The invasive grass Microstegium vimineum (Trin.) A. Camus grows in dense monocultures beneath the canopy of woodlands in eastern North America and along sunny forest edges. This annual matures seeds in chasmogamous (CH) spikelets on terminal racemes and cleistogamous (CL) spikelets on axillary racemes. The objectives were to determine whether density-dependent reductions in growth and reproduction occur, what patterns they show across densities in a sunny vs. shady environment, and whether the intensity of intraspecific competition depends on light environment. Seedlings of M. vimineum were grown at densities corresponding to 105, 526, 947, and 1368 individuals m⁻² in 11 cm diameter pots in a greenhouse exposed to full sun or under deep shade (5.8% of full sun). Mature, partially senesced target plants were harvested prior to seed drop. Shoot dry mass, CH and CL spikelet, and seed production, were significantly reduced under shade and at all densities relative to the control (no competition). However, in shade there were no density-dependent effects on reproduction via CL or CH. There was also no density-dependent mortality. Relative competition intensity, based on the density-dependent reduction in shoot mass of target plants increased with increasing mass of competitors and was greatest in the sunny environment. Reproductive mass (i.e., dry mass of CH or CL spikelets and seeds per target plant) was positively correlated with shoot mass across density treatments, suggesting that density-dependent effects on reproduction in competing monocultures were indirectly due to decreased shoot mass at higher densities. Density-yield curves were highest in the sunny environment where they quickly reached saturation, revealing that a solitary individual can produce the same biomass as a group of competing individuals at higher densities. The success of this invasive species in woodlands may be due to high tolerance to a range of intraspecific densities and to an ability to set seed under shady conditions even when densities are high.