About Each Cover Crop

Cover Crops are one of the best things you can do for your soil, but how do you choose which one is best for your unique situation? Here’s some information about the most common cover crops to help get you started.

Grasses for Winter

(in order of my favorites)

Winter Rye (Cereal Rye) – Easy to grow. Creates a thick cover. One of the best all-around cover crops. An excellent nitrogen scavenger, soil builder, weed suppressor, and erosion thwarter. Scavenges up to 100% of the nitrogen that might otherwise leach out of the soil. Germinates well even later in the season, just plant at a higher rate as it gets colder. Provides volumes of mulch and compost material if allowed to grow flower stalks in the spring. Kill by tilling in early, mowing and smothering at any stage, or simply by mowing at pollen drop (right around average last frost in our area).

Wheat – Easy to grow. Creates a thick cover. Possibly the second-best all-around grass cover crop. A very good nitrogen scavenger, soil builder, weed suppressor, and erosion thwarter. Can be planted later and still germinate well, just needs to be planted at a higher rate the later and colder it gets. Provides volumes of mulch and compost material if allowed to grow flower stalks in the spring. Kill by tilling in early, mowing and smothering at any stage, or simply by mowing at soft-dough stage (but I’ve killed it by mowing at pollen drop with rye).

Annual Ryegrass – Easy to grow. Creates a thick cover. A very good nitrogen scavenger, soil builder, weed suppressor, and erosion thwarter. Provides little or no mulch and compost material even if let mature in the spring. Kill by tilling in early or mowing and smothering at any stage, but, as an annual, will also die naturally after setting and dropping its seed.

Oats – Easy to grow. Creates a thick cover. An excellent weed suppressor, a very good scavenger of leachable nitrogen and erosion thwarter, and a good soil builder. Provides a small amount of mulch and compost material. Winter kills in zones 6 or 7 and below (even in the mountains of North Carolina, that’s been late November or early December), so possibly a good choice in winter-killed mixes in untilled beds. However, even though it is a thick cover in the fall, its cover decreases all winter long as the vegetation slowly degrades, leading to the need for additional mulch to keep the soil from seeing the effects of compaction and erosion from rainfall. To counter this, I use winter rye and just smother or mow it, depending on when I need to plant each area of the bed.

Barley – Easy to grow. Creates a thick cover. An excellent erosion thwarter, a very good soil builder, weed suppressor, and scavenger of leachable nitrogen. Does better in hotter weather than most other winter cover crop grasses. More akin to oats in that it winter kills in zone 6 or 7 and below, so may also be good in a winter-killed mix in untilled beds. However, I haven’t grown it because rye is winter hardy for my zone and does significantly better at most things (and I only plant wheat with the seed from the straw) and is easy enough to smother early for cool-season crops and kill by mowing later for warm-season crops. If it doesn’t winter kill in your zone, kill by tilling in early, mowing and smothering at any stage, or simply by mowing at mid- to late-bloom stage.

A Grass and a “Grass” for Summer

Buckwheat – Easy to grow. Can create a thick cover. An excellent weed suppressor, and a good soil builder. Grows and matures quickly, and is an excellent short-window (4-8 week) summer cover crop that quickly fills temporary empty spots in the garden. If planted thickly enough, creates a thick cover that chokes out summer weeds but also breaks down quickly once dead. Loosens the top soil and makes phosphorus and potassium more available to other plants. Flowers attract numerous beneficial insects, dramatically increasing their numbers in the garden while in flower. The most beneficial insects we see in the garden, by far, are during buckwheat flowering. Kill by weeding (weeds very easily), hoeing, tilling, or exposure to frost at any stage, or by mowing after 25% bloom or approximately 7-10 days after flowering starts. Sets seed quickly—so mow after flowering but before seed matures (hardens and turns brown) if you don’t want it to sprout up again in that area—but, again, young sprouts are easy to weed or hoe. Used culinarily like a grain, but not a true grain/grass.

