Yucca glauca




James L. Reveal @ USDA-NRCS PLANTS Database

Groen, Amy H. 2005. Yucca glauca. 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/ [].


Yucca angustifolia Pursh [70]


soapweed yucca
Great Plains yucca
small soapweed

The currently accepted scientific name for soapweed yucca is Yucca glauca Nutt. (Agavaceae) [39,40,49,57,70,91,137,161]. Varieties accepted by some authorities include:

Y. g. var. glauca, narrowleaf yucca
Y. g. var.gurneyi McKelvey [70], Gurney's yucca

Throughout this review, soapweed yucca refers to all varieties of Y. glauca. A distinction between the varieties will only be made in the Distribution and occurrence section. The Flora of North America [49] does not recognize soapweed yucca varieties.

Hybrids: Soapweed yucca hybridizes with soaptree yucca (Y. elata) in eastern Colorado, central New Mexico, and Arizona [160,161]. Soapweed yucca is also believed to hybridize with Buckley's yucca (Y. constricta), Navajo yucca (Y. baileyi), and narrowleaf yucca (Y. angustissima) [160].



Soapweed yucca is listed as imperiled in Missouri [102].


SPECIES: Yucca glauca
Soapweed yucca is one of the most widespread of all the yuccas [134]. Its range extends throughout most of the Great Plains, from Alberta to southeastern New Mexico and the Texas panhandle, and east into Iowa and Arkansas [57,137,151]. Distribution of narrowleaf yucca encompasses the entire range of soapweed yucca. Colorado, New Mexico, Kansas, and Oklahoma support populations of Gurney's yucca [151]. The Flora of North America provides a distributional map of soapweed yucca. Distributional maps for narrowleaf and Gurney's yucca can be found on Plants database.

FRES15 Oak-hickory
FRES21 Ponderosa pine
FRES29 Sagebrush
FRES30 Desert shrub
FRES31 Shinnery
FRES32 Texas savanna
FRES33 Southwestern shrubsteppe
FRES34 Chaparral-mountain shrub
FRES35 Pinyon-juniper
FRES36 Mountain grasslands
FRES38 Plains grasslands
FRES39 Prairie
FRES40 Desert grasslands

STATES/PROVINCES: (key to state/province abbreviations)



8 Northern Rocky Mountains
9 Middle Rocky Mountains
10 Wyoming Basin
11 Southern Rocky Mountains
12 Colorado Plateau
13 Rocky Mountain Piedmont
14 Great Plains
15 Black Hills Uplift
16 Upper Missouri Basin and Broken Lands

K016 Eastern ponderosa forest
K017 Black Hills pine forest
K023 Juniper-pinyon woodland
K024 Juniper steppe woodland
K031 Oak-juniper woodland
K032 Transition between K031 and K037
K037 Mountain-mahogany-oak scrub
K038 Great Basin sagebrush
K040 Saltbush-greasewood
K051 Wheatgrass-bluegrass
K053 Grama-galleta steppe
K054 Grama-tobosa prairie
K055 Sagebrush steppe
K056 Wheatgrass-needlegrass shrubsteppe
K057 Galleta-threeawn shrubsteppe
K058 Grama-tobosa shrubsteppe
K059 Trans-Pecos shrub savanna
K060 Mesquite savanna
K061 Mesquite-acacia savanna
K062 Mesquite-live oak savanna
K063 Foothills prairie
K064 Grama-needlegrass-wheatgrass
K065 Grama-buffalo grass
K066 Wheatgrass-needlegrass
K067 Wheatgrass-bluestem-needlegrass
K068 Wheatgrass-grama-buffalo grass
K069 Bluestem-grama prairie
K070 Sandsage-bluestem prairie
K071 Shinnery
K074 Bluestem prairie
K075 Nebraska Sandhills prairie
K081 Oak savanna
K082 Mosaic of K074 and K100
K085 Mesquite-buffalo grass
K086 Juniper-oak savanna
K087 Mesquite-oak savanna

46 Eastern redcedar
66 Ashe juniper-redberry (Pinchot) juniper
67 Mohrs (shin) oak
68 Mesquite
220 Rocky Mountain juniper
237 Interior ponderosa pine
238 Western juniper
239 Pinyon-juniper
242 Mesquite

101 Bluebunch wheatgrass
107 Western juniper/big sagebrush/bluebunch wheatgrass
109 Ponderosa pine shrubland
209 Montane shrubland
301 Bluebunch wheatgrass-blue grama
303 Bluebunch wheatgrass-western wheatgrass
310 Needle-and-thread-blue grama
314 Big sagebrush-bluebunch wheatgrass
401 Basin big sagebrush
402 Mountain big sagebrush
403 Wyoming big sagebrush
408 Other sagebrush types
412 Juniper-pinyon woodland
414 Salt desert shrub
501 Saltbush-greasewood
502 Grama-galleta
503 Arizona chaparral
504 Juniper-pinyon pine woodland
505 Grama-tobosa shrub
601 Bluestem prairie
602 Bluestem-prairie sandreed
603 Prairie sandreed-needlegrass
604 Bluestem-grama prairie
605 Sandsage prairie
606 Wheatgrass-bluestem-needlegrass
607 Wheatgrass-needlegrass
608 Wheatgrass-grama-needlegrass
609 Wheatgrass-grama
610 Wheatgrass
611 Blue grama-buffalo grass
612 Sagebrush-grass
703 Black grama-sideoats grama
704 Blue grama-western wheatgrass
705 Blue grama-galleta
706 Blue grama-sideoats grama
707 Blue grama-sideoats grama-black grama
709 Bluestem-grama
710 Bluestem prairie
713 Grama-muhly-threeawn
714 Grama-bluestem
715 Grama-buffalo grass
718 Mesquite-grama
720 Sand bluestem-little bluestem (dunes)
721 Sand bluestem-little bluestem (plains)
722 Sand sagebrush-mixed prairie
727 Mesquite-buffalo grass
729 Mesquite
730 Sand shinnery oak
733 Juniper-oak
734 Mesquite-oak
735 Sideoats grama-sumac-juniper
801 Savanna
802 Missouri prairie

