Picloram, fertilizer, and defoliation interactions on spotted knapweed reinvasion

Spotted knapweed (Centaurea maculosa Lam.) management may be enhanced by in tegrating strategies that stimulate and maintain competitive grasses. The o bjective of this study was to determine if picloram, fertilizer, and timing and frequency of grass defoliation could be integrated to minimize spotted knapweed reinvasion. Sixteen chemical treatments [4 picloram rates (0.00, 0.14, 0.28, and 0.42 kg a.i. ha(-1)) and 4 fertilizer rates (source: 16-20- 0, N-P-K; material: 0.0, 66, 132, 19& kg ha(-1))] were applied in the sprin g of 1994 to 4 by 4 m plots and factorially arranged in a randomized-comple te-block design. Within each plot, 6 grass defoliation treatments were rand omly applied to 1 by 1 m sub-plots. From 1994 through 1997, 60% of the abov e ground grass biomass was hand clipped and removed from the plots during t he spring, summer, fall, alternating spring/fall, all 3 seasons. A control received no grass defoliation. The experiment was replicated 4 times at 2 s ites dominated by spotted knapweed. At peak standing crop in 1997 spotted k napweed density, grass and spotted knapweed biomass; and percent cover of s potted knapweed, grass, litter, and bare ground were measured. Data were an alyzed as a split-plot using analysis of variance. Pour years after treatme nt all rates of picloram reduced spotted knapweed density, biomass, and cov er, and increased grass yield. Nitrogen and P fertilizer tended to increase spotted knapweed density and biomass. Nitrogen and P fertilizer plus defol iation in all 3 seasons caused a greater increase in spotted knapweed reinv asion at the site with Kentucky bluegrass (Poa pratensis L.) than the site with timothy (Phleum pratense L.) and smooth brome (Bromus inermis Leys.). Fall-only defoliation and no defoliation appear to deter spotted knapweed r einvasion better than defoliation in all 3 seasons and alternately in the s pring and fall.