AbstractBasic management practices, including ideal seeding rates, are still lacking for black oat (Avena strigosa Schreb.) in the southeastern United States. This study evaluated the performance of five seeding rates (15, 30, 60, 120, and 240 lb acre−1) on ‘Legend 567’ oat (Avena sativa L.) and ‘UF‐10’ black oat at three harvest dates (early, mid‐season, and late) per year. Seeding rates of 60, 120, and 240 lb acre−1 tended (P = .07) to increase total herbage accumulation [4,760 lb dry matter (DM) acre−1] compared with 15 lb acre−1 (3,945 lb DM acre−1). Tiller density was usually greater for black oat than for oat. Seeding rate was positively associated with tiller density and explained 84 to 98% and 96 to 98% of the variation for black oat and oat, respectively. Tiller mass was greater for oat (0.045 oz tiller−1) than black oat (0.034 oz tiller−1), and greater for the mid‐season harvest (0.043 oz tiller−1), compared with the early and late harvests (0.037 oz tiller−1 on average). There was a negative effect (P < .001) of seeding rate on tiller mass. Increasing the seeding rate had a negative effect on leaf length at the early and middle harvests (P < .05), and decreased leaf width from 0.56 to 0.39 inches in oat, and from 0.32 to 0.29 inches in black oat when changing from 15 to 240 lb acre−1. To maximize herbage accumulation at a lower seeding rate, we recommend 60 lb acre−1 for both oat and black oat for multiple harvests.