Technical Abstract: In the Mid-Atlantic region, forage radish (Raphanus sativus L. var. longipinnatus) winter cover crops planted prior to 1 September suppress winter annual weeds from fall until early April. Little is known about the mechanism of this weed suppression. Published research reports suggest that allelopathy and/or resource competition play a role in weed suppression by other Brassica cover crops. Controlled environment bioassays involving cover crop amended soil, aqueous plant extracts, and aqueous soil extracts along with a field experiment involving planted weed seeds did not provide evidence of allelopathy. Rather, forage radish amended soils in experiments involving soil bioassays and aqueous extracts of amended soil often stimulated germination and growth of test species. In residue moving experiments, few differences in spring weed suppression were observed if forage radish residues were removed prior to killing frost in November or were left in place to decompose in three of four site-years. These results were supported by planting date experiments in which fall ground cover and spring weed suppression was greatest for earlier planting dates of forage radish cover crops. Thus, rapid and competitive fall growth, rather than allelopathy, is the most likely mechanism of weed suppression by forage radish winter cover crops.