Winter cover crops (CC) can improve nutrient use efficiency by scavenging residual soil N. Shoot nitrogen accumulation (NA) of rye (Secale cereale L.), legume–rye, and mustard was determined in December to February or March during the first 8 yr of the Salinas Organic Cropping Systems (SOCS) trial focused on high‐value crops in Salinas, CA. By seed weight, legume–rye included 10% rye, 35% faba bean (Vicia faba L.), 25% pea (Pisum sativum L.), 15% common vetch (V. sativa L.), and 15% purple vetch (V. benghalensis L.); mustard included 61% Sinapis alba L., and 39% Brassica juncea Czern. 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. Early‐season NA was greatest in mustard. Nitrogen accumulation increased more gradually through the season in legume–rye than in other CC. Final NA (kg ha−1) was lower in rye (110) and mustard (114), than legume–rye (151), and varied by year. During December, SR increased NA in legume–rye by 41% but not for the other CC. Legumes contributed 36% of final NA in legume–rye, presumably from N scavenging and biological fixation. Nitrogen accumulation was highly correlated with shoot dry matter of legume–rye but not of rye or mustard. Seed costs per kg of NA were more than two times higher for legume–rye than rye and mustard. We conclude that high SR are necessary to hasten early season NA and minimize N leaching potential in legume–rye mixtures.