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 tnermis 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.