Throughout its range in Colorado soapweed yucca can often be found in plains grasslands in association with blue grama (Bouteloua gracilis) [32,96,105,121], needle-and-thread grass (Hesperostipa comata) [32,105], and in a variety of sagebrush (Artemisia spp.) communities including Bigelow sagebrush (Artemisia bigelovii) [130] and mountain big sagebrush (A. tridentata var. vaseyana) [31]. In 1906 it was reported as one of the most prominent plants in the region east of Pike's Peak and could be found alongside horned spurge (Euphorbia brachycera), mountain bladderpod (Lesquerella montana), nylon hedgehog cactus (Echinocereus viridiflorus), and prairie bluebells (Mertensia lanceolata var. lanceolata) in what was termed the "Yucca glauca society" [129]. It is often found among sandhill communities with prairie sandreed (Calamovilfa longifolia), sand sagebrush (Artemisia filifolia), and sandhill muhly (Muhlenbergia pungens) [96].

In the central Rocky Mountains of Colorado soapweed yucca is found in areas dominated by interior ponderosa pine (Pinus ponderosa var. scopulorum). Other shrubs include fivepetal cliffbush (Jamesia americana), common juniper (Juniperus communis), bearberry (Arctostaphylos uva-ursi), wax currant (Ribes cereum), shrubby cinquefoil (Dasiphora floribunda), and Wood's rose (Rosa woodsii) [120]. Along the Colorado Front Range it is associated with the New Mexico feathergrass (Hesperostipa neomexicana) community, where it is present with purple threeawn (Aristida purpurea), foothill milkvetch (Astragalus tridactylicus), Front Range twinpod (Physaria bellii), and shortstem buckwheat (Eriogonum brevicaule) [105]. Soapweed yucca is also found in tree cholla (Opuntia imbricata) associations with blue grama, sand dropseed (Sporobolus cryptandrus), and ring muhly (Muhlenbergia torreyi) [77], and in Colorado pinyon-juniper (Pinus edulis-Juniperus spp.) dominated systems [31].

Throughout Kansas soapweed yucca can be found with prairie graminoids such as little bluestem (Schizachyrium scoparium), blue grama, and needle-and-thread grass. Shrub associates include sand sagebrush and fourwing saltbush (Atriplex canescens) [83]. Soapweed yucca is found with velvetweed (Gaura mollis), nineanther prairie clover (Dalea enneandra), and hairy grama (Bouteloua hirsuta) in shortgrass prairie habitats [2] and with little bluestem, big bluestem (Andropogon gerardii var. gerardii), purple prairie clover (Dalea purpurea), azure blue sage (Salvia azurea), and western ragweed (Ambrosia psilostachya) in tallgrass prairie habitats [114].

Soapweed yucca is associated with open, dry habitat types in Montana. It occurs in interior ponderosa pine and limber pine (Pinus flexilis) stands with blue grama, sideoats grama (Bouteloua curtipendula), and plains prickly-pear (Opuntia polyacantha) [5], and can be found with skunkbush sumac (Rhus trilobata) and creeping juniper (Juniperus horizontalis) in the interior ponderosa pine/bluebunch wheatgrass (Pseudoroegneria spicata) vegetation type [45]. In the badlands of southeastern Montana, soapweed yucca is found with greatest frequency in the mountain big sagebrush-western wheatgrass (Pascopyrum smithii) community type and greatest abundance in the skunkbush sumac-western wheatgrass community type [25].

Soapweed yucca is found throughout most of Nebraska and is important for the ground coverage it provides in the Nebraska sandhill range [51]. It is commonly associated with sandy substrates, occurring on blowouts with sand bluestem (Andropogon gerardii var. paucipilus) and little bluestem [116], and on dune ridges and upper slopes with stiff sunflower (Helianthus pauciflorus), leadplant (Amorpha canescens), prairie rose (Rosa arkansana) [13], western sandcherry (Prunus pumila var. besseyi), and manystem pea (Lathyrus polymorphus) [12]. Soapweed yucca is found with western snowberry (Symphoricarpos occidentalis) and skunkbush sumac [149] on scattered and open woodlands and with blue grama, threadleaf sedge (Carex filifolia), and western wheatgrass in mixed-prairie habitat types [163].

In the high plains of Nebraska, soapweed yucca was found on 8 out of 9 surveyed plots. Relative abundance of soapweed yucca was highest on isolated canyon slopes, where it could be found with Rocky Mountain juniper (Juniperus scopulorum), eastern redcedar (Juniperus virginiana), and little bluestem. Soapweed yucca was also found on sand talus with big bluestem, little bluestem, needle-and-thread grass, and sagebrush (Artemisia spp.) [71].

