Hairy vetch (Vicia villosa Roth) is a legume grown for high biomass and N fixation. Climate, population density, establishment date, and termination timing affect biomass production; the combined effect of these factors has not been documented. We conducted an experiment in Massachusetts, New York, Pennsylvania, Maryland, and North Carolina across a range of hairy vetch seeding rates and dates and termination timings to define biomass production potential and determine minimum seeding rates. Hairy vetch was planted at two dates at rates of 6 to 50 kg ha–1. The cover crop was terminated at early and intermediate vegetative and 50% flowering stages. Across the factorial of planting and termination dates, biomass increased 529 kg ha–1 on average for every 100 growing degree days (GDD) accumulated. Maximum biomass across treatments was 460 to 2815 kg ha–1 in Massachusetts, 23 to 4523 kg ha–1 in New York, 72 to 6570 kg ha–1 in Pennsylvania, 1106 to 7117 kg ha–1 in Maryland, and 4314 to 7759 kg ha–1 in North Carolina. In Massachusetts, New York, and Pennsylvania, hairy vetch produced maximum biomass at seeding rates of 15 to 20 kg ha–1, at the low end of the currently recommended rate of 18 to 22 kg ha–1. In Maryland and North Carolina, hairy vetch produced maximum biomass when seeded at 5 to 10 kg ha–1. Our results show significant variation in optimal seeding rates across a latitudinal gradient, and illustrate the importance of site-specific management.