Figure 61. — Spines (arrow) 

 on Nuttall horsebrush 

 (T . nuttalliij. Speai- 

 gvowing in Antelope 

 Valley, Sanpete Co., 

 Utah. 



long. Blooming occurs from late May to early July. Seeds are about 50 percent larger 

 than those of T. canescens. 



Hybridization. --Only diploid (2n = 60) populations have been reported for 

 T. nuttallii (Strother 1974). 



Distribution and habitat . --}iut tall horsebrush occurs in the badlands of south- 

 western Wyoming and in dry barren hills and plains of northeastern Nevada, northern 

 half of Utah, and northwestern Colorado between 1,300 and 2,100 meters (4,300 and 

 6,900 feet) in elevation. It is usually associated with sagebrush and shadscale 

 vegetational types. 



Use . --The spiny growth habit precludes much use by animals except for cover. It 

 contributes to the vegetal cover in dry, hostile environments. 



Tetradymia spinosa Hook. & Arn. (spiny horsebrush^) 



Spiny horsebrush is a compact or spreading, much-branched spiny shrub to 12 dm 

 high. Unlike the other horsebrushes mentioned in this paper, the dense, evenly pannose 

 vestiture of the young stems is uninterrupted by streaks and is persistent as the stems 

 age. 



The primary leaves transform into stiff recurved spines 5 to 20 mm long. Fascicles 

 of essentially glabrous, linear to narrowly spatulate, secondary leaves 3 to 15 mm 

 long develop in the axils of the spines. 



Flowering heads are produced singly or in pairs on short peduncles 1 to 1.5 cm 

 long in the spine (primary leaf) axils formed the previous year. The heads usually 

 contain 6 pale to bright yellow disc flowers. Five or 6 oblong to ovate, conspicuously 

 tomentose, involucral bracts 8 to 12 mm long subtend each head. The achenes are covered 

 with long white woolly hairs that more or less conceal the pappus of small, white, 

 slender scales. Blooming normally occurs during May and June, but may extend into 

 August . 



Hybridization . --Only diploid (2n= 60) populations of T. spinosa have been re- 

 ported (Strother 1974). 



Other common names include cottonthorn horsebrush and catclaw horsebrush. 



62 



