Looking for a summer cover crop mix? Plant your own summer legume cover crops or other plants for weed suppression and a boost to soil organic matter.
Don’t sow your winter cover crops too early! Oats, barley, wheat and rye sown too early can head up and seed before you get to winter, making them less useful. Instead, sow fast-growing summer cover crops in any space possible, for weed suppression and a boost to soil organic matter. Keep live roots in the ground as much of the time as possible, to feed the microorganisms and anchor the soil, preventing erosion in heavy rains. Dead roots also have a role, providing drainage channels in the soil and letting air in deeper. Adding organic matter to the soil is a way of banking carbon, as well as providing nutrients for your crops.
Deep-rooted cover crops draw up nutrients, bringing them up where crop plants can access them. Leguminous cover crops provide nitrogen, saving imports of organic fertilizers or a big compost-making operation.
Advantages of Summer Cover Crops
Suppressing weeds. Weeds grow fast in summer, and fast-growing summer cover crops will suppress them. Sowing cover crops helps us stay on top of developing problems.
Growing biomass. Many summer cover crops can be mowed or scythed down (before flowering) to encourage regrowth. The cut biomass can be left in place, or raked out and used as mulch in another part of the garden. Some can even be used as feed or bedding for small livestock.
Feed the soil life. Cover crops are solar-power generators, transforming sunlight, water and carbon dioxide into leaves and roots. They also release carbohydrates and other nutrients that feed soil microbes, earthworms and other soil life forms that make soil fertile. This cycle of nutrients constantly passes through plants and back into the soil. When you aren’t growing vegetable crops, cover crops keep this cycle going.
Increasing biodiversity. Cover crops can attract beneficial insects, birds and amphibians to feed and reproduce. Biodiversity encourages ecological balance that can help reduce plant diseases and pest attacks.
Overcoming the Challenges of Summer Cover Crops
Finding time for the work. Hopefully you have a lot of harvesting to do at this time of year. You are probably also hoeing whenever you can find ten minutes. Give a thought to sowing some cover crops, which will reduce future weeding time and improve your soil. Buckwheat can be in and under in a month. Think of the time you’ll save in weeding.
Finding space. If you’ve been carefully filling every space with vegetables, you may think you have no room for cover crops, but because they feed the soil, it’s worth making space for them too. It’s part of the wholistic picture of sustainable food production. It’s worth making a priority to have one bed or one section of your garden in cover crops, because of what they can do for your soil.
Have a goal of No Bare Soil. Seek out odd spaces to fill with cover crops. Use space beside rows of sprawly crops short-term, until the vines start to run.
Take a cold, hard look at aging crops: better than keeping an old row of beans to pick every last bean, is to pull up those beans and sow a quick cover crop. It will be a more valuable use of the space.
Take a look at your planting plan. When is your next crop going in that space? Rather than till the soil to death, use a cover crop to manage the weeds. In the winter, see if you can re-arrange your crop rotation and planting plan to make more time windows of a month or more, with the plan of using more cover crops.
Undersow growing crops with a cover crop when the vegetable crop has been in the ground for about a month. The food crop will be big enough to resist competition from the cover crop, and the cover crop will still get enough light to grow. This way fewer weeds grow, and your cover crop is already in place when the food crop is finished, giving it longer to reach a good size. We undersow our sweet corn with soybeans (soy and oats for the last sweet corn planting). You can plant a short cover crop on the sides of a bed of any tall crop like tomatoes or pole beans. You will need to provide extra water, especially while the cover crop is germinating. Read my post on undersowing in late summer and early fall.
You can also undersow winter cover crops during late summer and early fall to last over the winter and even the next year. We broadcast clover among our fall broccoli and cabbage with the plan of keeping it growing for the whole of the following year, mowing once a month to stop annual weeds seeding.
High temperatures. Most summer cover crop seeds will germinate just fine at high temperatures provided they get enough water.
Drought. If it doesn’t rain much in your summers, or your irrigation water is limited, choose cover crops that are drought-tolerant once germinated. After sowing, work the seed into the soil and roll or tamp the soil so that the seed is in good contact with the soil, which will help it get the water it needs rather than drying out in an air pocket. To get good germination, keep the soil surface visibly damp. You can use shadecloth, a light open straw or hay mulch, or even cardboard to reduce evaporation. Be sure to check every day and remove the cardboard as soon as you see the first seedlings.
Sowing small spaces. You can sow cover crops in rows by hand in very small spaces, or use an EarthWay seeder. Or you can broadcast: tuck a small bucket of seeds in one arm, take a handful of seeds and throw them up in front of yourself in a fanning movement, trying not to spread seed into neighboring beds where you don’t want them. Aim for about two seeds/sq in (1 sq inch is 6.5 sq cm, I’ll leave you to think what it looks like). Don’t sweat the details, you will get better with practice! This isn’t brain surgery! For more even coverage, try broadcasting half the seed walking up and down the length of the patch, then sow the other half walking at 90 degrees to your original direction. Rake the seeds in, trying to cover most of the seeds with 0.5-1″ (1/2.5 cm) of soil. Water with a hose wand or sprinkler to keep the soil damp until germination.
Summer Cover Crops that Die with the Frost
Buckwheat is the fastest and easiest cover crop, a broadleaf annual that can be flowering within three weeks in very warm weather, 4 weeks in regular warm weather. Because it grows so fast, it quickly crowds out germinating weeds. Plant buckwheat after all spring frosts have passed, up to 35 days before the fall frost. If you have longer than 4 weeks for cover crops, you have the option of letting the buckwheat seed and regrow (only do this if you’ve finished growing vegetables for that year in that space). Another option if you are not close to the frost date is to incorporate the buckwheat in the soil and then sow fresh seed.
