Sunn hemp (Crotalaria juncea) has the potential to become an important green manure and cover crop in the South because it produces significant quantities of biomass and nitrogen in a short period of time. For successful incorporation into conservation management systems producers need a tool to predict potential biomass and N₂ fixation at different periods during the summer. We evaluated the influence of planting date (April to July) and harvesting date (30, 60, 90, and 120 days after planting (DAP)) on biomass production and N content at Watkinsville and Tifton, Georgia and related this information to DAP, cumulative degree days (CDD), and cumulative solar radiation (CSR). Predictions of biomass and plant N were similar for the three methods of time measurement. CDD was the best predictor of biomass (R² = 0.85) while the best prediction of N was with CSR (R² = 0.69). Biomass production at locations in Virginia and Alabama were simulated reasonably well with the three equations; however N content of the biomass was not closely simulated. Based on our results, these equations should provide producers a good tool for estimating potential biomass production from sunn hemp in the South.