Potential for Controlling the Spread of Centaurea maculosa with Grass Competition

Spotted knapweed (Centaurea maculosa Lam.) is a major rangeland and roadside weed of the northern Rocky Mountains. It is often found in plant communities dominated by Pseudoroegneria spicatum or Festuca idahoensis, but it rarely invades roadsides dominated by Bromus inerrnis Leyss. Aboveground biomass of the 3 grass species grown in mixture with Centaurea was compared to growth in monoculture at a range of nitrogen input levels. The results suggest that Bromus is capable of suppressing the growth of Centaurea with the degree of suppression increasing with increasing nitrogen levels. The 2 native grasses had no impact on Centaurea under the controlled environment conditions of this study. Centaurea maculosa Lam. (spotted knap­ weed) is a major weed associated with spring wet-summer dry areas of the northern Rocky Mountains (Forcella and Harvey 1981, Tyser and Key 1988, Weaver et aI. 1989). Centaurea dominates waste places, invades disturbed rangeland, and sometimes invades undisturbed range (Tyser and Key 1988). In contrast, it rarely invades roadsides dominated by Bromus inermis Leyss. (Weaver et al. 1989). This suggests that it may be excluded from waste places that are planted to Bromus before Centaurea invades. Alternatively, because planting exotics violates the charge of national park managers, one may ask whether Centaurea might also be excluded from disturbed areas by planting native grasses that naturally dominate either relatively dry (Pseudoroegneria spicatum (Pursh) Scribner and Smith = Agropyron spicatum) or more moist (Festuca idahoensis Elmer.) foothill habitats. Weed suppression may be accomplished by (1) preempting resources with more competi­ tive plant species or (2) using biocontrols or herbicides that selectively increase weed mor­ tality, decrease vigor, or prevent reproduction (Lindquist et al. 1995). This study considers management of Centaurea maculosa by compe­ tition rather than by common herbicide and biocontrol methods. This approach deserves attention because it may be less expensive and more effective than herbicides in the long term. Our objective was to measure the competi­ tive ability of 3 grass species against Centaurea in 2-way interaction experiments in sand cul­ ture. Mixture and mono culture treatments were tested for 12 wk at 5 positions on a nitro­ gen gradient to determine whether competi­ tive relations were influenced by differences in nitrogen availability. A plant's ability to com­ pete is related to its growth rate or ability to gain biomass relative to associated species (Harper 1977). We compared aboveground bio­ mass of each species grown in mixture with Centaurea to its growth in monoculture.