Cover crops play a vital role in crop rotations on organic farms along California’s central coast, influencing soil quality, nutrient cycling, and pest and disease management. Many organic farmers in this region plant cover crops in each field every 1 to 3 years. Common cover crops include monocultures of cereals or mustards, as well as mixtures of cereals and legumes. To maximize the benefits of cover crops, organic farmers require more information about how seeding rates, planting arrangements, and mix compositions impact cover crop biomass production and weed suppression.

This research focused on how seeding rate, planting arrangement, and mix composition affect cover crop biomass production and weed suppression during the winter cover cropping period on organic cool-season vegetable farms in central California. The studies were conducted on two commercial organic farms and the USDA-ARS certified organic research farm from October 2003 to April 2005, encompassing five trials.

Trials 1 to 3 examined the growth of rye and a legume-oat mixture planted at three different seeding rates, using both one-way and grid planting arrangements. Weed suppression improved with higher seeding rates. While biomass production was greater early in the season at higher seeding rates, final biomass production was unaffected by the seeding rate. The planting arrangement did not influence the growth of the legume-oat mixture, although rye biomass was higher and weed biomass lower in the grid-planted rye cover crop during one of the years.

Trials 4 and 5 assessed the growth of several monoculture cover crops and 17 mixtures of legumes, oats, rye, and mustard. Generally, cover crop biomass production was higher in non-legume monocultures compared to legume monocultures, and weed biomass was typically lower in non-legume monocultures. Biomass production and weed suppression for the mixtures varied significantly between years and locations. In the first year, weed biomass followed the order: legume-oat > legume-rye > legume-mustard at both sites, but this pattern did not hold in the second year. Increasing the non-legume proportion in a mixture usually enhanced weed suppression but also reduced the growth of the legume component. Further research is needed to better understand the complex competitive dynamics in cover crop mixtures, aiming to create mixes that effectively scavenge nitrogen, suppress weeds, and promote adequate growth and nitrogen fixation by the legume component.

The results from these trials were shared with farmers and the public through field days and presentations. The insights gained from this research will help farmers optimize the advantages of cover crops and lower weed management costs in subsequent vegetable crops.