In the middle Rio Grande valley of New Mexico, soapweed yucca occurs with soaptree yucca, tree cholla, and broom snakeweed (Gutierrezia sarothrae). Grasses in these desert grassland plant communities include black grama (Bouteloua eriopoda), big galleta (Pleuraphis rigida), burrograss (Scleropogon brevifolius), and threeawns (Aristida spp.) [86]. In the shrub-grassland areas, soapweed yucca occurs with velvet mesquite (Prosopis velutina), broom snakeweed, and sand shinnery oak (Quercus havardii). In the desert grasslands of northeastern New Mexico it is found primarily with blue grama, sideoats grama, and galleta (Pleuraphis jamesii) [44].

Soapweed yucca in the Dakotas can be found at the easternmost edge of interior ponderosa pine habitat with skunkbush sumac, western poison-ivy (Toxicodendron rydbergii), and chokecherry (Prunus virginiana) [66]. It occurs with prairie sandreed on less than 3 acres (1 ha) in Badlands National Park [36], and is associated with the sand bluestem-prairie sandreed habitat type in southwestern North Dakota [65]

Throughout its distribution in Texas, soapweed yucca is broadly associated with sand sagebrush and prickly-pear (Opuntia spp.) [9,154,155] and can also be found with redberry juniper (Juniperus erythrocarpa), Ashe juniper (J. ashei), sideoats grama, hairy grama, and blue grama [136]. It occurs with honey mesquite (Prosopis glandulosa), catclaw acacia (Acacia greggii), wait-a-minute (Mimosa aculeaticarpa var. biuncifera), and lotebush (Ziziphus obtusifolia) [98], and can reach densities exceeding 2,000 plants per acre on the panhandle of northern Texas [93]. Its occurrence in western Texas is not significantly (P<0.05) affected by the occurrence of Pinchot juniper (Juniperus pinchotii) [99].

In western Missouri soapweed yucca is found in prairie habitats with hairy grama, blue grama, large beardtongue (Penstemon grandiflorus), and downy paintbrush (Castilleja sessiliflora) [147]. In western Iowa it occurs with little bluestem, big bluestem, and purple coneflower (Echinacea angustifolia) [128].

Soapweed yucca can be found on east-facing slopes of the Big Horn mountains of Wyoming with little bluestem and bluebunch wheatgrass [34]. It can be found in the Powder River Basin with Wyoming big sagebrush (Artemisia tridentata var. wyomingensis), western wheatgrass, blue grama, and birdfoot sagebrush (Artemisia pedatifida) [48]. Soapweed yucca is often found with blue grama and sagebrush, with highest coverage and constancy percentages in shrub-steppe vegetation types [146]. At the Great Sand Dunes National Monument, it was found on 21% of research plots located on active and stabilized dunes [94].

Soapweed yucca is found with Rocky Mountain juniper and Colorado pinyon in juniper-pinyon woodlands that occur throughout the Great Basin. Other associates include Stansbury cliffrose (Purshia mexicana var. stansburiana), Apache-plume (Fallugia paradoxa), green ephedra (Ephedra viridis), Fremont's mahonia (Mahonia fremontii), fourwing saltbush, and banana yucca (Yucca baccata) [23]. In the Great Basin grasslands soapweed yucca is found with sacahuista (Nolina microcarpa), skunkbush sumac, wait-a-minute, and fourwing saltbush [123].

Shrubs such as soapweed yucca, fourwing saltbush, winterfat (Krascheninnikovia lanata), prairie sumac (Rhus copallinum var. lanceolata), rabbitbrush (Chrysothamnus spp.), and snakeweed (Gutierrezia spp.) are scattered throughout the plains grasslands. In areas subjected to heavy grazing, soapweed yucca is found with honey mesquite, sand shinnery oak, and sand sagebrush [24]. In the northern prairie wetlands region it can be found on blowouts with sand bluestem, hairy grama, and needle-and-thread grass in areas where there is no contact with the water table [108].


SPECIES: Yucca glauca


James L. Reveal @ USDA-NRCS PLANTS Database

This description provides characteristics that may be relevant to fire ecology, and is not meant for identification. Keys for identification are available (e.g., [57,91,137]).

Soapweed yucca is a native evergreen shrub [70] that grows to 3 feet (1 m) in height [137]. It supports short, branching, decumbent stems [91] and a taproot [142,143]. Root systems greater than 20 feet (6 m) in depth were reported in eastern Colorado [96]. Lateral roots extend 20 to 30 feet (6-9 m) [158]. Soapweed yucca supports 1 or 2 large diameter rhizomes from which a network of smaller oblique rhizomes protrude, forming a mat 4 to 24 inches (10 to 60 cm) below the soil surface [160].

Soapweed yucca has simple, concave, alternate leaves with coarse fibers along the margins and a sharp apex [137]. Leaf size ranges from 8 to 40 inches (20-100 cm) in length and 0.2 to 0.5 inch (0.6-1.2 cm) in width [91]. Leaves emerge from a woody caudex [39] and are situated at a steep angle that allows year-round photosynthesis [89]. Stomatal conductance and net photosynthesis are adapted to function at optimal capacity under given temperatures [124]. The appearance of the leaf head of soapweed yucca is contingent upon its age and available rainfall [160].

