includes large areas in southern Canada from British Columbia to 

 Manitoba, a few of the Lake States, and all the States west of North 

 Dakota and south into Mexico. 



This species prospers in a great variety of habitats, such as sage- 

 brush areas, canyon bottoms, glades, roadsides, and vacant lots. 

 It is also prevalent in brushlands, aspen, and open timber, but avoids 

 dense shade. It is comparatively drought-resistant and flourishes 

 in the sandy and gravelly loam soils of open flats, parks, and dry 

 meadows. The plants usually grow somewhat scatteringly, seldom 

 forming pure stands on areas larger than a few square rods. In 

 places where the natural plant cover has been but slightly or not at 

 all disturbed, western yarrow occurs only sparsely, but it is one of 

 the plants that invades readily and increases conspicuously when 

 overgrazing makes growing conditions unfavorable for more pal- 

 atable and less resistant species. 



The forage value of western yarrow varies greatly with different 

 localities and with different plant associates. On many ranges all 

 classes of livestock graze this plant moderately throughout the 

 season. Sheep (and sometimes cattle) often evince a fondness for 

 the flower heads of western yarrow. They do not relish the stems 

 when these parts become somewhat woody late in the season, al- 

 though they occasionally graze the dried leaves. The species rates 

 from poor to, rarely, good in palatability for sheep and from un- 

 palatable to fair, seldom fairly good, for cattle. Its chief value 

 for sheep appears to be in Nevada and New Mexico, although rank- 

 ing as fairly good in parts of Utah and Arizona. It appears to 

 be most valuable for cattle in parts of Colorado, Wyoming, Arizona, 

 and New Mexico. In California it is usually regarded as poor or 

 worthless. Horses graze it much less than do cattle. Deer, as a 

 rule, eat western yarrow very sparingly, but on the Kaibab Plateau 

 the species is regarded as fair mule-deer forage. 



Sampson, 1 in his plant succession studies on high summer range 

 in central Utah, shows that, when this type of range is undisturbed, 

 its climax vegetation is made up of wheatgrasses. With overgraz- 

 ing, however, this cover changes to a somewhat less stable mixed 

 grass and weed type, the porcupinegrass-yellowbrush consociation. 

 He reports further that when conditions unfavorable to growth are 

 sufficiently prolonged gradually to destroy the porcupinegrass- 

 yellowbrush cover, but not such as seriously to change the condition 

 of the soil, shallow-rooted perennial weeds of the second weed stage, 

 notably "blue foxglove" (Pentstemon procerus), "sweet sage" (Arte- 

 misia discolor), and yarrow (Achittea lanulosa) , are the natural 

 successors. Western yarrow is thus definitely placed by Sampson 

 as one of the three perennial weeds that, under certain definite 

 conditions of overgrazing, tend to dominate these high summer 

 ranges. Under such conditions, local abundance of western yarrow 

 would be an indicator of continued past overstocking and excessive 

 utilization. 



1 Sampson, A. W. PLANT SUCCESSION IN RELATION TO RANGE MANAGEMENT. U. S. Dept. 

 Agr. Bull. 791, 70 pp., illus. 1919. 



