Enhancing desired species establishment and persistence is central to rehabilitating invasive plant–infested rangeland. We hypothesized that nonnative desired species (alfalfa [Medicago sativa L., var. Arrow], intermediate wheatgrass [Thinopyrum intermedium {Host} Barkworth & D.R. Dewey], and crested wheatgrass [Agropyron cristatum {L.} Gaertn., var. Hycrest]) increase as desired species richness within seeding mixture increases, and spotted knapweed (Centaurea maculosa Lam.) decrease as desirable species richness increases. We simultaneously tested the degree of niche differentiation among desired species. Experiments consisted of 7 seeding monocultures and combinations. Treatments were monocultures of each desired species (3 plots), all combinations of 2 desirable species (2 250 seeds·m⁻² per species; 3 plots), and 1 plot containing all 3 desirable species (1 500 seeds·m⁻² per species). Monocultures or mixtures were replicated 4 times by seeding each treatment with four background densities of spotted knapweed (1 250, 2 500, 3 750, and 7 500 seeds·m⁻²; 7 treatments × 4 background densities = 28 plots). Analysis included regression with the 7 desired species monocultures or mixtures as a fixed effect and spotted knapweed sowing density as a continuous effect. All desired species established had either low or no negative influence on their neighbor, and differed in niche after 7 years of growing in association. Increasing richness of desired species led to increased productivity. Spotted knapweed density and biomass were low across all monocultures and mixtures at the productive site because shade and litter of desired species reduced light availability to the rosette-forming invasive weed. Combining crested wheatgrass and alfalfa provided lower spotted knapweed density and biomass more than did monocultures or grass mixtures because these 2 species appeared to occupy complementary niches. Increased niche occupation by nonnative desirable species may increase resource use and productivity, thus minimizing establishment and dominance of unwanted invasive plants during rehabilitation on arid, marginally productive rangeland sites.