This three year study examined the effect of a single grass defoliation on the establishment of diffuse knapweed (Centaurea diffusa) at two different sites east of Plymouth, Washington. The first site was dominated by crested wheatgrass and the second site was co-dominated by bluebunch wheatgrass and needle-and-thread grass. Diffuse knapweed was seeded at 500 and 1000 seeds per 1 m² plot in the fall. The following spring grasses on the site were defoliated (clipped) at six different levels (0, 20, 40, 60, 80 and 100) to mimic grazing. The number of flowering grass culms and weed seedlings were measured the first year of the study. The following May density and standing crop of diffuse knapweed were measured.
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