Cover Crops for April: before the last frost.

 

Beds of young buckwheat.
Photo Bridget Aleshire

In January I shared some resources to give the Big Picture of Cover Crops, including a compilation of slides for SARE (Sustainable Agriculture Research and Education) and my slideshow Cover Crops for Vegetable Growers.

In February, I described limiting winter annual weeds by sowing oats in spaces without a cover crop and no planned food crop for 6-10 weeks. Six–ten weeks (depending on your climate) is long enough in early spring to get worthwhile growth from oats before prepping for the food crop. Also see February’s post for the Stale Seedbed and Tarping Techniques.

In March I wrote about some options for cover crops you might be sowing then, and alternatives like a fast-growing hardy leafy vegetable or mixed Eat-All Greens, an idea from Carol Deppe, a great idea if you have more than eight weeks before your main vegetable crop goes in the ground. This is where using transplants really helps increase your total food output. While the frost-tender transplants are growing indoors, you could be growing a “catch crop” outdoors in spaces that didn’t get a winter cover crop. I also talked in the March post about incorporating cover crops. Remember that if you incorporate fresh green cover crops into the soil, you will need to wait two or three weeks to sow, to give the cover crop time to break down in the soil before it can be available for your crop. Especially, wait three weeks after turning under winter rye before sowing, as it produces allelopathic compounds that can inhibit the germination and growth of small seeds. Transplants don’t suffer the same problem.

Potatoes, weeds and standing water. Until the soil drains, the potatoes cannot be hilled, and the weeds here are already large. The yield will be reduced by weeds out-competing the potatoes. Potatoes may be flamed at 6″–12″ (15–30 cm) tall, to kill weeds without damaging the potato plants. After that, flaming is not recommended.
Photo Bridget Aleshire

Once we get to April here, it is too late to successfully grow oats (they will quickly head up after making very little growth). But in climates like ours, we can sow winter wheat or winter rye in April – they will not head up, but will “wimp out” when it gets hot. That is, they will stop growing, so you won’t get a lot of biomass, but you will have some live roots in the soil, holding it together, taking care of the soil microfauna, and discouraging weeds form germinating. One April when our spring-planted potatoes got flooded, we transplanted potato plants to the drier end of the patch and sowed winter rye in the lower end, once the floods had subsided. This kept the soil covered, scavenged the compost we had spread for the potatoes, and was easy to deal with in July when we harvested the potatoes. It was also much more hopeful to look at an area of green cover crop than an area of green weeds!

April is too soon for us to rely on frost-tender cover crops, but by mid-April, we can sow a mix of oats and buckwheat. The oats will protect the buckwheat somewhat from the cold. if the season is warmer than average, the buckwheat will survive and smother weeds, provide pollen and nectar for beneficial insects

Here’s a lovely quote from Barbara Pleasant in SW Virginia:

“It’s April and the soil is warming up and drying out. After loosening a clump of fall-sown wheat with a digging fork, you pull up a marvelous mop of fibrous roots and shake out the soil. What crumb! The soil’s structure is nothing short of amazing! These are the moments an organic gardener lives for.”

Root systems of four grass cover crops at early stages of growth (two months in a greenhouse). From left: annual ryegrass, barley, triticale (winter biennials) and sorghum-sudangrass (summer annual). Photos by Joseph Amsili. From SARE

Depending on the stage of the year where you are, you could revisit any of the earlier posts. Here are links for each of the cover crop posts in the past year.

May: Buckwheat and Other Summer Cover Crops

June: Sunn Hemp, Soybeans, Southern Peas, and Partridge Pea, Senna Ligustrina

July: Millets and Sorghum-Sudangrass (Sudex)

August: Oats, Barley and Other Winter-Killed Cover Crops

September: Winter Wheat and Crimson Clover

October: Winter Wheat and Austrian Winter Peas

November: Winter Rye (with Austrian Winter Peas early in November)

December: Planning Winter Cover Crops

January: The Big Picture, Ponder and Plan Your Cover Crop Strategies for the Coming Year

February: Oats if you have a 6–10 Week Gap

March: Sowing Options and Incorporating Cover Crops

Perennial and Native Cover Crops

I attended a workshop at the VABF-SFOP Summit on cover crops led by Cerruti R2 Hooks, Veronica Yurchak, from the Department of Entomology, University of Maryland, and Hanna Kahl of UC Davis. The UMD Eastern Shore IPM Center has lots of useful programs and publications. They focus on the most important pest problems and make science-based information available to everyone who contends with pests. This workshop discussed how cover crops influence weeds, plant diseases and insects. Cover crops can smother weeds, augment weed seed predators (lifeforms that eat weed seeds), create a weed-suppressive soil microbe community, release allelochemicals that are toxic to weed seeds, release nitrogen into the crop germination zone, boosting crop growth, cool the soil and compete with weeds for resources.

