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SPECIES: Tetradymia canescens

  Charles Webber, California Academy of Sciences       Plants Database [67]

Scher, Janette S. 2001. Tetradymia canescens. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: [].

Revisions: On 5 July 2017, the common name of this species was changed from: gray horsebrush
to: spineless horsebrush in FEIS.


Tetradymia canescens var. inermis (Nutt.) Gray [41]


spineless horsebrush
common horsebrush
gray horsebrush
grey horsebrush

The currently accepted scientific name of spineless horsebrush is Tetradymia canescens DC (Asteraceae) [14,25,31,32,64,71]. Putative spineless littleleaf horsebrush (T. glabrata) hybrids have been found in southern Idaho [64].


No special status

No entry


SPECIES: Tetradymia canescens

Spineless horsebrush is native throughout most of the Great Basin and adjacent areas [42,45,67]. It is distributed from the mountains of southern California to south-central British Columbia and east to the Rocky Mountains in western Wyoming, Montana, Colorado, and New Mexico. Spineless horsebrush is generally not abundant across its range [14,64]. The Plants database provides a distribution map of spineless horsebrush.

FRES20 Douglas-fir
FRES21 Ponderosa pine
FRES26 Lodgepole pine
FRES29 Sagebrush
FRES30 Desert shrub
FRES33 Southwestern shrubsteppe
FRES34 Chaparral-mountain shrub
FRES35 Pinyon-juniper
FRES36 Mountain grasslands
FRES38 Plains grasslands
FRES40 Desert grasslands


3 Southern Pacific Border
4 Sierra Mountains
5 Columbia Plateau
6 Upper Basin and Range
7 Lower Basin and Range
8 Northern Rocky Mountains
9 Middle Rocky Mountains
10 Wyoming Basin
11 Southern Rocky Mountains
12 Colorado Plateau
13 Rocky Mountain Piedmont
15 Black Hills Uplift
16 Upper Missouri Basin and Broken Lands

K005 Mixed conifer forest
K010 Ponderosa shrub forest
K011 Western ponderosa forest
K012 Douglas-fir forest
K016 Eastern ponderosa forest
K017 Black Hills pine forest
K018 Pine-Douglas-fir forest
K023 Juniper-pinyon woodland
K024 Juniper steppe woodland
K032 Transition between K031 and K037
K037 Mountain-mahogany-oak scrub
K038 Great Basin sagebrush
K039 Blackbrush
K040 Saltbush-greasewood
K050 Fescue-wheatgrass
K051 Wheatgrass-bluegrass
K053 Grama-galleta steppe
K055 Sagebrush steppe
K056 Wheatgrass-needlegrass shrubsteppe
K057 Galleta-threeawn shrubsteppe
K058 Grama-tobosa shrubsteppe,
K063 Foothills prairie
K064 Grama-needlegrass-wheatgrass
K066 Wheatgrass-needlegrass

210 Interior Douglas-fir
218 Lodgepole pine
219 Limber pine
220 Rocky Mountain juniper
237 Interior ponderosa pine
238 Western juniper
239 Pinyon-juniper
243 Sierra Nevada mixed conifer
247 Jeffrey pine

104 Antelope bitterbrush-bluebunch wheatgrass
107 Western juniper/big sagebrush/bluebunch wheatgrass
109 Ponderosa pine shrubland
210 Bitterbrush
212 Blackbush
302 Bluebunch wheatgrass-Sandberg bluegrass
303 Bluebunch wheatgrass-western wheatgrass
306 Idaho fescue-slender wheatgrass
314 Big sagebrush-bluebunch wheatgrass
315 Big sagebrush-Idaho fescue
316 Big sagebrush-rough fescue
317 Bitterbrush-bluebunch wheatgrass
318 Bitterbrush-Idaho fescue
319 Bitterbrush-rough fescue
320 Black sagebrush-bluebunch wheatgrass
321 Black sagebrush-Idaho fescue
322 Curlleaf mountain-mahogany-bluebunch wheatgrass
324 Threetip sagebrush-Idaho fescue
401 Basin big sagebrush
402 Mountain big sagebrush
403 Wyoming big sagebrush
404 Threetip sagebrush
405 Black sagebrush
412 Juniper-pinyon woodland
413 Gambel oak
414 Salt desert shrub
415 Curlleaf mountain-mahogany
416 True mountain-mahogany
421 Chokecherry-serviceberry-rose
501 Saltbush-greasewood
502 Grama-galleta
504 Juniper-pinyon pine woodland
607 Wheatgrass-needlegrass
608 Wheatgrass-grama-needlegrass
612 Sagebrush-grass
615 Wheatgrass-saltgrass-grama
708 Bluestem-dropseed
735 Sideoats grama-sumac-juniper

