''Acceptable'' levels of spotted knapweed (Centaurea maculosa) control

Competition between bluebunch wheatgrass and spotted knapweed was quantified using three addition series experiments in an environmental chamber. Using two density matrices, spotted knapweed plants were thinned by either 45 or 90% of their original densities 40 d after emergence. Another matrix of density combinations was not thinned. All plant material was harvested 90 d after thinning. Spotted knapweed was about four times more competitive than bluebunch wheatgrass. Reducing spotted knapweed by 45% did not alter the competitive relationship between the two species. We believe remaining knapweed individuals captured the majority of the newly available resources. Ninety percent reduction was necessary to shift the competitive relationship in favor of bluebunch wheatgrass. Successful integrated spotted knapweed management must exploit key mechanisms and processes directing plant community dynamics, in conjunction with weed density reduction, if communities are to be shifted toward those that are desired.