Most gardening experts are familiar with the concept of feeding plants by giving them nutrients in the form of compost, manure, or synthetic fertilizer. But the importance of feeding soil is less obvious to many people. In order for plants to thrive with as few outside inputs as possible, they need a healthy living soil full of nitrogen-fixing bacteria, mycorrhizal fungus, earthworms, and plenty of organic matter. The most effective way to achieve this is by planting a diversity of cover crops.
Aside from creating a living soil, cover crops are also excellent weed suppressors. Bare soil acts like an open wound in your garden. In nature, it’s immediately covered up with weedy annual plants to prevent the loss of soil, nutrients, and soil organisms. In order to simulate this ecological succession without introducing unwanted weeds to our gardens, we use cover crops. As I will outline below, each cover crop provides its own host of benefits, but generally they all help to hold the soil in place, add nutrients and organic matter, suppress weeds, and provide habitat for soil microbes and beneficial insects.
Consider where this cover crop is being planted, the effects it will have on the soil, and what season you will be planting in. These factors will vary between each of the following cover crops.
Legumes Example crops: Peas, beans, clover, hairy vetch Season: Early spring
Legumes provide the weed suppression and soil building benefits of other cover crops, but they also have a special trick up their sleeves—they can add nitrogen to the soil by taking it in from the atmosphere in a process called nitrogen fixation. To give credit where credit is due, the legume isn’t fixing the nitrogen itself, but instead plays host to bacteria called rhizobia that do the work in exchange for housing. The legumes form small nodules on their root hairs that act as homes for the bacteria as they take nitrogen gas from the air and convert it into a form of nitrogen that can be taken up by the plant’s roots. When it comes time to plant your vegetables and the legume cover crop is terminated, it will decompose and release most of that nitrogen back into the soil where other vegetables can access it.
One important note when using legumes like peas and beans as a cover crop is that in order to maximize the nitrogen that they are adding to the soil, make sure to kill them before they form mature seeds. Otherwise, much of the nitrogen that they generated will be tied up in proteins in their seeds. In other words, you can’t have your nitrogen and eat it too.
Tillage Radish Example Crop: Daikon radish Season: Late summer and early fall
If you’re gardening in heavy clay soils, especially if you’re trying to reduce mechanical tilling or other soil disturbance, tillage radish might be the cover crop for you. The most popular tillage radishes are daikon radishes, the Korean variety notably found in kimchi. In this case they aren’t being used for their crunch and mild mustard flavor but for their long, vigorous taproot that can break up compacted soils.
A tillage radish cover crop works best when planted in late summer or early fall, so that the root has a chance to grow while the weather is still warm. In winter, hard freezes will eventually kill the radish, leaving the long roots to slowly decompose and leave behind channels up to 8 inches deep in the soil for water, nutrients, and the roots of your vegetable crops to easily infiltrate. With their long taproots, tillage radishes are also nutrient “scavengers,” leaving behind nitrogen and other essentials pulled from deep in the soil to benefit the next crop’s growth.
Buckwheat Season: Summer
Buckwheat is an excellent summer cover crop. It’s great for that transition period between a late spring crop and a fall crop when there isn’t enough time to plant summer vegetables.
Buckwheat establishes itself quickly in warm weather to outcompete aggressive summer weeds and keep them from taking over your garden when it’s hardest to drag yourself outside to pull weeds in the scorching heat. The flowers of buckwheat provide a great nectar source for pollinators and beneficial insects that provide crucial support for your favorite summer vegetables elsewhere in the garden. Buckwheat also releases substances from its roots that help free up naturally occurring phosphorus in the soil for your next crop to utilize.
Grasses Example Crops: Annual ryegrass, cereal rye, oats, wheat Season: Winter
The dense root system of grassy cover crops is an outstanding tool for preventing soil erosion and nutrient runoff. Some grasses even have allelopathic abilities, meaning they release compounds that restrict the growth of other weedy plants. This can be effective at preventing weeds from establishing themselves in your garden beds in the offseason, but make sure to give the grass time to decompose after terminating if you are following it with a crop that might be susceptible to the allelopathy.
Many grasses are cold-hardy, so they are typically planted in the fall and grown throughout the winter and early spring when they are killed and incorporated into the soil. Be careful when selecting a grass to use as a cover crop because you do not want to introduce a typical lawn grass or invasive grass to your garden. Choose a grass variety that is meant to be killed after one season like annual ryegrass, cereal rye, oats or wheat. As with the legumes, be sure to terminate your grass cover crop before it forms mature seeds in the spring.
How to Manage Cover Crops For the above-mentioned cover crops, the most popular method of planting is to simply toss them evenly across the soil surface and then gently rake them in to cover with soil. The exception to this would be the larger seeded legumes like beans and peas which benefit from being tossed in a furrow and covered with around a half inch of soil.
Once the cover crops are grown out and you are ready to plant your vegetables, it’s time for the transition. For legumes, grasses and buckwheat, the ideal time to terminate them is during the flowering stage. At this point the plant is putting a lot of its energy into the flower buds so it is vulnerable and easier to kill but hasn’t yet produced mature seeds. In the case of the tillage radish, the best method is to plant the radish in the fall and allow the frost to kill it over the winter. If your winter does not get cold enough to kill this radish, you can use a black plastic tarp to suffocate it over a period of 2-3 weeks.
On our homestead the preferred method for terminating cover crops is to chop them down with a weedwhacker or lawn mower and then cover them with a tarp for 1 to 2 weeks to prevent them from regrowing. After terminating our cover crops, we leave the chopped up leaves and stems on top of the soil to act as a combination of a green mulch and a slow-release fertilizer. We then plant our vegetable crops directly into the fresh green mulch. This method allows us to leave the soil undisturbed and keeps the roots of our cover crops in the ground to break down and feed the soil organisms. While they may not be as glamorous as the bountiful fruits and vegetables of the garden, the humble cover crop has a lot to contribute to your soil health and therefore your dinner plate.