Spineless horsebrush is most frequently associated with sagebrush (Artemisia spp.), pinyon-juniper (Pinus-Juniperus spp.), ponderosa pine (Pinus ponderosa) [6,27,42,45,49,71] mountain brush, mixed conifer, and quaking aspen (Populus tremuloides) communities [71].

Green rabbitbrush (Chrysothamnus viscidiflorus) is a common associate of spineless horsebrush in big sagebrush (A. tridentata) communities of Utah, Idaho, Montana, and Nevada [37,70,76]. Threetip sagebrush (A. tripartita), rubber rabbitbrush (Chrysothamnus nauseous), and Idaho fescue (Festuca idahoensis) are associates in Idaho and Montana [37,70], while needle-and-thread grass (Hesperostipa comata) and Indian ricegrass (Achnatherum hymenoides) are associated in Montana and Nevada [70,76]. Other common associates of spineless horsebrush include antelope bitterbrush (Purshia tridentata) and mountain snowberry (Symphoricarpos oreophilus) in Idaho [37]; black sagebrush (A. nova) and bluebunch wheatgrass (Pseudoroegneria spicata) in Montana [70]; and bottlebrush squirreltail (Elymus elymoides), desert peach (Prunus andersonii), green ephedra (Ephedra viridis), and Thurber needlegrass (Achnatherum thurberianum) in Nevada [76].


SPECIES: Tetradymia canescens

Spineless horsebrush is a spineless, many-branched native subshrub or shrub [32,42,45,64,65]. Growth habit is low; the stiff, semiwoody to woody stems grow up to 6.5 feet (2 m) tall [31,42,43,45,64,79]. Horsebrushes (Tetradymia spp.) bear primary and secondary leaves: primary leaves are alternate along elongated shoots and are usually long lived. Secondary leaves develop in the axils of primary leaves and usually dry up within a few weeks [43]. Spineless horsebrush leaves are 0.4-1.2 inches (1-3 cm) long and 0.04-0.16 inch (1-4 mm) wide [14,32,42,45,65]. Medium-sized flowerheads are borne on tips of numerous short branches in small racemes [14,32,42,43,45,65]. The plant produces light achenes that are 0.12-0.16 inch (3-4 mm) long [14,32,42,45,64,65]. Horsebrushes (Tetradymia spp.) are taprooted [39].

Occasional dense stands of spineless horsebrush occur in the sagebrush zone [45], but stands with more than a few hundred individuals are rare. A typical colony size is 25 to 50 plants [64].


Breeding system: Spineless horsebrush is a monoecious species with perfect flowers [42,64].

Pollination: Spineless horsebrush is pollinated by generalists including moths, bees, flies, beetles, and other insects. Nearly all spineless horsebrush shrubs within a given colony flower within several days of each other. Flowering occurs relatively early in comparison to associated plants, minimizing competition for pollinators.  Pollen fertility is high [43,64].

Seed production: Seed production is high [67].

Seed dispersal: Seeds are light (120,000 seeds per pound) [67] and wind dispersed [79]. The hairs on horsebrush pappi aid dispersal [21].

Seed banking: No information

Germination: No information

Seedling establishment/growth: Spineless horsebrush seedlings are somewhat rare, indicating a long colony turnover rate. This may be due to the harsh environments in which spineless horsebrush grows [42,64]. Though seedling vigor is rated low [67], numerous seedlings have been observed in recently burned areas [42].

Asexual regeneration: Spineless horsebrush sprouts from the root crown after top-growth removal by fire or other disturbances [5,19,30,34,69,76].