Cover crops can decrease crop diseases by increasing the diversity of soil organisms, making soil more disease-suppressive; releasing compounds unfavorable to disease organisms; trigger plant immune responses; increase the number of beneficial organisms and forma physical barrier that reduces splash-back from the oil. A nice example is that sunn hemp interplanted in squash rows can cause aphids carrying virus particles in their mouth parts to drop them in the sunn hemp where they do no harm.

Sunn hemp at Nourishing Acres Farm.
Photo Pam Dawling

Cover crops can repel some insects and nematodes, as well as providing habitat, nutrients and protection from predators for the beneficial insects. This can help augment the population of beneficial insects. Cover crops can also act as trap crops for problem insects by being more attractive to them than the crop plants. Cover crops can also cause microclimate change within the crop, for example by acting as a windbreak.

The speaker gave examples with red clover, a short-lived perennial, sown in the previous fall between cucumber rows that were planted in spring. The population of striped cucumber beetles was lower, while populations of beneficials such as big-eyed bugs, minute pirate bugs and ladybugs were increased.

In November 2023, at the Carolina Farm Stewardship Conference, I went to an engaging workshop called On-Farm Cover Crops Research in the Carolinas by Justin Duncan from NCAT/ATTRA, Jason Lindsay from the Southeastern African American Farmers’ Organic Network, and Steve McAllan. See his YouTube  Cover Crops for Hot & Humid Regions. At the workshop, Justin Duncan explained Push-Pull Trap Cropping, invented in Kenya, combining a companion plant that repels a pest with a trap crop nearby that attracts it, making pest control easier.

Pigeon Pea as cover crop. Photo https://conservationist.wordpress.com/2008/11/03/pigeon-pea-as-cover-crop/

He advocated for pigeon peas (Cajanus Cajan) as a cover crop for warm droughty climates, that will also keep the soil cooler. When mean temperatures rise 1 Celsius degree, soils in warm areas burn up 10% of their OM, and cool areas lose 3%. Loss of water leads to loss of OM, leading to more water loss. Hot humid areas need twice as much OM as cooler ones to maintain fertility. No-till can cut the loss of OM by half compared to conventional tillage. Other cover crops Justin Duncan recommended include Perennial Peanut, good in orchards, Chamaecrista rotundifolia (round-leaved cassia) and Scarlet Runner beans. Cover crops are a way of growing Organic Matter in place.

Patrick Johnson, RVA Permaculture. Photo https://rvapermaculture.com/about-us/

Patrick Johnson, a Virginia permaculturist, also gave a presentation on native cover crops. See his Proposal and Project Overview:  https://projects.sare.org/sare_project/fs22-345/

And read the Feb 2024 SARE report Using a Native Legume as a Cover Crop for Soil and Vegetable Production Benefits in Small Scale Vegetable Production.

No-Till Cover Crops

I have not covered these yet, and don’t have much personal experience, apart from our one-year-in-ten growing of paste tomatoes in a mow-killed rye, hairy vetch and Austrian winter peas dying mulch. I’ll make a separate post for next week about combining cover crops and no-till methods.

Cover Crop Training Videos from SARE

See SARE for a series of ten training videos.

Weeds Next

For my next annual series of blogposts, starting at the beginning of May, I will cover Weeds of the Month.

Cover Crops for May: Buckwheat

Buckwheat in flower in September.
Photo Pam Dawling

This is the first of a monthly series on cover crops, which will take us through a whole year, to April 2024.

Why Grow Cover Crops?

<a title=”USDA, Public domain, via Wikimedia Commons” href=”https://commons.wikimedia.org/wiki/File:Soil_food_webUSDA.jpg”><img width=”512″ alt=”Soil food webUSDA” src=”https://upload.wikimedia.org/wikipedia/commons/thumb/8/89/Soil_food_webUSDA.jpg/512px-Soil_food_webUSDA.jpg”></a>

See my post Cover Crops in Summer.

  • Cover crops suppress weeds and provide a boost to soil organic matter.
  • Keep live roots in the ground as much of the time as possible, to feed the microorganisms in the Soil Food Web.
  • Here is a diagram of the Soil Food Web: https://upload.wikimedia.org/wikipedia/commons/8/89/Soil_food_webUSDA.jpg
  • Roots 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 sequestering 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.
  • Too often, gardeners grow cover crop only in the off-season, to protect the soil in winter, and assume it’s not a summer option. But it is, and planting summer cover crops provides many benefits.

Focus Cover Crop for May: Buckwheat

A bed of young buckwheat with a cosmos plant to attract beneficial insects. Photo Pam Dawling

I have a blog post Buckwheat, a wonderful summer cover crop, introducing an article I wrote for Growing for Market magazine. See that post for basic details I mostly won’t repeat here.