Buckwheat is very easy to incorporate into the soil. Use a mower or scythe to cut it down, and then either let the dead plants die into a surface mulch and plant into that, or rake it up and compost it, or dig or till it into the soil. For small areas, buckwheat can simply be pulled up by hand – this is what we do in our hoophouse.
Buckwheat can be used as a nurse crop for fall-planted, cold-tolerant crops, which can be difficult to germinate in hot weather. Sow a combination of buckwheat and a winter vegetable to shade and cool the soil. When frost kills the buckwheat, the vegetable crop can continue growing with no competition.
Sorghum-Sudangrass. One of our favorite summer cover crops is sorghum-sudangrass, a hybrid that grow 5 to 12 feet (1.5-3.6 m) tall and produces an impressive amount of biomass. You’ll need big machinery, at least a big BCS mower, to deal with sorghum-sudangrass. If you have only hand tools and a lawnmower, I recommend growing one of the millets instead.
Plant sorghum-sudangrass about two weeks after your first sweet corn planting date and anytime onward until six weeks before frost. After it’s established, sorghum-sudangrass is highly drought-resistant and thrives in summer heat. Plant in rows 8 inches (20 cm) apart, with seeds 1 inch (2.5 cm) deep, 1.5 inches (4 cm) apart. Sorghum-sudangrass will smother weed competition, and make big improvements to the soil texture and the levels of organic matter.
When the sorghum-sudangrass reaches 4 feet (1.2 m) tall, cut it down to 1 foot (30 cm) to encourage regrowth and more, deeper, roots growth that will loosen compacted soil. The cut tops make a good mulch, or you can leave them in place.
Sorghum-sudangrass roots exude allelopathic compounds that suppress damaging nematodes and inhibit small seeds (weeds and crops) from germinating and inhibits the growth of tomatoes, lettuce, and broccoli. Wait at least 6 weeks after killing sorghum-sudangrass before planting another crop in the same spot. Plant earlier at your own risk — I think we’ve had some success despite the warnings. Be careful if feeding to livestock. Read up about prussic acid poisoning from this cover crop.
Soybeans. These are a quick easy leguminous cover crop for warm weather. Buy organic seed if you don’t want GMOs, as almost all non-Organic soybeans in the US are GMOs. We plant these whenever we have a minimum of six weeks for them to grow before frost or before we’ll need to turn them under. They aren’t the highest N-producing legume, but they are very fast-growing.
Southern peas. Also known as cowpeas, although I have heard this might be perceived as insulting by African-American families who use them as food. Southern peas grow fast, thrive in heat, and are very drought-tolerant. Their taproots can reach almost 8 feet (2.4 m) deep. They grow well in almost any soil, except highly alkaline ones. Southern peas attract beneficial insects.
Sow southern peas 1 to 2 weeks after your sweet corn, when the soil has warmed up. You can continue sowing until 9 weeks before a killing fall frost. Sow seeds 2 inches (5 cm) apart, 1 inch (2.5 cm) deep in rows 6 inches (15 cm) apart (give vining types more like 15 inches (40 cm) between rows. Close planting is needed to shade out weeds.
Because they are fast-growing, southern peas can follow spring vegetable crops and fix nitrogen in time to feed heavy-feeding, fall-planted onions or garlic.
Sunn hemp is a nitrogen-fixing legume from the tropics, which can grow as much as 9 feet (2 m) tall in just weeks. Sow sunn hemp from 1 to 2 weeks after your sweet corn sowing date, up to 9 weeks before a killing frost. It tolerates a wide range of soils (but not if waterlogged), and dies with the frost. Plant inoculated seed (use the same inoculant as for southern peas) 1 inch (2.5 cm) deep, with seeds 1.5 inches (4 cm) apart in the row, and with rows 6 inches (15 cm) apart. Sowing densely will smother the weeds.
If you sow in a summer gap between spring and fall vegetable crops, it will provide a nitrogen boost for the fall crop. In dense plantings, it can fix more than 120 pounds (54 kilograms) of nitrogen and 12 pounds of biomass per 100 square feet (0.56 kilograms per square meter). 60 days after sowing, the stems thicken and become fibrous and high in cellulose; cutting at this stage produces long-lasting mulches that increase soil carbon. If you cut the crop back at a younger stage, this will stimulate branching (more biomass) and more root penetration (better drainage).
Late Summer Cover Crops for Winter: Oats and Barley
In late summer you can sow oats for a winter cover crop that will be killed at 6 degrees F (-14 C). We sow in late August and early September in Zone 7. Inexpensive and easy to grow, oats are a standard fall cover crop: a quick-growing, non-spreading grass, oats will reliably die in Hardiness Zone 6 and colder, and nine years out of ten in Zone 7.
Barley grows even faster than oats, and on average it will get killed later in the winter. It usually dies at 17 degrees F, making barley another choice for gardeners in regions where oats are used.
Cover Crop Resources
For more about using cover crops at other times of year, see my post All About Growing Cover Crops
My book Sustainable Market Farming has a chapter on cover crops and 9 pages of charts about particular options.
The book Managing Cover Crops Profitably (third edition) from the Sustainable Agriculture Research & Education Program (SARE), is the best book I know on the subject. You buy the book for $19 or download it as a free PDF from SARE.
Pam Dawling has worked at Twin Oaks Community in central Virginia for more than 27 years, growing vegetables for 100 people on 3.5 acres and training many members in sustainable vegetable production. She is the author of Sustainable Market Farming and The Year-Round Hoophouse. Pam often presents workshops at MOTHER EARTH NEWS FAIRs and at sustainable agriculture conferences.
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