Native cover crops suppress exotic annuals and favor native perennials in a greenhouse competition experiment

In a greenhouse experiment, we examined the effectiveness of four native cover crops for controlling four exotic, invasive species and increasing success of four western North American grassland species. Planting the annual cover crops, annual ragweed (Ambrosia artemisiifolia) and common sunflower (Helianthus annuus), reduced the biomass of the exotic species cheatgrass (Bromus tectorum), Japanese brome (Bromus japonicus), Canada thistle (Cirsium arvense), and whitetop (Cardaria draba). The annual cover crops also reduced the desired species biomass in competition with the perennial exotics, but either increased or did not affect the desired species biomass in competition with the annual exotics. Planting the perennial cover crops, Canada goldenrod (Solidago canadensis) and littleleaf pussytoes (Antennaria microphylla), rarely inhibited exotic species, but did increase the desired species biomass. Field experiments are needed to test the cover crops under more ecologically relevant conditions, but our results suggested that the annual cover crops may be effective for controlling invasive annuals and for facilitating native perennials.