Spotted Knapweed and Grass Response to Herbicide Treatments

Abstract

Picloram at 0.28 kg ai ha-1 , clopyralid plus 2,4-D at 0.21 kg ai ha-1 plus 1.12 kg ai ha-1 , or dicamba plus 2,4-D at 0.56 kg ai ha-1 plus 1.12 kg ai ha-1 were applied to spotted knapweed (Centaurea maculosa Lam.) at the spring-rosette, bolt, bud, flower, or fallrosette growth stages in 1991 on 2 sites in Montana. Treatments (3 herbicide treatments, 5 growth stages) were applied in a randomized-complete-block design and replicated 3 times at each site. Effects of herbicides on mature and seedling spotted knapweed density depended upon spotted knapweed growth stage at the time of application and the number of years after application. Picloram consistently reduced mature spotted knapweed density to low levels (<5 plants m-2), regardless of growth stage, and its effect persisted through 1994. Clopyralid plus 2,4-D applied at the bolt or bud stage reduced spotted knapweed densities similar to that of picloram (95%) at the Avon site, while providing about 50% reduction in density 3 years after application at Missoula. This treatment may provide an alternative to picloram in environmentally sensitive areas. Dicamba plus 2,4-D was most effective when applied during the bud and bolt growth stages, and least effective when applied during the spring-and fall-rosette stages. In most situations, picloram and clopyralid plus 2,4-D provided greater control of spotted knapweed than dicamba plus 2,4-D. Herbicide treatments increased perennial grass biomass from 173 kg ha-1 in the nontreated controls to 494, 880, and 1,309 kg ha-1 for dicamba plus 2,4-D, clopyralid plus 2,4-D and picloram, respectively.