Soapweed yucca has a raceme inflorescence [57] that is nearly sessile, 12 to 80 inches (30-200 cm) in length [61], and composed of 25 to 30 drooping flowers [137]. Fruits are oblong, dehiscent, erect, and approximately 1.2 inch (3 cm) in length [61]. Soapweed yucca flowers house a single style and 6 stamens [40]. Seeds are glossy [151], and grow to 9 12 mm [49].


Soapweed yucca reproduces by seed [142] and rhizomes [165]. The rates at which soapweed yucca can reproduce through rhizomes is likely dependant upon mechanisms such as rates of branching, rhizome length, and the number of rhizomes. Soapweed yucca rhizomes form from the seedling rhizome in a rebranching pattern, spreading horizontally. After 4 to 6 years, buds grow above the rhizomes and produce leaves after reaching the soil surface. Connections with the mother plant eventually become dormant and decay [160]. In a study conducted on cloned soapweed yucca plants, total number of flowering days was not an indication of reproductive success and was less important than the time at which flowering occurs. It was also beneficial for the plant to have branched flower stalks for reproductive success [106].

Pollination: Pollination of soapweed yucca is dependent upon the yucca moth (Pronuba yuccasella) [165] or other pollinating insects such as small flies (Pseudocalliope spp.) [38]. The yucca moth transfers pollen from the anther, depositing it deep into the style and leading to the production of many seeds, some of which are fed upon by the larvae [38]. The number of yucca moths available for pollination is related to the abundance of open flowers [106].

Soapweed yucca benefits from wood ants (Formica spp.). They are natural predators of the nonpollinating yucca moth (Tegeticula corruptrix), which feeds on soapweed yucca seeds while in the larval stage [111].

Breeding system: Moth pollination [165] assures that soapweed yucca is mostly outcrossing. Anthers are remote from the cavity encapsulated stigma and sticky pollen [11], assuring that autogamous pollination of soapweed yucca is rare. Pollen is viable for a minimum of 4 days, as are the flowers. Autogamous pollination occurs as the flower begins to wilt and stamens come into direct contact with the stigma [38].

Seed production: The number of viable soapweed yucca seeds is dependant upon the transfer of high quality pollen, sufficient resources for development, and ovules that are not damaged by the insertion of the yucca moth's ovipositor during the transfer of pollen [1]. It can take several years for rosettes to bloom [79]. Despite the high number of seeds consumed by moth larvae, large numbers are left unharmed for dispersal [11].

In a study to determine the relationship between soapweed yucca and the yucca moth, 124 plants with a mean of 291 ovules per plant had 138 viable seeds after consumption by adult and larval yucca moths. The maximum number of larvae per fruit was 19 [1]. In Colorado, the number of surviving seeds increased with elevation despite fewer fruits. This was likely due to decreased predation by the yucca moth, which prefers the warmer temperatures of the Great Plains over those experienced at higher elevations [38].

Seed dispersal: Seed pods become erect and split longitudinally, exposing seeds to dispersal by wind and gravity [11].

Seed banking: Laboratory studies exploring germination rates of soapweed yucca suggest that seed banking is likely [38,149,160]. Further research is needed to determine longevity and viability of soapweed yucca seeds in a field setting.

Germination: Soapweed yucca experiences higher rates of germination when stratified. In a laboratory setting soapweed yucca successfully germinated under a variety of stratification treatments. Following 1 to 3 months of stratification, germination occurred after exposing seed to a constant 70 F (20 C) or alternating temperatures between 70 F (20 C) and 40 F (5 C). Germination also occurred after seed was stored at 40 F (4 C) and then exposed to 70 F (20 C) temperatures. A maximum of 27% of seeds germinated after being exposed to 90 F (30 C) or by alternating 90 F (30 C) and 70 F (20 C). Sixty-seven percent germination was achieved at 50 F (10 C) with 3 months stratification. The highest germination rates (91%) were experienced when 4-month-old seed was dry stored at 70 F (20 C) and then wet treated for 3 months at 70 F (20 C). Six days were required for 50% germination. See Emerson [43] for further seed germination information.

A study on the germination of Nebraska sandhill plants revealed 67.1% germination rates for soapweed yucca seed that had been dry stored, 44% germination for seeds vernalized for 1 month, and 86.6% germination for seeds vernalized for 2 months. Seed vernalized for 3 months sprouted while in storage under winter temperatures [149]. Soapweed yucca germinated in 4 days after being soaked in water for 24 hours and kept at temperatures between 80 F and 90 F (28 C and 32 C) in moist cotton [160]. Seeds from low-elevation soapweed yucca plants are thought to have stronger tendencies towards dormancy than those found at high elevations [38].

Seedling establishment/growth: Reproduction of soapweed yucca by seed is limited to the percentage of viable seeds remaining after seed predation by larvae. Soapweed yucca had the highest number of seedlings of 19 observed yucca species over a 4-year period, with 72 seedlings observed [160]. Soapweed yucca seedlings have reduced growth rates compared to ramets. Seedlings are important for colonizing new sites, after which clones take over as primary reproducers. As the carrying capacity is reached, seed production increases in order that new populations may emerge outside the range of vegetative reproduction. The ratio of seedlings produced vegetatively versus those produced by seed varies across populations [76].

