Hairy vetch (Vicia villosa). To look at this spindly legume, you'd never guess that it's the Mercedes of cover crops for most parts of the United States. Hairy vetch not only has the widest range of adaptability, but offers consistently high rates of nitrogen fixation and biomass.

There are other cover crops you can consider though. Common vetch, bigflower vetch, crimson clover, subterranean clover, arrowleaf clover, austrian winter pea, and berseem clover are all possibilities.