Cover crops are tilled under in late winter or early spring.
In our garden, we see the soil level drop about 3 – 4” in each bed after the vegetable crops are harvested. This is a visible reminder that growing a food crop consumes considerable organic matter.
Cover crops, such as fall rye, crimson clover, buckwheat and others are easy to grow. When they are digested by soil microorganisms they restore organic matter and nutrient levels in the soil. Because they are sown thickly, they also help to outcompete weeds. Cover crops also control erosion from heavy winter rains, and help prevent the soil from compacting over winter.
Depending on your growing region, some cover crops will die during the coldest weather. The crop residue is still a valued supplement in the spring.
What cover crops should you plant?
Seed companies sell cover crop seeds as individual crops or as mixes of grasses and legumes. Common annual cover crops suitable for fall planting are:
These nitrogen-fixing crops provide a fertilizer as well as organic matter. Planted in fall, they grow slowly until late winter when growth speeds up. Legume crops may not mature until May in some regions. Cut down these cover crops in spring before they go to flower, then till them under.
Hairy vetch (Vicia villosa): Grows to 2 feet high; hardy to -15° F. Hairy vetch is considered the hardiest annual legume. Vetch tolerates poor soil, and matures late. Sow 1 to 2 pounds per 1,000 square feet.
Field pea (Pisum arvense and P. sativus): Grows 6 inches to several feet tall. Field peas are hardy to 10 to 20°F. ‘Austrian Winter’ pea is low growing and late maturing. ‘Magnus’ grows to 5 feet. Sow 2 to 4 pounds per 1,000 square feet.
Berseem clover (Trifolium alexandrinum): Grows 1 to 2 feet high. Berseem clover is hardy to 20° F. Produces high amounts of nitrogen. Sow 2 pounds per 1,000 square feet.
Crimson clover (T. incarnatum): Grows 18 inches high, and is hardy to 10° F. Crimson clover matures late and fixes less nitrogen than other clovers. Sow 1/2 to 2 pounds per 1,000 square feet. If allowed to go to seed, Crimson clover can become an invasive weed.
Cover crops from the grass family grow quickly, tolerate cold, and improve the structure of compacted soils. Thickly sown grasses add increased organic matter in comparison to legumes. Grasses also control erosion which is a real benefit in wet regions. Grasses do not have the benefit of fixing nitrogen as legumes do. Annual grass cover crops are cut down or mowed in spring before seeds set, and then tilled under.
Annual ryegrass (Lolium multiflorum): Grows 2 to 3 feet high; hardy to -20° F. Fast growing and tolerates flooding. The seeds are inexpensive, and the grass is very hardy. Ryegrass can become weedy. Sow 1/2 to 2 pounds per 1,000 square feet.
Winter rye (Secale cereale): Grows 4 to 5 feet tall; hardy to -30° F. Best grass for cold winter climates: tolerant of low fertility, acidic soils. Sow 2 to 3 pounds per 1,000 square feet.
Oats (Avena sativa): Grows 2 to 3 feet tall: hardy to 10 to 20° F. Produces the least organic matter of grasses, but is tolerant of wet soils. Oats usually succumb to winterkill, but the residue is still beneficial to the soil. Sow 2 pounds per 1,000 square feet.
Barley (Hordeum vulgare): Grows 2 to 3 feet tall; hardy 0 to 10° F. Fast maturing and tolerant of dry and saline soils: intolerant of acidic soil. Sow 2 to 3 pounds per 1,000 square feet.
A common mix offered by seed companies is hairy vetch and annual rye. The vetch fixes atmospheric nitrogen, while the rye uses leftover nitrogen. Nitrogen from the decomposing vetch will cause the rye to decompose more quickly and not tie up nitrogen as long.
To plant cover crops, sow seeds at least 30 days before the first expected fall frost date in your growing region. For cover crops that are only marginally hardy in your area, push back the sowing date to 60 days before the first frost. The more established a cover crop is before winter the more likely it will overwinter successfully.
Are there drawbacks to planting fall cover crops?
Grasses, such as fall rye, can attract the adult wireworm beetles to lay eggs. If you have wireworm problems you may want to reconsider planting fall grasses as cover crops.
Since cover crops shade the soil, they can keep the soil wet and cool in a late spring, which can lead to poor seed germination. We turn under cover crops early in these conditions.
Attention must be paid to the timing of digging in cover crops. If the plants get too mature, the stalks can get woody and be harder to cut. Cover crops are much easier to turn under while the leaves are still lush and green.
What if you have no time to plant a fall cover crop?
An alternative to planting fall cover crops is to add leaves, plentiful in autumn, in a thick layer over the soil. Leaves will block air-born or smother soil-born weeds during the winter months, and are easier to incorporate into the soil in the spring. Cover crops usually require more work to chop up and hoe into the soil, by comparison.
Using leaves instead of cover crops has one obvious drawback – they blow away in autumn winds and winter storms! When we use leaves to cover a garden bed, we cover the leaves with fishnet and weight down the corners of the fishnet with rocks.
How to ‘harvest’ your cover crop
Cover crops are mowed or cut in early spring, at least 3-4 weeks before the bed is planted with the intended crop. We use a garden shears to chop the cover crop to a few inches above the soil. Once the mass is bunched on the bed, it gets chopped again with the shears. The process takes only a few minutes per bed.
Turning under the cut cover crop requires more effort. We use a sharp hoe to chop into the stubble and to cut the root mass which surfaces during the hoeing. Some stubble and plant residue will still be visible, but this breaks down quickly.
Building healthy soil is an ongoing project for every gardener. Even where topsoil is organically rich, it will become depleted within a few crop rotations. Gardeners need to revitalize the soil in garden beds regularly. Even buying “new” topsoil to develop additional growing space is no guarantee that it will contain organic matter. You may need to ‘feed’ the soil to get it up to gardening standards. Cover crops are very useful in this regard.
One final observation about over-winter cover crops may seem trivial, but I find satisfaction in seeing something growing in the garden during winter. Even when cover crops succumb to harsh winter weather, the residue lying on the beds is a valued soil amendment. The same beds, left empty through the winter, would be gradually occupied by native weeds which need pulling in the spring.
This winter you can have your garden continue to work for you, and come spring you’ll feel you have a head start on the season, thanks to your over-wintered cover crops!