-western hemlock||Thuja plicata-Tsuga heterophylla||> 200 |
Fire may kill some French broom seeds in the soil seed bank, although seed
bank densities are not substantially decreased immediately following fire [3,49].
Observed reductions in French broom seed banks following fire (see
Plant Response to Fire)
are more likely due to subsequent germination rather than direct fire-induced seed mortality .
DISCUSSION AND QUALIFICATION OF FIRE EFFECT:
The effects of fire on broom seed in the soil seed bank depends on temperature maxima and duration of heating that occurs during fire and the depth of seed burial, as well as seed bank density. Seed bank densities seem to increase with soil depth with the age of the broom stand .
PLANT RESPONSE TO FIRE:
Germination of French broom from the soil seed bank appears to be induced by fire, as high seedling densities occur after a single fire [1,12,13,20,49]. Laboratory tests support the notion that heat from fire scarifies French broom seed and stimulates germination ([1,11,49] and references therein) (see Germination).
D'Antonio and Haubensak  found that more French broom seeds germinated in burned than in unburned grassland habitat in Marin County, and that growth and survival of seedlings were enhanced in burned areas. Data from various studies on French broom and Scotch broom indicate a reduction in broom seed bank densities of 43% to 97% after one fire [9,11,24]. Similarly, a small roadside population of French broom in Australia was burned, and seedling emergence following burning was 174 seedlings/m². French broom seed continued to germinate without further disturbance for up to 3 years after the fire. Some postfire sprouting was also observed .DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:
Frequency: Substantially lower germination rates in the 2nd and 3rd postfire growing seasons compared to those of the 1st postfire growing season suggest that most French broom seed in the soil seed bank is depleted after one fire . Similarly, postfire germination response of French broom decreases after sequential burning . Alexander and D'Antonio  examined the effects of fire on the seed bank under French broom stands of varying ages, comparing broom (mostly French broom, with small amounts of Scotch broom), native perennial grasses, native forbs, nonnative annual grasses, nonnative perennial grasses, and nonnative forbs. They used 2 different approaches. First they surveyed the soil seed bank across stands that had either never been burned or had burned from 1 to 4 times. Next they sampled the seed bank in 2 sites immediately before and after prescribed burning to look at direct effects of fire on the number of viable seeds in the soil. They found a large decrease in the number of broom seeds between the never burned and the burned sites, though there was no statistical difference between sites burned once versus sites burned more than once. The French broom seed bank did not change with repeated burning, although the seeds of other species appear to have been affected. The size of the nonnative seed bank (other than brooms) in twice-burned plots was equal to or greater than the seed bank of the never-burned stand. However, a 3rd burn decreased nonnative perennial grasses and nonnative annual grasses while increasing native forbs. The results of comparisons of pre- and postburn samples show similar trends; however, because of the low replication of direct burning treatments, it is not clear whether these patterns are due to repeated burning or to the nature of the individual fires .
Severity: Control efforts and observations by Boyd [12,13] indicate that when broom is hand-cut and left on site to cure, subsequent prescribed burning is severe enough that postfire sprouting is prevented. The timing of burns in Marin County is dependent on the occurrence of climatic conditions that provide lower risk of an escaped fire (cooler, wetter, low-wind days). Though fuel loads are increased by cutting and drying broom biomass before burning, fire intensity and severity are rarely high (personal observation, cited by ).
Odion and Haubensak  studied seed bank depletion and site alteration in French broom stands in relation to burning. Pre- and postburn sampling was conducted in relatively young and old stands of broom in which fuel was added, removed, or left undisturbed. Comparisons were made with an adjacent unburned control stand. Addition of fuel in old stands did not improve seed bank depletion and it resulted in much lower native species cover compared to leaving broom slash where it fell. In contrast, if stands are burned at a young enough age, with supplemental fuel, the seed bank can be more rapidly exhausted . Additional information is available on the response of Scotch broom plants and seed to varying heat treatments and varying fire severities.