Spineless horsebrush is found in dry, open places on hills, ridges, and plains [14,24,32,42,65] at elevations ranging from 1,300 to 10,800 feet (400-3,300 m) [19]. In the southern part of its range, spineless horsebrush is found at higher elevations (5,800-9,700 feet, 1,800-3,000 m), while in the northern areas of its range, British Columbia and Washington, it is found at lower elevations (1,300-1,900 feet, 400-600 m) [64]. It is adapted to medium to coarse soils with a pH of 7 to 8 [67] and is most often found in gravelly or sandy loams [6,32,65] in semi-desert saline or alkali loam range sites [32]. However, 1 source characterizes spineless horsebrush as salt intolerant. The annual moisture requirement of spineless horsebrush is 5-12 inches (130-300 mm); it is considered highly drought tolerant [67]. The plant tends to occur least frequently on south-facing slopes but occurs equally on north, east, and west slopes [33].

Elevation ranges for spineless horsebrush in some states in its range include:

Arizona 6,000-7,000 feet   (1,829-2,134 m) [32]
California 4,000-10,000 feet (1,219-3,048 m) [25,49]
Colorado 4,500-9,500 feet   (1,372-2,896 m) [24]
Montana 4,000-7,000 feet   (1,219-2,134 m) [16]
New Mexico 6,000-7,000 feet   (1,829-2,134 m) [41]
Utah 5,000-10,335 feet (1,525-3,150 m) [71]
Wyoming 4,000-9,000 feet   (1,219-2,743 m) [16]

Though spineless horsebrush can be found in nearly all successional stages, it is primarily a weedy, seral species [17,31,64]. One study reported that the species comprised greater than 5% of the cover only in early-mid successional stages [33]. It is not shade tolerant [67].

Spineless horsebrush begins growth in spring and blooms in spring or summer [30]. It begins flowering in mid-June in the northern part of its range (British Columbia), while blooming begins in late July or early August in southern California and northern Arizona [42,45]. Blooming may continue until September in New Mexico [41] and as late as October in Arizona [32]. Leaf fall occurs in autumn [14]. Flowering dates in several states are as follows:

Arizona July-October [32]
California July-August [49]
New Mexico July-September [41]

The following spineless horsebrush phenology data are from a study in the Upper Snake River Plains of southern Idaho [8]:

Leaf growth starts Twig growth starts Flower buds visible 1st bloom Full bloom Bloom over Seed ripe Dissemination over Leaves drying Leaf drop
April 17 May 27 June 18 July 17 July 25 Aug. 7 Aug. 15 Sept. 7 Sept. 7 Oct. 6


SPECIES: Tetradymia canescens

Fire adaptations: Spineless horsebrush is a fire resistant shrub [69] that is only briefly harmed by burning [30,34]. After a fire the plant responds by rapid sprouting from adventitious buds on the root crown [5,19,30,34,69,76]. A single burned spineless horsebrush shrub may produce as many as 5 to 7 sprouts [76], which may appear to be entirely new plants [19]. In addition, the plant reproduces abundantly from a heavy seed crop following fire [51,79]. These postfire responses often result in dense stands of spineless horsebrush [19] and can result in dominance by spineless horsebrush in sagebrush-grass communities that are burned with a frequency of 20 to 25 years [74].

Fire regimes: Fires in the desert shrub and grassland ecosystems in which spineless horsebrush occurs were historically stand replacing. Fires in the forest types (ponderosa pine and Douglas-fir) were usually frequent surface fires, whereas fires in pinyon-juniper types were of mixed severity [52]. Fire return intervals for plant communities and ecosystems in which spineless horsebrush occurs are summarized below. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find Fire Regimes".

Community or Ecosystem Dominant Species Fire Return Interval Range (years)
sagebrush steppe Artemisia tridentata/Pseudoroegneria spicata 20-70 [52]
basin big sagebrush A. tridentata var. tridentata 12-43 [57]
mountain big sagebrush A. tridentata var. vaseyana 15-40 [3,11,44]
Wyoming big sagebrush A. tridentata var. wyomingensis 10-70 (40**) [68,77]
saltbush-greasewood Atriplex confertifolia-Sarcobatus vermiculatus < 35 to < 100
desert grasslands Bouteloua eriopoda and/or Pleuraphis mutica 5-100
plains grasslands Bouteloua spp. < 35 
blue grama-needle-and-thread grass-western wheatgrass B. gracilis-Hesperostipa comata-Pascopyrum smithii < 35 
blue grama-buffalo grass B. gracilis-Buchloe dactyloides < 35 
grama-galleta steppe Bouteloua gracilis-Pleuraphis jamesii < 35 to < 100 
blue grama-tobosa prairie B. gracilis-P. mutica < 35 to < 100 
western juniper Juniperus occidentalis 20-70 
Rocky Mountain juniper J. scopulorum < 35 
pinyon-juniper Pinus-Juniperus spp. < 35 
Colorado pinyon Pinus edulis 10-49 [52]
Pacific ponderosa pine* P. ponderosa var. ponderosa 1-47
interior ponderosa pine* P. ponderosa var. scopulorum 2-10
Arizona pine P. ponderosa var. arizonica 2-10 [2]
mountain grasslands Pseudoroegneria spicata 3-40 (10**) [1,2]
Rocky Mountain Douglas-fir* Pseudotsuga menziesii var. glauca 25-100 [2]
*fire return interval varies widely; trends in variation are noted in the species summary

