Tips for spring cover crop planting

Author: Adria Fernandez, Researcher, Grossman Lab

As you make your field plans for this year, consider complementing your later-planted vegetable crops
with an early spring cover crop. In fields where you plan to grow a vegetable with a summer planting
schedule, like transplanted broccoli or cauliflower for fall harvest, or even a late succession of beans or
sweet corn, you may have time to grow a cover crop to build and protect your soil until the vegetable is
ready to go into the field. 

Cover crops can serve several purposes in a short spring growing window:

• They can suppress weeds and keep the soil covered in the field while you’re waiting for transplanting time
• They can take advantage of the soil moisture and sunlight available in April and May to build organic matter that will become part of your soil reserves when the cover crop is incorporated.
• If the cover is a legume (like a pea or clover), it can also contribute to long-term nitrogen fertility by fixing atmospheric nitrogen that will be gradually released as the cover crop breaks down.

We all know how quickly an “extra” like cover cropping can get crowded off the to-do list when the
fieldwork rush hits, so now is the perfect time to make plans and order seeds so you can be prepared to
get your cover crop into the ground as soon as conditions are right. To get good biomass production
early in the season, you’ll need to plant as soon the soil is dry enough to be worked without causing
compaction.  

Species that establish well in cool soil include field pea, red clover, crimson clover, annual
ryegrass, and small grains (spring wheat, barley, or oats).

These plots of crimson clover are blooming in early May at the University of Missouri Extension Bradford Research Center. Image from SARE cover crop image library

A few tips to help a spring cover crop work for you:

1. Stand establishment is key to realizing cover crop benefits. Many of the farmers we talk to find that it’s helpful to use seeding rates at the top of the seed supplier’s recommended range, or even higher. Make sure that your cover crop seed has good contact with the soil, and plenty of moisture during emergence. Larger-seeded cover crops in particular tend to establish better with drill planting, but you can also broadcast seed your covers. If you’re broadcast seeding, prepare a clean, soft seedbed, and rake or harrow after broadcasting to improve soil-seed contact. If your field was tilled in the fall or has winter-killed residues on it, you may be able to drill-seed directly into the residues without tilling first. However, it’s important not to let weeds get a head start on your cover crop, so if perennial or winter annual weeds are already starting to green up in your field, you should till them down before seeding the cover.
2. Avoid nitrogen immobilization. Nitrogen immobilization happens when soil microbes, in the process of consuming a carbon-rich input, take up available soil nitrogen and make it temporarily unavailable for plant use. This tends to happen when the C:N ratio of the input material is over about 24:1. Legumes tend to have C:N ratios around 12:1, while the C:N ratios of grasses can range much higher, so one way to minimize the risk of your transplanted crop being affected by nitrogen immobilization is to choose a legume species for your spring cover crop. You can also consider giving your transplants a jump-start by applying a little bit of supplemental fertility (such as fish emulsion) in the transplant water.
3. Don’t wait too long to terminate. It’s generally recommended that you terminate your cover crop at least 10-15 days before you plan to transplant, to allow the cover crop residues time to start breaking down in the soil. This gives soil microbes time to start re-releasing the nutrients that the cover crop took up from the soil, and also reduces the chance that coarse residues will interfere with the transplant’s ability to explore the soil with its roots.

Author: Adria Fernandez, Research scientist, Grossman lab