Even with high seed mortality following high-severity fire, some French broom seeds will probably remain in the soil. Often enough broom seeds remain to replace a French broom  or Scotch broom [24,62] stand.FIRE MANAGEMENT CONSIDERATIONS:
Fire as a control agent: Unlike mowing or slashing, fire may remove most of the aboveground vegetation and leave the soil bare, thereby providing a large number of open sites for the establishment of species, such as French broom, from the seed bank . In many places in California, prescribed fire is used to control French broom by consuming aboveground vegetation (which is typically cut and left on site to cure before burning) and stimulating germination from the seed bank [3,12,13,49,73]. The dry fuel helps carry the fire, but fires often do not burn evenly and are affected by weather and microtopographic conditions . Follow-up treatments (burning, pulling, herbicides) are then used to kill seedlings. After the 2nd treatment, seedling emergence is usually reduced to low levels; however, follow-up monitoring and treatment are required indefinitely [3,12,13,49,73]. Swezy and Odion  provide fire management principles and guidelines to consider for managing French broom in wildlands.
Control of French broom with repeated prescribed burning, preceded by cutting adult broom (both French and Scotch broom) plants, was first reported from Mt. Tamalpais State Park by Boyd . Continued efforts to remove broom from the Park allow some conclusions to be drawn regarding the effectiveness of this method there . When the control project began, French broom occurred on about 70 acres (28 ha) of the 91-acre (37-ha) project area. One stand was almost continuous French broom, while others formed a mosaic with surrounding grassland and native shrubland. Large stands of Scotch broom were also present in some areas. All areas were grassland sites before the establishment and spread of broom. Adult broom was cut to within 3 to 24 inches (8-60 cm) of the ground using chainsaws. Cut stems were left in place, providing a 2 to 3-foot- (0.6-0.9 m) deep fuel bed. Because of the long-term viability and abundance of the broom seed bank, reestablishment of broom stands must be prevented by killing postfire germinants before they produce seed. At Mt. Tamalpais, this is accomplished by establishing a cover of annual grasses (agricultural barley (Hordeum vulgare) and a 'Blando' brome (Bromus hordeacus)-'Zorro' rattail sixweeks grass (Vulpia myuros) mix) to provide fuel for subsequent prescribed burns to kill broom seedlings. Treatments on 3 sites were as follows :
|Site||Date cut||Date burned||Date seeded||Grass burned|
|April 1994||May 1994||Oct.-Dec. 1994||July 1995, July 1996|
|Upper Muir Woods (12 acres)||Dec. 1994-March 1995||June 1995||Nov.-Dec. 1995||Oct. 1996, Sept. 1997|
|Lower Muir Woods (20 acres)||Nov.1995||April 1996||Dec. 1996||Sept.1997|
All burns resulted in complete mortality of mature broom, broom seedling establishment, and vigorous coyote bush sprouting. Some differences in fire behavior and response of broom to timing of cutting and burning were observed. None of the cut broom stems cut in April 1994 sprouted before burning in May 1994. Broom burned in May was very dry and extremely flammable; the weather was moderate (73 °F (40 °C), 52% relative humidity), but very intense upslope headfires were possible. Rainfall occurred soon after burning, and numerous broom seedlings established and survived the summer. Due to the abnormally wet winter, the site cut in December 1994 could not be burned until June 1995. Substantial stem sprouting had occurred by that time. The fires were less intense, but still consumed all of the cut material. The April 1996 burn was conducted during relatively cool conditions due to rainfall, but again resulted in complete consumption .
The results of seeding grass for fuel were not uniform. Agricultural barley did not provide an adequate fuel bed, while the 'Blando' brome-'Zorro' rattail sixweeks grass mix did. Both of these species, while nonnative, are already present in the larger area. After the planted annual grasses had cured, there were patches of broom seedlings that would clearly survive the fire in areas where grass fuels appeared inadequate to produce mortality. These patches were cut with weed-eaters before burning. The fires that burned the grass fuel killed almost all of the broom seedlings, even though many stems were not consumed. Seedling size seems to influence vulnerability to fire with minimum heat needed to kill small plants. Limited basal sprouting of larger seedlings after burning was observed in some cases, but the sprouts were very small compared to native coyote brush and nonnative sweet fennel (Foeniculum vulgare) sprouts. The number of broom seedlings that established after the 2nd burns were far fewer than after the 1st, suggesting a reduction in seed bank abundance. If seedlings are too large to be vulnerable to burning, an herbicide spot treatment may be necessary [12,13].