Asexual regeneration: Soapweed yucca reproduces vegetatively through sprouting from the caudex and from horizontal rhizomes [38,160], which yield rosette-baring offshoots [11]. Projecting from the upper portion of one or two large diameter rhizomes are horizontal branches forming a netlike pattern from which sprouts emerge. Depriving the plant of shoots through fire or mechanical means results in regeneration rates that equal or exceed previous regeneration rates within a year or 2. In a study to confirm successful vegetative propagation, sprouts were separated from the mother plant by removing 2- to 4-inch (5-10 cm) sections of rhizome while in the field. Normal growth of the sprouts continued during the following year [160]. Vegetative reproduction can be stimulated after top-kill of soapweed yucca [93].

Climate/topography: Soapweed yucca is adapted to a wide range of environments. Average daily and annual variations in temperature can be great, such as in the deserts of New Mexico [33]. Air temperatures can drop to -44 F (-42 C) during winter months, and escalate to 106 F (41 C) in the summer. Yearly precipitation averages range from a low of 1.2 to 1.6 inches (300-400 mm) in Wyoming [48] to a high of 24.0 inches (610 mm) in Texas [136]. Humidity levels in Wyoming are at least 25% during July and August [48] while on the east slope of the Colorado Front Range, mean annual relative humidity is 69% [90]. Topography includes hills, plains [61,107], bluffs [69,83], rocky outcrops and ridges [134], and areas of shifting sands [96].

Soils: Soapweed yucca is most commonly found on sandy sites [27,32,65,134] or rocky areas with coarse-textured soils [43,146,161]. In the Great Plains it is associated with limestone soils [71,137] and alluvial terraces [105]. In New Mexico soapweed yucca is found in sandy soils underlain by caliche [33]. Soils in Texas are relatively deep clays or clay loams [98]. On the Colorado Front Range, soil pH is approximately 8.0 to 8.2 [105]. Soapweed yucca does best in well-drained soils with sunny exposures [135].

Elevation: Soapweed yucca tolerates a wide range of elevations:

CO 2,500 to > 8,500 feet (760-2,600 m) [38,92]
MT 3,000 to 4,400 feet (900-1,300 m) [25]
NM 3,600 to 6,500 feet (1,100-2,000 m) [86,91]
UT 4,000 to 7,000 feet (1,200-2,100 m) [144]
Great Plains 660 to 7,200 feet (200-2200 m) [73]

Soapweed yucca occurs in all stages of succession [76,125]. In a blowout that occurred in eastern Colorado, vegetational succession data revealed that soapweed yucca returns during stage 4 of a 6-stage process. Sandhill muhly was the primary species to return before soapweed yucca. Once it became established, soapweed yucca persisted on the site [96].

Soapweed yucca can be found on undisturbed sites [32] and sites affected by land management activities [160] such as plowing [96]. It is a late-seral species on sandy range sites and in grassland associations (50% cover) in Montana [125]. In southeastern Montana, soapweed yucca is considered an understory species in the interior ponderosa pine/skunkbush sumac vegetation type [55], indicating possible shade tolerance. It also occurs on open sites [135].

Soapweed yucca flowers in May and June in New Mexico [91] and May through July in the Great Plains region [57]. The fruit ripens in July and August, and seed disperses in September [3]. Ramets are produced from lateral buds or rhizomes near rosettes that have senesced in late summer [76].


SPECIES: Yucca glauca
Fire adaptations: Despite immediate adverse effects of fire on soapweed yucca [64], populations recover from fire [60,160]. Extensive rhizomatous mats 4 to 24 inches (10-60 cm) below the soil surface [160] are likely protected from fire damage. Vegetative reproduction (sprouting from the caudex and rhizomes) can be stimulated when soapweed yucca is top-killed [93]. Documented sprouting from the caudex after cutting [160] suggests that soapweed yucca can sprout following top-kill by low- to moderate-severity fire.

Recently disturbed sites provide an opportunity for soapweed to germinate; however, germination rates of soapweed yucca seed decreased after being exposed to high temperatures (180-250 F (80-120 C)) in a laboratory setting [73]. This suggests that on-site seeds require burial in the seed bank in order to germinate following fire. Wind and gravity dispersal [11] of off-site seed sources is also possible after fire. Further research is needed to explore fire adaptations of soapweed yucca.

Fire regimes: Soapweed yucca is primarily associated with grassland environments. Historical fire return intervals within these vegetation types were probably dependant upon drought cycles, the prevalence of lightning strikes, and anthropogenic uses. Soapweed yucca is primarily found throughout ecosystems characterized by fire return intervals of less than 35 years [109]. Where fire has been excluded from sagebrush communities and desert grasslands, invasion of nonnative annual grasses such as red brome (Bromus madritensis ssp. rubens), lovegrass (Eragrostis spp.), and cheatgrass (Bromus tectorum) has often occurred, enhancing the potential for fires to start and spread, and increasing fire frequency [14,28,164,169]. Fire return intervals have also been reduced as a result of overgrazing which can lead to dense overstories forming closed canopies that carry fire more efficiently. Exotic annual grasses are likely to invade these sites if a seed source is available [112].

