The relevance of diversity—invasibility studies to the conservation of biodiversity has been questioned on grounds that species-rich assemblages may not deter invasion by competitively superior invaders. Few studies have compared the competitive effects of invaders on native species individually vs. in mixture. Using field experiments, we measured competition between an invasive grass, Microstegium vimineum, and six species of large native groundcover plants. We first examined whether planting six native species was more effective than planting an equivalent number of a single species in competitively suppressing Microstegium. Using a split-plot design, naturally occurring patches of Microstegium were treated with one of the two planting treatments or a control. We monitored Microstegium emergence and percent cover through 2015 and into the spring of 2016. We then tested the competitive effect of Microstegium on plantings of native species within Microstegium patches using a removal experiment in 2016. Although initial transplant survival was high in spring 2015, subsequent mortality during the growing season was also high in both planting treatments and, thus, there was no evidence of competitive suppression of Microstegium. In contrast, removal of Microstegium benefited the growth (and flowering) of native transplants in 2016, and all native species were more or less equally affected. These results suggest that neither high native diversity nor the presence of certain native species is likely to be effective in constraining the abundance of invaders demonstrated to have strong competitive effects on most or all native species under conditions that favor both native diversity and the invader.