A review by Bossard  indicates that using fire to remove uncut French broom in late spring or early summer has also had some success at Mt. Tamalpais State Park. Repeated burning is necessary 2 and 4 years after the initial burn to kill broom sprouts and seedlings. Follow-up burning is most successful if there are either naturally occurring or seeded grasses to carry the fire .
The effectiveness of fire as a control agent for French broom depends on a number of factors, including stand age, seed bank density, and vertical distribution of seeds in the soil profile [49,73]. It is important to identify what stage an infestation is in and the density and depth of the soil seed bank before initiating control treatments.
Odion and Haubensak  found French broom stands about 10 years old or younger have a small, relatively shallow persistent seed bank and can be treated with cutting and repeat prescribed burning. Addition of fuel before burning helps deplete the soil seed bank in young French broom stands, but not in older stands. French broom seed accumulation at soil depths from which seedlings do not emerge (>3 inches (8cm)) requires regular monitoring and treatment to eliminate emerging plants . Some researchers in Australia suggest that older broom stands should be left intact (not burned) unless follow-up treatment is assured [23,55]. Results in California presented by Alexander and D'Antonio  indicate that older broom stands pose similar challenges to control as stands of younger ages (similar broom seed bank size); however, lower densities of native species in the soil seed bank under older stands increase concern that restoration will become increasingly difficult as stands age.
In some ecosystems, prescribed fire may have the added benefit of promoting native species (see Fire Ecology). However it is not clear that repeated burns are the most effective means of both reducing the broom seed bank and also promoting native species. Burning generally encourages native forbs in California grasslands, but the fire frequencies required for broom control may have negative effects on native species overall, and alternative follow-up treatments (e.g. spot-burning, cutting, or herbicides) might be considered .
Ken Moore (personal communication, cited in ) reports that California State Parks has been very successful (100% mortality) using a propane torch to remove French broom seedlings, up to 8 inches (20 cm) tall, that emerge from the seed bank after removal of adult brooms. This is done at the end of the rainy season when seedlings have emerged and fire danger is low. The torch is set so it is hot, but not flaming, and it is passed over the French broom seedlings. The heat does not burn the seedlings, but within a day the seedlings are wilted and dead. Flame throwers have also been used to spot-treat roadsides or small areas of seedlings emerged from the seed bank after removal of adult brooms in Redwood National Park and in New Zealand ( and references therein).Another approach for using fire to control broom, which was the most effective removal treatment tested by the CalEPPC broom committee, involves integrating prescribed fire and herbicides. Mature shrubs in a dense infestation were killed in early July with a basal bark application of triclopyr herbicide. After 4 weeks, all the shrubs were dead and were cut down, left on site, and burned. A flush of French broom seedlings followed the next rain. For the next 2 years seedlings were either killed with glyphosate or cut with a brush cutter. Mean percent cover of French broom was reduced from 87% to less than 0.2% in plots treated with basal bark triclopyr, cut, burned, and seedlings treated with glyphosate. French broom seed banks in burned plots were reduced to less than 5% of their original size 3 years after prescribed burns. Seed banks of unburned plots otherwise treated the same were reduced to 15.5% of their original size, and control plots exhibited no substantial decrease in seed bank size (Bossard and others 1995 as cited by ).
© Barry A. Rice/The Nature Conservancy
Palatability/nutritional value: According to Hickman , French broom flowers, and perhaps all parts, are toxic. French broom foliage and seeds contain a variety of quinolizidine alkaloids, especially in young leaves. In some livestock, ingestion of plant parts can cause staggering followed by paralysis. French broom foliage can cause digestive disorders in horses (, and references therein).
