Grass pea is a cool-season legume commonly grown for human consumption and livestock feed in Asia and East Africa. A better understanding of the agronomic importance of planting date and the influence of photoperiod may lead to improved management strategies for cultivation of grass pea in the southern Great Plains. Experimental plots (3 by 20 m) were disked and fertilized with 60 kg ha-1 P2O5 during late February, and no N fertilizer was applied. Seeds of the cultivar AC-Greenfix were treated with a commercial liquid inoculum (Rhizobium leguminosarum) and planted at 25 to 30 kg ha-1 on March 15 (D1), March 30 (D2) and April 15 (D3) of 2004, 2005 and 2006. Effects of planting dates and photoperiod on number of days to flowering, physiological maturity and biomass production were determined. Average growing degree-days (GDD) to reach flowering were 779 (53 days), 761 (45 days) and 762 (40 days) and GDD to reach physiological maturity were 2067 (108 days), 2247 (102 days) and 2033 (91 days) respectively for planting dates D1, D2 and D3. Average biomass production at maturity across years and sampling dates was greater for D1 (2806 (±267) kg ha-1), compared to D2, and D3 (2557 (± 289) and 2120 (± 243) kg ha-1, respectively). Maximum biomass production across years and planting dates (5140 (±434) kg ha-1) was observed on the sixth sampling date (first week of July). Delaying seeding till D2 and D3 reduced biomass yield by 16 kg ha-1 day-1 and 29 kg ha-1 day-1, respectively, compared to D1. These results suggest that early planting of grass pea provides a longer growth period, resulting in greater biomass yield.