EFFECT OF WEED SEED RATE AND GRASS DEFOLIATION LEVEL ON DIFFUSE KNAPWEED

Diffuse knapweed (Centaurea diffusa Lam,), an invasive weed, has reduc ed forage production and biodiversity, and increased soil erosion on o ver a million hectares of rangeland in the western United States, This study evaluated the effects of a single grass defoliation on establis hment of diffuse knapweed seeded at 2 rates into a bluebunch wheatgras s (Pseudoroegneria spicata [Pursh.] Scribn and Smith)/needle-and-threa d (Stipa comata Trin, & Rupr,) community and a crested wheatgrass (Agr opyron cristatum (L,) Gaertn,) community, Six defoliation levels (0, 2 0, 40, 60, 80, 100%) and 2 seeding rates (3,000, 6,000 diffuse knapwee d seeds) were applied to 1 m(2) plots in a randomized-complete-block d esign (n=4). Diffuse knapweed was seeded in the fall of 1992, and gras ses were defoliated on 28 April 1993, The number of flowering culms an d weed seedlings were counted in September 1993, Densities of diffuse knapweed seedlings, juveniles, and adults, as well as plant standing c rop, were determined in May 1994, Seed rate had minimal effect on diff use knapweed density, By May 1994, densities of diffuse knapweed were about 20 and 30 plants m(-2) on undefoliated bluebunch wheatgrass and crested wheatgrass plots, respectively, indicating that defoliation is not required for this noxious weed to become established, Higher leve ls of grass defoliation (>60%), especially of bluebunch wheatgrass, en hanced diffuse knapweed establishment, indicating that moderate (less than or equal to 60%) defoliation would not necessarily accelerate inv asion by diffuse knapweed.