The Great Garden Cover Up
Written onFebruary 09 , 2016
Cover crops and green manures may be terms you usually associate with farming, but they’re important for even a small-scale home gardener. “Cover crops” usually refers to grains or legumes grown in fall to “cover” the soil in winter. They’re usually tilled or turned under in spring. Green manures are plants grown during the growing season and turned under in summer or fall to enrich soil.
Why Cover Crops?
Some of the reasons to grow a cover crop or green manure are obvious and some a little more obscure. Cover crops hold the soil in place, preventing erosion. (This can be particularly important if you garden on a slope—and we have plenty of those in Vermont.) Cover crops also help build soil fertility in a number of ways:
- When tilled under, these crops add organic matter to soils, improving water drainage in clay soils and water retention in sandy soils.
- plants can use. Cover crops can also help break up clay soils. On our Addison County clay soil, we’ve noticed changes in the workability of our soil just from a few winters of cover cropping with winter rye.
- Some cover crops and green manures, such as buckwheat, can literarily choke out weeds with their lush, thick growth. Others, such as winter rye, exude chemicals from their roots that inhibit weed growth.
- Cover crops can also attract beneficial insects to your garden. I’ll never forget seeing a community garden plot cover cropped with buckwheat one summer. When that buckwheat was in bloom is was loaded with bees, wasps, and other beneficial insects.
- Finally, you can eat your cover crops, too! Not all cover crops have to be grasses or unusual legumes. I’ve used extra fava bean seeds as a cover crop, enjoying the beans in summer and letting the taproots break up the clay soil. Red clover is our state flower and has a tasty bloom that’s great in salads. Even winter wheat shoots can be harvested when young in fall and made into wheat grass juice.
Planning for Cover Crops
Growing cover crops is one of those good gardening chores that we all know we should do, but that often gets overlooked. One of the biggest challenges for the average gardener is where to grow them. Here are three ways and places to grow cover crops in your garden.
Rotate beds. If you have enough beds or room in your garden, consider rotating a green manure planting into different beds each year. Letting a bed grow a green manure will help rejuvenate it for next year. If you plant fast-growing green manures such as buckwheat, you can even plant in spring, turn it under in late summer, and still have time for an early fall planting of kale or other greens.
Succession plant green manures. Another way to integrate green manures into your growing beds is to succession plant. Succession planting is when you plant two to three crops in a bed in one season. For example, plant arugula in spring; after harvesting, plant bush beans in early summer; and once the bush beans finish in late summer, plant a fall kale crop. You can apply the same technique with a green manure crop: After your lettuce has stopped growing in one bed, consider planting annual rye grass or buckwheat as a summer green manure instead of planting another vegetable. Even small patches of green manures growing in part of a bed will benefit the garden.
Cover crops in fall. The most common way gardeners use cover crops is to plant them in fall after most vegetables and flowers have passed. Winter rye, winter wheat, oats, field peas, and hairy vetch are some of the most popular fall cover crops. These cover crops fall into two categories in Vermont: cover crops that survive the winter (winter rye, hairy vetch) and have to be turned or tilled under in spring, and crops that will grow well in fall, but die in winter (oats, field peas, summer alfalfa). This second group can be tilled or turned under in spring—or, because the tops have died, you can just lightly fluff the soil in spring and plant right into the bed. The cover crops that naturally die off in winter are particularly good choices for gardeners who are practicing no-till gardening.
Planting the Cover Crop
As with any seed crop, you need to properly prepare the seed bed before planting. Till or lightly loosen the soil, removing old plants and roots. Broadcast the cover crop or green manure seed at the recommended rate and cover with soil. On sandy sites, cover the seed with a deeper layer of soil than on clay soils. Keep well watered if Mother Nature doesn’t do it for you. Sow most cover crops in fall, ideally 30 to 40 days before a normal hard-frost date. The earlier you can plant, the better your crop will cover the soil in fall.
Best Cover Crops for Vermont
While many plants can be used as cover crops, they generally fall into two categories: grains and legumes. The grains hold the soil and produce an abundance of organic matter, while the legumes help the grains grow by fixing nitrogen. If you’re growing cover crops or green manures, look for mixes with a little of both. If you’re just looking to smother weeds or get the most organic matter possible in summer, grow all one type, such as buckwheat or annual ryegrass.
Here are some of the best cover crops and green manures to grow in Vermont. Check local garden centers or mail-order catalogs for seed.
Winter rye/winter wheat—These grains grow 4 to 5 feet tall at maturity and grow on low fertility and acidic soils. They are hardy to -30 °F, so good choices for Vermont. Winter rye can be sown as late as early November in Vermont and still germinate and grow. Turn these grains under as soon as possible in spring before they reach maturity or they will turn into a thick grass patch that will have to be mowed before being tilled. Sow 2 to 3 pounds per 1,000 square feet.
Hairy vetch—This cold-tolerant legume survives to -15 °F, so it usually makes it through a Vermont winter. It’s best paired with winter rye for that reason. It grows in poor soil, but accumulates phosphorous from the soil so is a good choice in soils naturally high in that nutrient. Sow 1 to 2 pounds per 1,000 square feet.
Oats—This grain is only hardy to 10 °F and doesn’t produce as much organic matter as wheat or rye. But it does tolerate wet soils. Because it dies off in winter it’s a good choice for no till gardeners. Sow 2 pounds per 1,000 square feet.
Field peas—Peas love growing in cool weather and are a good choice for a fall crop paired with oats. They not only fix nitrogen for plants but use the oat stalks to grow on. They are annuals. Sow 2 to 4 pounds per 1,000 square feet.
Annual ryegrass—This annual form of perennial ryegrass looks like a lawn when growing. It’s fast growing and hardy to -20 °F so it may survive the winter. However, it will flower and die in spring so you won’t have to till it under to kill it. It’s particularly good at absorbing and holding nitrogen in the soil so it’s a good cover crop to grow following a legume. Sow 1/2 to 2 pounds per 1,000 square feet.
Buckwheat—This is the best crop to grow as a green manure in summer, to add organic matter and to smother weeds in your garden. It’s fast growing, going from seed to a 3- to 4- foot tall plant in flower in roughly 40 days. You can sow two to three crops per summer. Cut down the stalks at flowering and use them as mulch around plants or till them under to directly add organic matter. Wait one to two weeks and sow another crop. Sow 3 pounds per 1,000 square feet.
Last, it should be said that cover crops will not only provide all the benefits I mention above; they can also save you money over time. Instead of annually buying yards of compost or bags of fertilizer, you can let cover crops and green manures provide your plants with some of the vital nutrients they need.