Warm Season Cover Crops

by Harold Schrock | Aug 1, 2022 | 0 comments

Sunn hemp flowers
Sunn hemp. Photo © Dreamstime.com.

One of the most important rules of gardening is to always be gardening. That doesn’t mean we need to keep ourselves in the garden every working day planting, weeding, fertilizing, or harvesting. What it does mean is to have something growing as much of the year as possible. God designed soil and plants to work together. Soil cannot remain healthy without plants any more than plants can remain healthy without soil. In fact, the possibility exists of growing a plant hydroponically without soil, but I’m not aware the possibility exists to maintain the productivity of soil without plants.

Any plant is better than no plant for soil function (this includes weeds), but some plants have notable characteristics that make them valuable for improving soil health. Let’s look at some of the individual species and the characteristics they bring to the garden.

The biggest drivers of soil carbon accumulation and biomass production are warm-season grasses such as corn, sorghum-sudangrass, sudangrass, and millet. These grasses have a metabolic system, known in scientific terms as C4, that allows them to produce more sugars with less water and at higher temperatures than do most plants. With the exception of corn and some species of millet, these do not directly produce food for people. However, in my experience, they are still worth using in spite of that fact. I’ve seen that between a quarter and a third of any garden space can be used for summertime cover-cropping without loss of food crops being produced, when evaluated over several years of time.

Corn, whether sweet, field, or popcorn, can be counted toward the C4 component of our cover crop mixture, but there is a lot of benefit in using the sudans and millet as well. Corn has a relatively coarse root system that, while providing a lot of sugar for soil microorganisms, is not known for creating soil tilth and breaking up compaction to the degree that the smaller grasses do. Harvesting the corn ear also removes some nutrition from the garden mineral profile.

Sunflowers are another warm-season crop that should be regularly grown for the soil benefit, besides the beauty and food they provide. Sunflowers develop a massive root system, as well as good above-ground biomass. There are many different sunflower varieties, and I recommend planting several in any summer cover crop mix for diversity and beauty.

Next let’s examine some warm-season legumes. Legumes have a unique interaction with nitrogen-fixing bacteria that allows them to harvest nitrogen from the atmosphere and fix it into soil. For this reason and others, they are very valuable components of our cropping mix. It’s interesting to dig up a legume and gently shake or wash the soil from the roots and examine the nitrogen-fixing nodules. These will appear as little balls attached to the roots, ranging in size from almost microscopic to the size of a large pea or even larger. I like to take a sharp knife and cut some of these nodules in half. If they are functionally fixing nitrogen, the interior will be pink.

When planting legumes, it is a good idea to apply an inoculant to the seed before planting. Without inoculation, you may not have enough of the specific bacteria in the soil to create good nitrogen fixation. Once you use any species of legume several years in a row, inoculation becomes less important, as some of the bacteria associated with that plant will likely survive from year to year. Commercial inoculants are commonly available in a powder form to mix with the seed, or, in the case of soybeans, some brands are available in liquid form.

Soybeans are the most common and widely grown summer legume. Unfortunately, most soybeans in North America are genetically modified to tolerate certain herbicides. That trait is not needed or desired for backyard cover crop use, so I recommend purchasing soybean seed at an organic supplier or from an organic farm to avoid the genetic engineering. Typically, farm-harvested organic soybeans will work just fine for cover crop use. You don’t have to purchase the more expensive certified seed. Soybeans have a specific inoculant that is recommended to use on the seed.

Cowpeas are a common legume grown throughout the South as both cover and food, but they can be grown for cover crop use in northern regions as well. They tend to be more viny than most soybeans and are well-suited for mixing in with corn and sudangrass, as they grow for the sun and don’t shade out as quickly.

Sunn hemp is an interesting legume that deserves a place in many cover crop mixtures. It has an upright stalk that develops pretty yellow flowers a couple months after planting. Sunn hemp seems to enjoy being mixed with other plants and grows better as a companion than planted by itself. Those not familiar with the plant would likely not pick out sunn hemp as a legume, but it has excellent nitrogen-fixing capability. The largest nitrogen-fixing nodules I have ever seen were growing on sunn hemp roots. Both sunn hemp and cowpeas are related to peanuts and use inoculant labeled for peanuts.

Speaking of peanuts, they are a less common garden crop but are also legumes and noted for improving soil. I have not personally tried them in a cover crop mixture. I suspect they would grow well since their cousins discussed above are excellent for companion mixes. However, harvesting peanuts from among the roots of sudangrass or sunflowers might prove challenging.

Clovers are not known for warm-season production, but Berseem clover is an annual clover that can go into warm-season mixtures.

There are several more non-legume plant species that deserve mention for cover crop use. Linseed or flax was once widely grown throughout North America. Its commercial use has been largely concentrated in dryer regions, but as a cover crop it can be grown almost anywhere. Linseed is known for the soil-conditioning benefit of its roots.

Phacelia is another interesting species that works well in cover crop mixtures. Phacelia blooms are noted for supporting pollinators and help add diversity to the soil microbial profile.

Most cover crop mixtures should also contain a brassica. Daikon radish is often used for this purpose. These are known as “tillage” radishes due to their large size and deep tap roots. These radishes are edible, although, like most radishes, they tend to get woody when very large. Other brassicas are also good companion plants in a mixture and have food value as well. While turnips and kale reach their best flavor in cool weather, the partial shade of a cover crop mixture can improve their quality.

No discussion of summer cover crops is complete without buckwheat. Buckwheat can go into mixtures or be used as a stand-alone, short-term cover. Buckwheat can outgrow and out-shade most other species, so, when used in a mixture, the seed percentage should be kept low. Buckwheat is the fastest-establishing cover and only needs thirty to forty days to be worth planting. It’s also fairly easy to terminate because it doesn’t have a massive root system, yet it makes nutrients available to the next crop.

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