Long‐term research on cover crops (CC) is needed to design optimal rotations. Winter CC shoot dry matter (DM) of rye (Secale cerealeL.), legume–rye, and mustard was determined in December to February or March during the first 8 yr of the Salinas Organic Cropping Systems trial focused on high‐value crops in Salinas, CA. By seed weight, legume–rye included 10% rye, 35% faba (Vicia fabaL.), 25% pea (Pisum sativumL.), and 15% each of common vetch (V. sativaL.) and purple vetch (V. benghalensisL.); mustard included 61% Sinapis albaL. and 39% Brassica junceaCzern. Cover crops were fall‐planted at 1x and 3x seeding rates (SR); 1x SR were 90 (rye), 11 (mustard), and 140 (legume–rye) kg ha−1. Vegetables followed CC annually. Cover crop densities ranged from 131 to 854 plants m−2and varied by CC, SR, and year. Year, CC, and SR affected DM production, however, the effects varied across the season and interactions occurred. Averaged across years, final DM was greater in rye and legume–rye (7 Mg ha−1) than mustard (5.6 Mg ha−1), and increased with SR through January. Dry matter production through the season was correlated significantly with growing degree days (GDD). Legumes contributed 27% of final legume–rye DM. Season‐end legume DM was negatively correlated with GDD at 30 d, and legume DM in the 3x SR increased during years with frequent late‐season rainfall. Seed costs per Mg of final CC DM at 1x SR were approximately three times higher for legume–rye than rye and mustard.