Spotted knapweed response to season and frequency of mowing

Spotted knapweed (Centaurea maculosa Lam,) is a non-indigenous weed that ha s invaded millions of hectares of rangeland in the United States. Mowing ma y be useful for reducing this weed. Our objective was to investigate the re sponse of spotted knapweed and grasses to season and frequency of mowing. R esponse of grass and spotted knapweed to 16 mowing treatments applied annua lly for 3 years was studied at 2 sites. Treatments consisted of combination s of spring, summer, and fall mowing. Treatments were arranged in a randomi zed-complete-block design with 4 replications (16 treatments; 4 replication s; 2 sites = 128 plots). After repeating mowing treatments for 3 years, a s ingle fall mowing when spotted knapweed was in the flowering or seed produc ing stage reduced;its cover and adult density as much as any treatment cons isting of repeated mowing. Fall mowing decreased adult density 85 and 83% b elow that of the control at Sites 1 and 2, respectively. Treatments reduced seedling density at Site 2, but the response was not consistent between ye ars or among treatments. Spotted knapweed cover was decreased by several mo wing treatments at each site (10-36%), while grass cover was only decreased by 3 mowing treatments (18-23%) at Site 1 in 1998, We recommend a single a nnual mowing, applied at the flowering or seed producing stage, for the par tial control of spotted knapweed.