The following table provides fire return intervals for plant communities and ecosystems where soapweed yucca is important. For further information, see the FEIS review of the dominant species listed below.

Community or Ecosystem Dominant Species Fire Return Interval Range (years)
bluestem prairie Andropogon gerardii var. gerardii-Schizachyrium scoparium <10 [78,109]
Nebraska sandhills prairie Andropogon gerardii var. paucipilus-Schizachyrium scoparium <10
bluestem-Sacahuista prairie Andropogon littoralis-Spartina spartinae <10 [109]
silver sagebrush steppe Artemisia cana 5-45 [63,118,167]
sagebrush steppe Artemisia tridentata/Pseudoroegneria spicata 20-70 [109]
basin big sagebrush Artemisia tridentata var. tridentata 12-43 [127]
mountain big sagebrush Artemisia tridentata var. vaseyana 15-40 [8,26,101]
Wyoming big sagebrush Artemisia tridentata var. wyomingensis 10-70 (=40) [156,170]
saltbush-greasewood Atriplex confertifolia-Sarcobatus vermiculatus <35 to <100
desert grasslands Bouteloua eriopoda and/or Pleuraphis mutica <35 to <100 [109]
plains grasslands Bouteloua spp. <35 [109,167]
blue grama-needle-and-thread grass-western wheatgrass Bouteloua gracilis-Hesperostipa comata-Pascopyrum smithii <35 [109,126,167]
blue grama-buffalo grass Bouteloua gracilis-Buchloe dactyloides <35 [109,167]
grama-galleta steppe Bouteloua gracilis-Pleuraphis jamesii <35 to <100
blue grama-tobosa prairie Bouteloua gracilis-Pleuraphis mutica <35 to <100
juniper-oak savanna Juniperus ashei-Quercus virginiana <35
Ashe juniper Juniperus ashei <35
western juniper Juniperus occidentalis 20-70
Rocky Mountain juniper Juniperus scopulorum <35 [109]
cedar glades Juniperus virginiana 3-22 [59,109]
wheatgrass plains grasslands Pascopyrum smithii <5-47+ [109,118,167]
pinyon-juniper Pinus-Juniperus spp. <35 [109]
Colorado pinyon Pinus edulis 10-400+ [50,56,72,109]
interior ponderosa pine* Pinus ponderosa var. scopulorum 2-30 [7,10,84]
galleta-threeawn shrubsteppe Pleuraphis jamesii-Aristida purpurea <35 to <100 [109]
mesquite Prosopis glandulosa <35 to <100 [97,109]
mesquite-buffalo grass Prosopis glandulosa-Buchloe dactyloides <35
Texas savanna Prosopis glandulosa var. glandulosa <10 [109]
mountain grasslands Pseudoroegneria spicata 3-40 (=10) [6,7]
oak-juniper woodland (Southwest) Quercus-Juniperus spp. <35 to <200 [109]
oak savanna Quercus macrocarpa/Andropogon gerardii-Schizachyrium scoparium 2-14 [109,157]
shinnery Quercus mohriana <35 [109]
live oak Quercus virginiana 10 to<100
little bluestem-grama prairie Schizachyrium scoparium-Bouteloua spp. <35 [109]
*fire return interval varies widely; trends in variation are noted in the species review

Rhizomatous shrub, rhizome in soil
Secondary colonizer (on-site or off-site seed sources)
Caudex/herbaceous root crown, growing points in soil


SPECIES: Yucca glauca
Soapweed yucca can suffer adverse effects from fire [64]. It is likely that sustained surface fires top-kill soapweed yucca while underground structures survive. Reduced germination rates of seed exposed to increasing temperatures in a laboratory setting [73] indicate soapweed yucca is unlikely to regenerate from seed following a sustained, high-temperature fire, but that seed will survive lower temperatures of short duration, such as those typical of grass fires.

Soapweed yucca populations were reduced by 13% after burning with a propane torch in a study to determine the effects of burning, shredding, and herbicide treatments. Surface fire temperatures simulated those previously recorded for combustion of fine fuel loads of 5,400 lb/ac [93].

Wright and Bailey [167] report that Yucca spp. are weak sprouters after fire, but are able to hold their position in plant communities due to sprouting ability. As a result of burning with a propane plant burner, soapweed yucca suffered an 85% reduction in total biomass and sent out an average of 1.5 new shoots per plant at postfire month 18 [93]. Observations suggest that when damage from a burn is too great or rodent populations too high, soapweed yucca is not able to recover [160]. Populations in the Nebraska sandhills prairie declined as a result of wildfires in late spring [21].

Soapweed yucca seeds gathered from the Texas-Oklahoma border were exposed to heat treatments in a laboratory setting. Results indicate that soapweed yucca seeds are susceptible to heat and may not provide a viable regeneration source if exposed to temperatures over 250 F (120 C) [73].

Soapweed yucca populations have been observed recovering from fire [160]. After a prescribed burn in sand shinnery oak communities in Oklahoma, frequency and occurrence of soapweed yucca was 0.41% and 0.08% after the 1st postfire growing season, 0.12% and 0.12% after the 2nd postfire growing season, 0.40% and 0.40% after the 3rd postfire growing season, and 0.82% and 0.40% after the 4th postfire growing season. No significant differences were noted in shrub species composition after the prescribed burn (P=0.55) [60].