Cover value: No information is available on this topic.OTHER USES:
Several reviews suggest that French broom invades native vegetation, displaces native plant and forage species, and dominates the invaded community, sometimes forming dense, monospecific, almost impenetrable stands [11,43,57]. Experimental evidence indicates that French broom invasion causes changes in plant community composition by displacing vegetation and decreasing local native plant alpha diversity . A comparison of 12 "old-growth" French broom plots and 12 uninfested California grassland plots indicates that the number of native species (11.7 species + 0.8 s x, versus 3.1 + 0.3) and cover (15.1 + 5.0% versus 5.1 + 1.7%) are higher in plots where broom is absent . A study of seed banks in California grassland indicates a decrease in seed banks of native species in invaded stands, even in French broom stands as young as 5 years .
Some authors suggest that French broom invasion leads to changes in soil properties that may have implications for restoration and rehabilitation efforts. Invasions may change microclimate conditions on and in the soil . French broom invasion is thought to be responsible for reducing arthropod populations by 1/3 in Golden Gate National Recreation Area (Landforn and Nelson 1992, ctied by ). As a nitrogen-fixing species, French broom may enrich soil nitrogen levels and alter nitrogen dynamics in the invaded system .
French broom along roadsides obstructs views, requiring expensive ongoing road maintenance. Since it grows more rapidly than most trees used in forestry, it shades out tree seedlings in areas that are revegetated after harvest and makes reforestation difficult .
While it has been suggested that French broom invasion increases fire hazard (e.g. [11,54]), this relationship may be site-specific, as others authors (e.g. ) indicate that French broom is difficult to burn (see Fire Ecology).
Control: As with other broom species, the best method for removal of a French broom infestation depends on climate, topography, age and size of the infestation, importance of impact to nontarget species, and type, quantity, and duration of resources available to remove and control broom at the site. All methods require appropriate timing and follow-up monitoring. Because of the seed bank, monitoring removal sites to locate and kill new seedlings is essential. Location and treatment of sprouts is also necessary. Sites should be examined annually following broom seed germination (usually late spring) for 5 to 10 years, and every 2 years thereafter .
Controlling the seed bank is an important obstacle for French broom control. To do this it is necessary to determine the conditions for stimulating germination of the greatest number of seeds to exhaust the seed bank. In the case of French broom, a sizeable seed bank may remain dormant in the soil indefinitely in the absence of fire or other treatments that break dormancy .
The following is a general review of control methods and includes information that may be applicable for better understanding of French broom's fire ecology. Given their similar biology and lack of information on all of the broom species, much of this information comes from research on Scotch broom and is generalized to apply to all brooms. More research is needed, however, to understand how this information applies to French broom, Portuguese broom, and Spanish broom. The reader is referred to other reviews [4,10,37,38,58] and the Weed control methods handbook for more details on control methods.
Prevention: The most effective method for managing invasive species is to prevent their establishment and spread. Some methods of prevention include limiting seed dispersal, containing local infestations, minimizing soil disturbances, detecting and eradicating weed introductions early, and establishing and encouraging desirable competitive plants .
Invasive broom species (French, Scotch, Portuguese, and Spanish brooms) should not be used for horticultural and landscape purposes. McClintock  indicates that several ornamental brooms are not invasive. An effective step in preventing further introduction and spread of the weedy brooms may be in asking nurseries to carry only the nonweedy species of broom .
Integrated management: Integrated management approaches for French broom involve combining prescribed fire with other control methods. See the Fire Management Considerations section of this summary for more information.
Physical/mechanical: Manual methods of broom removal, such as hand-pulling and removal with hand tools, have the benefit of being highly selective, thus allowing removal of weeds with minimal damage to surrounding desirable vegetation. Hoshovsky  suggests the Bradley method, as described by Fuller and Barbe , as a sensible approach to manual control of brooms. He also suggests some logistics for securing and managing volunteers for manual control programs. Manual removal must be repeated regularly, since broom seedlings continue to establish from the seed bank after removal of adult plants . See the Nature Conservancy's Element Stewardship Abstract on French and Scotch broom for more information on management of these species.