Tall shrub, adventitious bud/root crown
Initial off-site colonizer (off-site, initial community)
Secondary colonizer (on-site or off-site seed sources)


SPECIES: Tetradymia canescens

Spineless horsebrush is top-killed by fire [40].

No entry

Spineless horsebrush rapidly establishes by sprouting from the root crown and profuse seed production after fire [7,10,40,51,65,74,75,79]. Though it is usually reduced the 1st postfire year, it quickly recovers and exceeds prefire herbage production [54]. Its coverage may double by the 3rd year after fire, and after 12 years, it may increase to 5 times its prefire coverage [9,47,51,55,74]. Spineless horsebrush colonizes sagebrush communities after fire [75], sometimes establishing in thick stands [7]. Approximately 30 years after fire, spineless horsebrush begins to decline [47,51,55,74], although its production may still be as much as 60% higher than in unburned areas [74].

A study of big sagebrush-grass communities in Wyoming reported that while individual spineless horsebrush plants' production increased following a fire, an overall increase in density of the species was not observed by postfire year 2 [61]. Research in a big sagebrush community on the Upper Snake River Plains reported profuse sprouting by spineless horsebrush after a prescribed burn in August. By postfire year 3, sprout height exceeded height of prefire stems, and sprouts were producing seeds. Fifteen years later, spineless horsebrush production was significantly (p=0.05) higher on burned than on unburned plots, and highest yields tended to be associated with areas that burned more severely [7]. Results from a study of big sagebrush-Idaho fescue communities in Salmon National Forest, Idaho, suggest that "cool" burns may also enhance spineless horsebrush [37].

Results from 1 postfire study in a big sagebrush-grass range on the Upper Snake River Plains, Idaho, are below. The table compares spineless horsebrush annual herbage production (lb/acre, air-dry) on unburned and burned areas for selected years following a 1936 burn. Overall, it shows that higher yields of spineless horsebrush were produced on burned plots than on unburned plots [23].

  1936 1937 1939 1948 1966
Unburned 12 12 8 8 8
Burned 33 10 38 70 36

No entry

Fire exclusion on desert steppes has tended to favor sagebrush species over fire-tolerant shrubs such as horsebrush [12,23,26]. Increased fire frequencies due to invasion of cheatgrass, red brome, and other annual grasses, may favor spineless horsebrush. If the management objective is to reduce spineless horsebrush, fire treatment of spineless horsebrush populations is generally not recommended as it may result in the species becoming dominant [10,51,54,74]. Repeated burning will likely produce thick stands of the shrub [7]. Therefore, consideration of resprouting and seedling establishment of spineless horsebrush is recommended when planning prescribed fires, especially on sagebrush rangelands [10].


SPECIES: Tetradymia canescens

Spineless horsebrush has little value as domestic forage. It is toxic to domestic sheep and is generally considered undesirable on rangelands [30,53,66,73].

Poisoning by spineless horsebrush  has resulted in losses of as many as 1,000 domestic sheep at a time [15,64]. All parts of spineless horsebrush contain compounds (furanoeremophilanes) that are toxic to sheep, but they generally consume only young stems and buds. Spineless horsebrush is therefore most dangerous in its bud stage. After flowering, its toxicity appears to decrease [63]. Stems and leaves equaling as little as 0.5% of the animal's body weight [45] can cause liver damage and related photosensitization, leading to capillary breakage, edema, and swelling of the head known as bighead disease [9,28,45,63,64]. Photosensitization occurs when domestic sheep consume black sagebrush (Artemisia nova) and littleleaf horsebrush together [46,58,63,66], which seems to synergistically increase sheep sensitivity to the furanoeremophilanes [30]. Other effects include a lowering of wool quality [15] and abortion [15,30]. Domestic cattle are not poisoned by spineless horsebrush [9,30,45,64].

