Effects of defoliation, shading and competition on spotted knapweed and bluebunch wheatgrass

Abstract

Spotted knapweed (Centaurea maculosa Lam.) is a noxious plant that has invaded many native ranges in the Northern Intermountain Region. Although the use of livestock to control knapweed is intuitively appealing, feasibility of the strategy has received little attention. This greenhouse study was conducted to evaluate response of spotted knapweed to defoliation, light, and competition. Although total knapweed biomass (g/plant) was not altered by defoliation treatments, several of the more severe treatments adversely affected root, crown, and final harvest foliage. Root and crown growth were also adversely affected by increasing competition from bluebunch wheatgrass (Pseudoroegneria spicata). Foliage, root, and crown growth of spotted knapweed increased significantly when plants received full, rather than half light. Spotted knapweed was less sensitive to defoliation than was bluebunch wheatgrass. Although the feasibility of using livestock to control spotted knapweed cannot be completely disregarded, data suggest that the knapweed would have to be selectively and repeatedly grazed during the growing season.