Soapweed yucca seeds exposed to various temperatures in a convection oven indicate a reduced ability to germinate from seed exposed to high-temperature fire [73]:



2 hours of heat

5 minutes of heat
Temperature Room temperature 180 F (80 C) 190 F (90 C) 190 F (90 C) 210 F (100 C) 230 F (110 C) 250 F (120 C)
Germination (%) 79 57 42 41 59 28 3

Soapweed yucca was monitored after spring, mid-summer, and fall fires on hilltops, north-facing, and south-facing slopes in the Nebraska sandhills prairie to determine its response to fire. Prefire canopy cover was measured in 1984 and postfire cover was measured each of the 4 years following the prescribed burns, which took place on 1-2 May, 19 July, and 1 October [22]:

Topographic Location Treatment

Mean Canopy Cover 1 s x, by year

1984 1985 1986 1987 1988
Hilltop Spring 1 1.2 1 0.5 1 0.5 1 0.5 1 0.5
Summer 4 1.8 3 2.1 3 2.2 2 1.4 1 1.2
Fall 5 2.5 -- 1 0.5 1 0.5 1 0.7
Unburned 0.0 0.0 0.0 0.0 0.0
North-facing Slope Spring 0.0 0.0 0.0 0.0 0.0
Summer 2 2.1 2 1.3 1 0.7 1 0.7 2 1.3
Fall 1 0.7 -- 1 0.5 1 0.5 0.0
Unburned 3 1.8 3 1.8 2 0.8 3 1.8 2 0.8
South-facing Slope Spring 3 2.8 2 2.1 3 2.8 3 2.8 3 2.1
Summer 0.0 0.0 0.0 0.0 0.0
Fall 4 1.8 -- tr. 1 0.5 2 1.3
Unburned 2 1.3 2 1.4 2 1.4 2 1.4 1 0.7

Due to a system of netlike rhizomatous branches protected from the effects of heat by overlaying soil [160], fire will not likely eliminate soapweed yucca. Seeds that were exposed to temperatures of 230 F (110 C) and 250 F (120 C) for 5 minutes in a laboratory setting experienced 28% and 3% germination rates, respectively [73]. This may indicate that soapweed yucca establishment from on-site, soil-stored seed is possible except following fires of longer duration and/or higher temperatures.

In order to circumvent adverse effects to the prairie-obligate skipper butterfly, land managers in western Iowa should consider the extent of butterfly habitat in the Loess Hill prairies prior to conducting prescribed burns [128].


SPECIES: Yucca glauca
Soapweed yucca is utilized by a variety of mammals, primarily during drought years when more desirable forage is not available. Livestock and big game browse flower and seedlings when accessible [107,165]. Cattle prefer the fleshy parts of the plant and can decrease soapweed yucca populations if allowed access to a single site for multiple years [143]. Soapweed yucca is considered poor forage for cattle by some [51]. It can be used as a feed supplement during severe drought years and will return after extensive harvest [160]. On the shortgrass ranges of Colorado, cattle feces revealed that soapweed yucca consumption was highest in April and August [133].

Mule deer consume soapweed yucca throughout the year [80] with heaviest use in spring [35]. Rumen samples in prairie habitat indicate consumption in summer, winter, and spring months [42]. In New Mexico, soapweed yucca makes up 0% to 3% of mule deer diets [88], accounting for 0.5% of their overall diet [103]. Bighorn sheep consume soapweed yucca in North Dakota [47] and the Colorado Rockies, where soapweed yucca constitutes an average of 5.3% of their diet [148]. Soapweed yucca is also utilized by mule deer in South Dakota [168], white-tailed deer in Montana [4], pronghorn [117,142], and bison [110].

Soapweed yucca provides food, shade, and nesting sites for birds and small mammals [143]. Seeds are an important food source to small mammals [137], and leaves are consumed during times of drought and snow [122,142]. Black-tailed jackrabbits in Colorado and Kansas and white-tailed jackrabbits in Colorado utilize soapweed yucca as a food source [41]. Soapweed yucca is used by the southern plains woodrat as construction material for houses built to shelter nests and store food [145]. It is found in areas that support cotton rat populations in New Mexico [104].

Palatability/nutritional value: A plant composition analysis was conducted on soapweed yucca in South Dakota. Moisture content throughout the year varied with 4 samples, from 55.7%-64.7%. Carotene had ranges of 18.0-43.5 mg/g, ash ranged from 1.5%-1.9%, crude fat from 0.73%-1.3%, crude protein from 3.6%-4.8%, and crude fiber from 14.6%-16.0% [53]. The average percentage of crude fiber, ash, and protein as measured in leaves from soapweed yucca plants in New Mexico was 42.3%, 4.86%, and 5.57%, respectively [19].