Though spineless horsebrush is generally low in palatability, cattle graze the shrub heavily in some areas of Nevada [42]. Mule deer reportedly browse it moderately in winter and lightly in spring [36]. Pronghorn also browse the shrub sparingly [60]. Spineless horsebrush's palatability increases during the bud stage, when young tissues - buds, shoots, and leaves - are frequently browsed by domestic sheep, domestic goats, pronghorn, mule deer, and cattle [42].

The degree of use shown by livestock and wildlife species for spineless horsebrush is rated as follows [16]:

Cattle Poor Poor Poor Poor
Domestic sheep Poor Fair Poor Poor
Horses Poor Poor Poor Poor
Pronghorn ---- Poor Poor Fair
Elk Poor Poor Poor Fair
Mule deer Poor Poor Poor Good
White-tailed deer ---- ---- ---- Fair
Small mammals ---- ---- Poor Fair
Small nongame birds ---- ---- Poor ----
Upland game birds ---- ---- Poor ----
Waterfowl ---- ---- Poor ----

Spineless horsebrush is rated low in energy [16] and protein value [16,67]. See 'Value to Livestock and Wildlife' section for information about spineless horsebrush toxicity.

Spineless horsebrush provides critically needed cover in dry, sparsely vegetated desert ranges [42]. The value of spineless horsebrush as cover for domestic animals and wildlife is rated as follows [16]:

Pronghorn ---- Poor Poor Fair
Elk ---- Poor Poor Poor
Mule deer ---- Poor Poor Poor
White-tailed deer ---- ---- ---- Poor
Small mammals Fair Poor Fair Good
Small nongame birds ---- ---- Fair Good
Upland game birds ---- ----- Fair Fair
Waterfowl ---- ---- Poor Poor

Spineless horsebrush may be effective at establishing cover on severely depleted soils such as mine spoils [42]. The plant can be propagated by cuttings or seeds [67].

Native Americans have used spineless horsebrush as a physic, a disinfectant, a treatment for sexually transmitted diseases [45], and a tonic [31,32].

Although spineless horsebrush is not favored browse, domestic sheep are more likely to consume the plant during stormy weather or otherwise stressful periods when animals' grazing habits often change. Also, since spineless horsebrush may be abundant near watering holes, it is frequently browsed shortly after trailing sheep are watered. During stressful situations or after watering, it is best to avoid trailing sheep through overgrazed trails or rangelands where spineless horsebrush is abundant [27,30,63]. See the 'Importance to Livestock and Wildlife' section above for more information on spineless horsebrush toxicity.

Control: Root cutting, bulldozing [53,72], and tebuthiuron application, can reduce spineless horsebrush density [13,50]. Application of 2,4-D in spring when plants are actively growing can help control spineless horsebrush; however, retreatment is necessary to control sprouts [72].

In rangelands where big sagebrush control is desired, care should be exercised with selection of methods. Unless spineless horsebrush is also controlled, it may increase in density [38,51,53,54,74].  Some control treatments may actually increase spineless horsebrush density. Single applications of  atrazine [18] or 2,4-D do not effectively control spineless horsebrush [18,19,53]. In a big sagebrush community near Reno, Nevada, May application of 2,4-D the spring after a July wildfire did not significantly (p=0.05) reduce spineless horsebrush sprouting (plots were remeasured at posttreatment year 4). Livestock grazing significantly increased spineless horsebrush density on both sprayed and unsprayed plots. Seeding with the perennial bunchgrasses desert wheatgrass (Agropyron desertorum) and intermediate wheatgrass (Elytrigia intermedia) significantly reduced spineless horsebrush density on grazed and ungrazed plots [19]. Desert wheatgrass and intermediate wheatgrass are exotic species [25]: native bunchgrasses may be more desirable rehabilitation alternatives. The following control methods are also generally ineffective at controlling spineless horsebrush: prescribed burning, cutting, beating, shredding, harrowing, railing, and rolling brush cutting [53]. In 1 Utah study, prescribed burning, rotobeating, application of 2,4-D, or railing each resulted in at least a 2-fold increase in spineless horsebrush density compared with untreated plots [48].


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