Pulling with weed wrenches is effective for small infestations or in areas where an inexpensive, long-duration labor source is dedicated to broom removal . Weed wrenches remove the entire mature shrub, eliminating sprouting. Wrench removal allows targeting broom plants and minimizing impact on neighboring species and can be done on slopes. However, soil disturbance caused by weed wrenches tends to bury broom seeds and create microsites that favor broom seed germination. Sites must be monitored annually and seedlings removed before reaching maturity (2-3 years) until the broom seed bank is depleted .
The following procedures are recommended for control of brooms by cutting: Cut broom shrubs with a saw or brush cutter, at or below ground level, after broom has gone to seed and soil moisture is at a seasonal low. Remove or burn cut plants the following spring. The following summer, after grasses are dry and have dispersed their seed, mow French broom seedlings as close to the ground as possible with a heavy-duty brush cutter. Repeat for 5 or 6 seasons, until the seed bank is exhausted. Timing and height of cutting is critical in this technique to minimize sprouting. Cutting French broom in June in Mendocino County at 2 to 3 inches (5-8 cm) above the soil surface resulted in extensive sprouting. Brush hogs twist off aboveground plant material and are more destructive to perennating tissues than is cutting; however, sprouting still occurs .
Mulching with 3 to 4 inches (8-10 cm) of straw (certified weed-free) during winter or spring (before seedlings are over an inch tall) may prevent broom seedling emergence. A controlled study by the Habitat Restoration Team in California demonstrated that mulching with rice straw was 99% effective in preventing French broom seedlings from emerging through straw throughout the germination period from December to April. Mulching also increased the mortality of brush-cut French broom in the same study when applied during winter (Alvarez unpublished data, cited by ).
Fire: See the Fire Management Considerations section of this summary.
Biological: Biological control of invasive species has a long history, and there are many important considerations before the implementing a biological control program. Tu and others  provide general information and considerations for biological control of invasive species in their Weed control methods handbook. Additionally, Cornell University, Texas A & M University, and NAPIS websites offer information on biological control.
Foreign exploration for biological control agents for French broom began in 2000, and several promising agents have been found. As of 2003, at least one insect was undergoing host specificity testing. An insect purposely introduced for control of Scotch broom, the Scotch broom bruchid (Bruchidius villosus) , also attacks French broom. See Coombs and others  for more information on this insect, its distribution, and effects.
The native moth Uresiphita reversalis, which is known as "the genista caterpillar," has built up high numbers on local French broom populations in central California. Large genista caterpillar populations can be very damaging to local French broom populations; however, most populations of the moth are transitory and thus do not provide long-term control .
Heavy grazing by domestic goats for 4 or 5 years during the growing season is reported to effectively control broom in New Zealand , and has been tried at a few sites in Marin County (Archblad, personal communication cited by ). Rice  reports that goats were "very effective at reducing heavy brush, including that of...French broom" in the Hill Area watershed of San Francisco Bay." The disadvantage is that goats are not selective, and native species may also be eaten .
Chemical: Herbicides are effective in gaining initial control of a new invasion (of small size) or a severe infestation, but are rarely a complete or long-term solution to invasive species management because they do not change conditions that allow infestations to occur . Herbicides are more effective on large infestations when incorporated into long-term management plans that include replacement of weeds with desirable species, careful land use management, and prevention of new infestations. See the Weed control methods handbook  for considerations on the use of herbicides in natural areas and detailed information on specific chemicals and adjuvants. Also see the broom Element Stewardship Abstract  and other reviews  for more detailed information on chemical control.
Glyphosate is effective in some situations, but is likely to impact nontarget species, and broom sprouting occurs. Triclopyr ester is effective in killing French broom with a low-volume, basal bark application to the main stem. This application does not impact nontarget species but is time consuming. Used alone and in combination with other herbicides, 2,4-D has been used to control French broom ( and references therein).Cultural: No information is available on this topic.
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