Growing cover crops is one of the best methods we can employ to bring organic nutrients to our garden soil, yet it’s often not practiced by home gardeners. This is mostly because information on using cover crops is often tailored to farmers, who use tractors to mow down or turn under cover crops. But that doesn’t mean home gardeners should forgo this tried-and-true technique to build healthier soil and plants. When your main tools for taking down plants have wooden handles and you measure your space in feet rather than acres, you simply need a special set of cover crop plants and the right methods for using them.
A cover crop is any plant grown for the primary purpose of improving soil, as opposed to food or ornamental crops. Since the early 1900s, farmers have used cover crops to restore fertility to worn-out land. In addition to helping bulk up soil with organic matter, cover crops prevent erosion, suppress weeds, and create and cycle soil-borne nutrients using the power of the sun. Recent advances in soil biology have revealed two more ways cover crops can improve soil.
Rhizodeposition offers a special advantage to those working with cover crops. Many plants actually release sugars and other substances through their roots. They are like little solar engines, pumping energy down into the soil. With vigorous cover crop plants, this process goes on much more deeply than you would ever dig—six feet down for oats and rye. If you leave your garden beds bare in winter, you are missing the chance to use cold-hardy crops such as rye and oats to pump up your soil throughout the season, leading to healthier, more biodiverse soil the following spring. Thanks to this release of sugars, the root tips of many plants host colonies of helpful microorganisms, and as the roots move deeper, the microbes follow.
Bio-drilling is what happens when you use a cover crop’s natural talents to “drill” into compacted subsoil. For example, you might grow oilseed or daikon radishes as a cover crop where their spear-shaped roots will stab deep into tight subsoil. Bio-drilling action also takes place when deeply rooted cover crop plants penetrate subsoil and die. The next crop grown can then follow the rooting network mapped out by the cover crop. Maryland researchers were able to track this process using special camera equipment (called a minirhizotron), which took pictures of the interactions between cover crop (canola) and crop plant (soybean) roots. As the canola’s deep roots decomposed, soybean roots followed the trails they blazed in the subsoil. In addition to reduced physical resistance, the soybean roots probably enjoyed better nutrition and the good company of legions of soil-dwelling microcritters, all compliments of the cover crop.
Dozens of plants have special talents as cover crops. If you live in an extremely hot, cold, wet or dry climate, check with your local farm store or state extension service for plant recommendations—especially if you want to use cover crops under high-stress conditions. Also be aware that many cover crop plants can become weedy, so they should almost always be taken down before they set seed.
One of the only difficulties with cover crops comes when it’s time to take them down, which is why it’s a good idea to start small with your first cover crop plantings. On farms, cover crops are plowed under, but home gardeners must chop them into the soil or pull them out by hand (or assign the task to a flock of pecking poultry). In some ways, pulling plants and then adding them to the compost pile is the best method. Composting cover crop plants may produce a more balanced soil amendment compared with simply chopping the crop residue directly into the soil. You might also want to pull plants rather than chopping in to avoid possible negative reactions between rotting plant residues and the plants you plan to grow. For example, the cover crop known as sudex (a fast-growing sorghum-sudangrass hybrid) produces gargantuan amounts of biomass (leaf, stem and roots), but fresh sudex residue in the soil inhibits the growth of tomatoes, lettuce and broccoli. Oats, wheat and other cover crop plants also produce allelopathic substances that can temporarily hinder the germination and growth of other plants. However, chopping in plant material will also benefit your soil. If you chop in instead of pulling up the plants, simply plan to wait two to three weeks before sowing crop seeds, and any potential interaction problems will disappear.
The following cover crops work well in a wide range of climates and situations, and they’re not hard to take down, even in home gardens. We’ve selected these six because they are easy to manage using hand tools, grow during different seasons, and provide multiple benefits in the garden.
During summer, buckwheat is in a class by itself in terms of speed of growth and effectiveness at killing weeds. Buckwheat seeds sown in moist soil turn into a weed-choking sea of green within a week, with many plants growing 2 feet tall and blooming in less than 30 days. Should you need to reclaim space that has been overtaken by invasive species, buckwheat can be your best friend. In my garden, buckwheat has been a huge ally in cleaning up a spot overrun by dock, bindweed and other nasties that grow in warm weather. For two years, each time the noxious weeds grew back, I dug them out and planted more buckwheat. Throughout the battle, the buckwheat attracted bees and other pollinators in droves. Fortunately, even mature buckwheat plants are incredibly easy to take down—simply pull the succulent plants with a twist of the wrist, or use a hoe or scythe to slice them off at the soil line. You can let the dead plants die into a surface mulch and plant through them, gather them up and compost them, or chop them into the soil.
In late summer, while the soil is still warm, you have a fine opportunity to try barley, a fast-growing grain that’s great for capturing excess nitrogen left over from summer crops, which might otherwise leach away during the winter. Barley often suffers from winter injury in hardiness zone 6, and is often killed altogether in zone 5 and above. This is good! In these cold zones, the dead barley residue shelters the soil through winter and dries into a plant-through mulch in spring.
Early fall is the best time to grow the dynamic duo of soil-building cover crops—oats mixed with cold-hardy winter peas. When taken down just before the peas start blooming in spring, an oat/pea combination cover crop is the best way to boost soil’s organic matter and nutrient content using only plants. Both make a little fall growth when planted in September, and in spring the peas scramble up the oat stalks. On the downside, one or both crops can be winter-killed before they have a chance to do much good north of zone 5, and in more hospitable climates it will take some work to get the plants out of the way in spring. Take down plants by mid-April because the job gets tougher as the plants get older. Cut or mow them down first, and then pull and dig your way through the planting. A heavy-duty chopping hoe works well for this.
Hairy vetch needs a good head start on winter, too, but it’s hardy to zone 4 and gives a huge payback in terms of soil improvement, plus saved time and labor. Unlike many other cover crop plants, hairy vetch can be quickly killed by slicing just below the crown with a sharp hoe. When hairy vetch is beheaded about a month before it’s time to plant tomatoes and peppers, you can open up planting holes and plant through the dried mulch—no digging required.
Even late fall is not a lost season for cover crops. Cereal rye is the cold-hardiest of them all. Rye will sprout after soil has turned chilly, but be sure to take it out early in spring before the plants develop tough seed stalks. Or if you have chickens, let them keep it trimmed; leave the birds on the patch longer in spring and they will kill the rye for you.
In any season, you may find many more great cover crops in seed catalogs or among your leftover seeds. As you consider possibilities, think about plants that quickly produce an abundance of leaves and stems, but are easy to pull up or chop down. Bush beans, leafy greens and even sweet corn can be grown as short-term cover crops, along with annual flowers such as calendulas and borage in early spring, or marigolds and sunflowers in summer. Teaming up a flower with a cover crop is always fun, whether you’re planting sulphur cosmos with cowpeas in summer, oats with dwarf sunflowers in late summer, or bachelor’s buttons with crimson clover in the fall. Whatever you do, don’t leave your soil bare or you’ll miss out on the chance to use these amazing plants to recharge your food web.
This article originally appeared in our sister publication Mother Earth News as Use Cover Crops to Improve Soil.
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