Defoliation effects on Spotted Knapweed seed production and viability

Abstract

Spotted knapweed (Centaurea stoebe L.) is a deeply taprooted perennial forb infesting millions of hectares of rangeland in western North America. Spotted knapweed forms large monocultures, which lowers plant diversity, reduces livestock and wildlife forage, and increases surface water runoff and sediment yield. It can produce 5,000-40,000 seeds m-2 year-1, and often produces new flowers after prescribed sheep grazing or mowing defoliates spotted knapweed plants during the bolting or flowering stage. Research has yet to determine if new flowers produced following spring/summer defoliation produce viable seeds by the end of the growing season. The purpose of this 2-year study was to determine the appropriate timing(s) or combination(s) of timings of defoliation on spotted knapweed to reduce viable seed production. Ten spotted knapweed plants, located on spotted knapweed-infested rangeland in west-central Montana, were hand-clipped for each of the following treatments: 1) 35-40% relative utilization of above-ground biomass when plants were in the bolting stage; 2) 100% of buds removed at late-bud/early-flowering stage; 3) 100% of flowers removed at full-flowering stage; 4) Treatment 1+Treatment 2; 5) Treatment 1+Treatment 3; 6) Treatment 2+Treatment 3; 7) Treatment 1+Treatment 2+Treatment 3; and 8) unclipped control. The number of buds/flowerheads per plant, number of seeds per plant, percent viability of seeds, and number of viable seeds per plant were determined when seeds were in the well-developed stage, but seedhead bracts were still tightly closed (mid-August through September). Clipping at any timing or combination of timings reduced the number of buds/flowerheads per plant (P < 0.01), number of seeds per plant (P < 0.01), percent viability of seeds (P < 0.01), and number of viable seeds per plant (P < 0.01) both years compared with the unclipped control. Clipping during the bolting stage reduced the number of viable seeds by nearly 90% compared with no clipping. Clipping during the late-bud/early-flower or full-flower stage reduced the number of viable seeds by nearly 100% compared with no clipping. Defoliation of spotted knapweed via prescribed sheep grazing or mowing in summer should effectively suppress viable seed production of spotted knapweed.