Relationships among idaho fescue defoliation, soil water, and spotted knapweed emergence and growth.

Abstract

Developing rangeland management strategies to minimize spotted knapweed (Centaurea maculosa Lam.) invasion will require understanding the effects of intensity, frequency, and season of grazing on weed establishment. We studied the effects of hand-clipping 2-year-old Idaho fescue (Festuca idahoensis Elmer) plants 0, 30, 60, or 90%, 1, 2, or 3 times (14-day intervals) on spotted knapweed seedling emergence and growth in pots watered with 150 ml once weekly or 50 ml 3 times weekly. Pots were seeded with 5,000 spotted knapweed seeds m-2, replicated twice, and placed in a growth chamber in a completely randomized design. The experiment was repeated once. Plants were harvested after 50 days. Treatment effects on soil moisture, Idaho fescue and spotted knapweed shoot and root weight, and leaf area were compared using analysis of variance and regression analysis. At final harvest, Idaho fescue shoot weight and leaf area decreased with increasing defoliation level and frequency. Idaho fescue root weight was not affected by any treatment. A single Idaho fescue defoliation at 30% and 90% increased spotted knapweed weight and numbers per pot respectively, over those pots with undefoliated plants. The level of defoliation necessary to enhance spotted knapweed numbers was lower as defoliation frequency increased. As defoliation level and frequency increased, soil water content increased resulting in a corresponding increase in spotted knapweed emergence and growth.