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Preventing the invasion of uninfested rangeland is central to managing spotted knapweed (Centaurea maculosa Lam.). Intensity, frequency, and season of grass defoliation determine the ability of grasses to tolerate grazing and resist weed invasion. We hypothesized that as grass defoliation intensity increases, spotted knapweed cover, density, and biomass would increase, that increasing defoliation frequency would increase the intensity effect, and that spring defoliation would cause a greater increase in spotted knapweed than summer defoliation. We hand clipped grasses in 1 m2 plots at 2 spotted knapweed infested Idaho fescue (Festuca idahoensis Elmer) sites in western Montana. Clipping treatments were 4 intensities (0, 30, 60, and 90% of the foliage), 3 frequencies (1, 2, and 3 at 14-day intervals), and 2 seasons (spring beginning in mid-May and summer beginning in mid-July), factorially arranged in a randomized-complete-block design with 4 replications for a total of 24 treatments per replication. Treatments were applied in 1995 and 1996. By 1997 grass cover and density were reduced by defoliation intensity of 90%. Defoliation frequency greater than once caused a reduction in grass cover and density at the 60% intensity. Spring defoliation caused a greater reduction in grass cover and density than summer defoliation. Grass biomass was reduced by the 30% defoliation treatment. Grass defoliation intensity greater than 60% caused an increase in spotted knapweed cover and density. Defoliation more than once increased spotted knapweed cover. Spring defoliations increased spotted knapweed cover compared to summer defoliations. Spotted knapweed biomass was not affected by defoliation treatments. Our study suggests that an annual single grass defoliation of 60% or less, regardless of the season, will not increase spotted knapweed invasion on rangeland. The Journal of Range Management archives are made available by the Society for Range Management and the University of Arizona Libraries. Contact lbry-journals@email.arizona.edu for further information. Migrated from OJS platform August 2020