Distribution and amount of four knapweed (Centaurea L.) species in eastern Washington.

Abstract

Distribution and Amount of Four Knapweed (Centaurca L.) Species in Eastern Washingtonl Abstracl Since their introduction in the early 1900's, four knapweed (Centaureo L.) species have become proninent weeds o{ pastures, .angeland,and nonnrigated hav in the Northern Internounrain Region.The reducrion in forage follo{ing invasion by rhese weeds lhreatens fie stabilily of lhe ranching indus!ry.Planning an effective integrated control prograh requires infornarion on $e distribution and anount of each species.An eastern Washington knapweed survey $as compleled in 198?.Diffuse knapwed (Cer,ourea diJfusa Lan.) Nas reporred in all 20 eastern Vashinston coun!ies, for a total of l?3, l5l ha of infested area.Ninety percent of lhe infestation vas in the six counties that border ihe Columbia River Basin lo the north and west: Stevens, Ferry, Okanogan, Chelan, Kitlilas, and Yakina.Of rhe l6 counlies reporring yellow srarrhistle (C roArn'dl'r L.), those 5 in southastern Vashington had 93 percent ofthe 54,172 ha.A lotal of 10,??? ha ofsponed knap{eed (C nd.Llosd Lan.) was reported from l9 counties.Ninety-tr.yopercent was reporred fron the northeastern counries ofSpokane, Pend oreille, and Stevens.Russian knapwed (C.repens L.), reported from 19 counties, was esrinated at 3,259 ha.The annual loss in forage production on pastu.e and rangeland was estinared to be 59,190,540 kg, a value of $951,000 in Animal Unit Monrhs o.E2.9 million in hav.