The nutritional value (%) of soapweed yucca in the Black Hills of South Dakota was as follows [53]:

  Moisture  Carotene Ash Crude fat Crude protein Crude fiber N-free extract Ca P Fe Mn
18 January 60.10 27.60 1.59 1.13 4.78 14.81 17.59 0.33 0.100 40.76 23.20
16 May 55.68 22.10 1.57 1.28 3.79 15.97 21.53 0.43 0.070 67.31 16.54
27 June 64.71 18.04 1.87 0.73 3.93 14.62 14.14 0.31 0.117 32.82 7.84
22 October 61.42 43.48 1.49 1.24 3.56 14.67 17.62 0.28 0.077 47.11 13.34

The chemical composition (%) of soapweed yucca from 3 sites in North Dakota was as follows [47]:

  January February March April May June July August September October November December


Ca 0.79 1.33 0.96 1.00 1.14 1.17 0.99 1.10 1.09 1.18 0.66 1.18
Mg 0.43 0.42 0.36 0.52 0.57 0.77 0.62 0.69 0.78 0.53 0.60 0.69
K 0.96 0.56 1.08 0.67 0.65 1.12 1.23 1.22 0.94 0.84 0.74 0.59

Flat Top Ridge

Ca 1.56 2.12 1.90 1.18 1.45 1.66 1.11 1.18 1.18 1.39 1.31 1.24
Mg 0.32 0.23 0.33 0.47 0.26 0.38 0.35 0.48 0.43 0.46 0.40 0.67
K 0.39 0.37 0.44 0.55 0.61 1.23 1.63 1.04 0.94 0.77 0.68 0.57

Side hill

Ca 1.13 1.20 1.40 1.10 0.87 1.75 0.80 1.25 1.38 0.82 1.05 1.24
Mg 0.23 0.19 0.35 0.42 0.33 0.58 0.51 0.47 0.49 0.77 0.45 0.60
K 0.50 0.48 0.42 0.59 0.80 1.26 0.99 1.04 0.80 0.93 0.60 0.42

A study conducted in eastern Colorado to determine the effect of geologic refuges on the occurrence of species found that areas inaccessible to cattle supported 1.96% soapweed yucca cover as opposed to 1.45% in browsed areas. Soapweed yucca was listed as having relatively low palatability with the potential to become more palatable at different times in the season [100].

Cover value: Dittberner and Olson [37] report the cover value of soapweed yucca for wildlife species as follows:

Pronghorn poor   fair
Elk     poor
Mule deer poor fair poor
White-tailed deer     poor
Small mammals fair   poor
Small nongame birds fair   poor
Upland game birds poor   poor
Waterfowl     poor

Birds, small mammals, and reptiles utilize soapweed yucca for shade and nesting sites [137]. Scaled quail in Texas utilize it for night roosting [141] and sharp-tailed grouse broods use it for cover [58]

The use of soapweed yucca for disturbed site rehabilitation has yielded varying results. Soapweed yucca is considered a "common plant" by the USDA Natural Resource Conservation Service and can be purchased from suppliers for conservation purposes [150]. It responds favorably to propagation, experiencing shoot proliferation and rooting from explants of the shoot tip, flower bud, and rhizome [16]. Instructions on producing soapweed yucca as container seedlings can be found in [82].

Soapweed yucca seeds obtained from mid-October through early spring were used for reclamation in southeastern Montana. Seeds were harvested from dehiscent capsules while they were still intact or after being dried. Best results were obtained from seed stored at 40 F (4 C) over winter and planted in spring and seed planted immediately after being collected in the fall [43]. On previously mined land in southeastern Montana, reseeding occurred with either 4-species or 16-species mixtures to determine rates of perennial plant regeneration under various treatments. Soapweed yucca returned to plots seeded with the 4-species mixture at a density of 0.3 plant/m 5 years after seeding, and did not return on plots seeded with the 16-species mixture. For additional information on methods utilized in this project see [67].

Of 28 soapweed yucca plants planted in 1988 on a prairie reconstruction site in Kansas, 79% survived the 1st year and 64% survived the 2nd year. None of the plants flowered within that 2-year period. Of 19 plants planted in 1985, 100% survived the 1st year. No record was made beyond 1986 [115].

Viable soapweed yucca seeds are black. Seeds with a white seed coat lack an endosperm and are not viable [1].

Soapweed yucca has been utilized by Native Americans for nearly 10,000 years [75]. Both the fruiting portion and flower stalks are consumed [15,29,30,143], and various plant parts are processed for medicinal uses [62,153]. Soapweed yucca is also used to produce items such as paper [19], rope, baskets, brushes, soaps [15,143], dyes, poison for arrows [153], and sticks for fire [62].

The response of soapweed yucca to browsing is variable. Reports have been made of its increases [27,81,143] and decreases [21,71,85] following grazing practices. The amount of soapweed yucca consumed by cows in eastern Montana was influenced by pregnancy [113].

Eradication of soapweed yucca from rangelands in favor of more palatable species has been achieved through various methods. One study found repeated aerial applications of the herbicide silvex controlled soapweed yucca [20], while another achieved control through the burning or cutting of plant tops in combination with the use of tebuthiuron, picloram, and 2,4,5-T [93] (2,4,5-T is now banned by the E.P.A. [152]). Grazing forage was increased by 37% and soil water content significantly reduced (P=0.05) on a site where chemical control of soapweed yucca was achieved [136].

Soapweed yucca populations increased significantly (P<0.05) after aboveground portions were crushed by off-road vehicles, and the plants sprouted from belowground portions [131]. Fluoride and sulfur from coal-fired plant emissions could have adverse effects on soapweed yucca, but damage to plants would likely not be detected prior to overstory effects such as reduced needle length and tissue necrosis [55].

When inoculated with mycorrhizae in a laboratory setting, the mean dry mass of inoculated soapweed yucca plants was 87% greater than nonmycorrhizal control plants [166].

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