Buckwheat (Fagopyron esculentum) is a fast-growing warm-season broadleaf annual that is a very useful cover crop. Its special strengths are in weed-suppression, attracting beneficial insects, improving the soil tilth (aggregate structure) with its fibrous roots, and extracting potassium, calcium and phosphorus from the soil to the benefit of following crops. Buckwheat is almost three times as good as barley in scavenging phosphorus, and more than ten times better than rye (a poor phosphorus scavenger). Because buckwheat is not related to any of the common food crops, it is simple to include in rotations.

Buckwheat can be sown up from your last frost up to 35 days before first fall frost. Buckwheat can close its canopy in just two weeks, preventing the soil baking in summer conditions. Because it matures quickly, and self-sows, it can be used in several successions with tilling between, to suppress some perennial weeds.

Flowering buckwheat in September.
Photo Pam Dawling

Buckwheat can do fairly well on poor soils, is tolerant of a range of soil pH and is an easy crop to deal with manually or with small-scale equipment. Even mature buckwheat plants are easy to deal with using manual or small-scale equipment. You can just pull up the plants by hand, or use a hoe or scythe to slice them off at the soil line. You can chop them into the soil, or gather them up and compost them. Or you can use a no-till method, let the dead plants die into a surface mulch and plant through them.

Buckwheat yields only a couple of tons per acre, but does it in only six to eight weeks. If you want to increase the (admittedly sparse) biomass, cut down buckwheat just before it reaches 25% bloom, to a height above the lowest leaf node. Buckwheat will regrow rapidly and you may even be able to make a second cutting.

Buckwheat also makes good food for poultry or rabbits, and chickens love the seeds. It does not provide good forage for larger livestock.

Beds of young buckwheat.
Photo Bridget Aleshire

Barbara Pleasant, in her 2009 article Cover Crops, described Rhizodeposition as a special advantage of using cover crops. Many plants release sugars and other substances through their root hairs into the soil. They are solar-powered pumps, sending energy down into the soil, causing the root tips to host colonies of useful microorganisms. As the roots move deeper, the microbes follow. With vigorous winter cover crop plants, like oats or rye, this process goes on down to 6 feet (much more deeply than you should dig). Buckwheat doesn’t go very deeply at all, but it can be working for you, which is much better than leaving the soil empty and drying out.

Buckwheat in flower in June. Photo Pam Dawling

Buckwheat Resources

Secondary Cover Crops for May

Soy, mustard, sunn hemp, southern peas are all also good summer cover crops, and I will say more about them in the next few months. See Cover Crops in Summer.

Sunnhemp cover crop at Nourishing Acres Farm, NC.
Photo Pam Dawling

Sunn hemp, a nitrogen-fixing legume from the tropics, can grow as tall as 9’ (2.7 m) in a few months. Sow sunn hemp from a week after your sweet corn sowing date, up to 9 weeks before your first fall frost, which will kill it. It tolerates a wide range of soils (but not if waterlogged). Plant inoculated seed (use the same inoculant as for southern peas) 1” (2.5 cm) deep, with seeds 1.5” (4 cm) apart in the row, and with rows 6” (15 cm) apart. Sowing densely (as with all cover crops) will work better to smother the weeds.

If you sow sunn hemp in a summer gap between spring and fall vegetable crops, it will provide a nitrogen boost for the fall crop, because it is a legume. In dense plantings, it can fix more than 120 lbs (54 kg) of nitrogen and 12 pounds of biomass per 100 sq ft (0.56 kg/sq m). 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).

Sunn hemp cover crop at Nourishing Acres Farm, NC
Photo Pam Dawling

We have taken to sowing sunn hemp as a summer cover crop in our hoophouse, and lopping it periodically with hedge shears to an ergonomic elbow height. This is because we don’t want it to shade crop plants further back (north). The fallen tops make a nice “forest floor” carbonaceous mulch.

Mustard we don’t grow as cover crops, although I do have experience of growing it in England, where it is one of the favorite cover crops for short crop gaps, or in preparing areas reclaimed from pasture or lawn. We have too many harlequin bugs, and we hope to break their lifecycle by having a summer month without any visible brassicas. (We do often have fall brassica seedlings growing under insect netting.) Also, our “crop portfolio” has plenty of brassicas already, and we’d rather have a better rotation, with brassicas less often.

Mustards can decrease weeds, or certain pest nematodes, if you grow the right kind.

Soybeans as a cover crop
Photo agcrops.osu.edu

Soybeans are a great summer cover crop and they are also a legume, so they add nitrogen to the soil. They have good shade tolerance and tolerance to foot traffic (that is, people harvesting crops on either side. Because of this, we like using soy to undersow in sweet corn.

Southern peas are another warm weather cover crop option. They are also a legume, and so will add nitrogen to the soil. Iron and Clay is the sprawl variety best known for cover crop use, but other varieties also work.

Iron and Clay southern peas flowering in September. Photo Pam Dawling

Cover Crop Planning

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.

See Harvey Ussery  Four Outstanding Cover Crops for Summer.