Author: Nick Andrews, Small Farms Program, Oregon State University

Publish Date: Summer 2014

Lots of vegetable crops (i.e. winter squash, some brassicas, tomatoes, peppers and eggplants, etc.) are harvested late.  This makes it difficult to establish over-wintering cover crops after harvest. On many farms, these fields are left bare over the winter. Sometimes there are opportunities to establish cover crops by relay seeding (inter-seeding) them into established vegetable crops. This allows the farmer to protect soil from compaction, surface runoff or erosion, build organic matter, and provide cover crop nitrogen (N) to the following year’s crop.

Cover crops can be relay seeded when the vegetable crop is established, and most early-season weed control has been completed, but before the canopy of the vegetable crop has closed, preventing light from reaching the soil.

In collaboration with the West Multnomah Soil and Water Conservation District (http://www.wmswcd.org/) we have tried relay seeded and post-harvest cover crops with several growers. This article shares some lessons we are learning. A couple caveats though.

    • We don’t have much experience relay seeding into drip irrigated vegetables. If the drip irrigation can provide enough moisture to germinate the cover crop, this may work. If overhead irrigation is available especially during the few weeks after relay seeding, cover crop establishment would probably be more even. Monitor cover crop growth to make sure it isn’t suffering from drought stress. See comments below about cover crop establishment.
        • Plastic mulches are popular for weed control, and increased crop performance, especially in Solanaceous crops like tomatoes. Clearly, the plastic mulch makes it impossible to establish cover crops in the bed. There is some discussion about establishing cover crops in the alleys between mulched beds. I haven’t seen this in practice, but it does seem feasible, and these cover crop strips may provide some benefit compared to fields left completely bare over the winter.

          Overhead irrigated vegetables grown without plastic mulch provide opportunities to relay seed cover crops. To choose the best species, consider your cover cropping objectives (see table 1). Are you trying to grow your own N for your next crop by using legume covers? Are you trying to take up residual plant-available N (PAN) before it is leached by winter rains? Cereals or grasses are better at this than legumes.  Are you trying to provide flowers in the spring for pollinators and other beneficial insects? Legumes and other flowering broadleaf covers can do this if allowed to flower the following spring.

          Timing of seeding is critical with relay seeded cover crops. If seeded too early, weeds or excessive cover crop growth may compete with the vegetable crop. If seeded too late, the cover crop may not establish under the vegetable canopy. Our trial plots have been on commercial farms, so we typically relay seed when the farmer is doing their last weed cultivation with tractors. In our experience, this is sometimes a little too late, and shade from the vegetable crop canopy can prevent the cover crop from establishing well. We were initially concerned that vining cover crops like common vetch might grow into the vegetable canopy and interfere with harvest. In practice, vetch got established, but didn’t grow large enough to be a problem. Cereals grow more quickly, and while we did not have any major issues with cereals interrupting harvest, this seemed more likely to be a potential challenge.

          Table 1. Functions provided by popular winter hardy cover crops in the maritime PNW. Adapted from information provided by, The Xerces Society for Invertebrate Conservation (http://www.xerces.org/).

          Cover Crop Lifecycle N-Fixer Beneficial insects N scavenger
          Alfalfa Perennial x x  
          Clover, crimson Annual x x  
          Clover, red Perennial x x  
          Fava bean Annual x x  
          Vetch, chickling Annual x x  
          Vetch, common Annual x x  
          Vetch, hairy Annual x x  
          Buckwheat Annual   x x
          Mustard, tame Annual   x x
          Phacelia tanacetifolia Annual   x x
          Radish Biennial   x x
          Turnip Biennial   x x
          Annual ryegrass Annual     x
          Cereal rye Annual     x
          Winter oats Annual     x
          Winter wheat Annual     x
          Triticale Annual     x