Sorghum-sudangrass – Easy to grow. An excellent soil builder, erosion thwarter, and nitrogen scavenger, and a very good weed suppressor. An excellent summer cover crop that’s renowned for its biomass production, providing three or four cuttings of material for composting or mulch. Grows quickly and chokes out weeds. Great combined with cow peas and buckwheat. Mow when plants reach 3-4′ (~1m) to encourage deeper, fibrous root systems that greatly loosen and improve the soil. Leave 4-6″/10-15 cm of stalks (i.e., mow at 4-6″/10-15 cm height) to aid regeneration of tillers. Stems and leaves look like thinner versions of corn plants, and stems break down similarly slowly but not as slowly as corn stalks. Best composted after cut—and provides wonderful volume to compost if a little slower breakdown—but also works well left on the field as a decaying, soil-building mulch. Can mow to ground level and plant winter cover crop over in weeks before fall frost. Will resprout some, but will kill with frost. Can kill by tilling or smothering at any time, but is easiest to let frost kill in the fall.

Legumes for All Seasons

Legumes grow nodules in their roots that house the rhizobia bacteria that fix inert atmospheric nitrogen (N2) into plant usable forms. In some species, like those below, they fix a significant amount of plant usable nitrogen each growing season, making these legumes a wonderful natural source of nitrogen.

For all of the below species (and possibly some above), kill after flowering and before seeds form to avoid creating a weed problem or introducing an exotic species to your area.

Alfalfa – An excellent weed suppressor, erosion thwarter, nitrogen scavenger, soil builder, and source of fixed nitrogen. A great, perennial summer cover crop, producing volumes of high-quality green compost material over several summer cuttings. Works well as a soil builder using a three-year rotation on a fraction (e.g., ⅓, ¼, ⅕…⅛, etc.) of the garden, with a ⅓ of this fraction’s area seeded on new ground and the oldest ⅓ tilled in and returned to vegetable production each year. Persistence (its ability to come back year after year) can be an issue and is tied to dormancy, so find varieties with the ideal fall dormancy for your zone. Reseeding poor stands does not work because alfalfa is autotoxic, meaning it produces secondary metabolites that inhibit the growth of its own seedlings. In the compost bin, we typically layer a few inches of freshly cut alfalfa between the dry leaves and other “brown” compost material we set aside the previous fall and add a little soil to each set of layers to add microbes. In this way, growing alfalfa on 1/4–1/3 of the garden has given us massive volumes of high-quality cured compost, revolutionizing our ability to “grow our own soil fertility” right in our garden.

Berseem Clover – An excellent weed suppressor and source of nitrogen and a faster grower, and a very good soil builder, erosion thwarter, and nitrogen scavenger. One of the ideal legumes for winter-killed mixes in zone 7 and below. Also, shows promise as a summer cover, maturing quickly (60 days) and producing large volumes of nitrogen and compost material over several cuttings. Mowing, even at ideal stages, only kills a percentage of the stand. Kill by smothering, tilling, or winter-killing.

Cowpeas – An excellent weed suppressor, erosion thwarter, and source of nitrogen, and a good soil builder. A very good summer cover crop paired with sorghum-sudangrass and buckwheat. Kills with frost.

Crimson Clover – A very good weed suppressor, erosion thwarter, soil builder, and source of nitrogen, and a good nitrogen scavenger. Good paired with annual ryegrass. Will winter kill in zones 5 and below. Kill after flowering and before seeds form to avoid creating a weed problem or introducing an exotic species to your area. Supposed to kill if mowed after flower buds form, but does not in my experience. Instead, grows back vigorously after mowing and needs to be laboriously weeded or tilled. Therefore, is probably best in areas that are tilled or smothered in beds so doesn’t become a weed problem.

Field Peas – An excellent source of nitrogen, a very good erosion thwarter, and a good soil builder and weed suppressor. One variety, Austrian winter peas, is supposed to be hardy to 10º F (-10º C), but, even in zone 7, I haven’t had much success getting it established well before it winter kills. Kill by mowing after full bloom.

Hairy Vetch – An excellent source of nitrogen (one of the best), a very good soil builder, and a good weed suppressor and erosion thwarter. Produces lots of biomass (one of top legumes). Pairs well with rye or wheat because of similar hardiness and its climbing habit on their stalks. Hardy to zone 4. Kill by mowing after flowering but before it sets seed, smothering, or crimping/rolling and leaving the residue to smother remaining plants.

Red Clover – A very good soil builder, weed suppressor, and source of nitrogen, and a good nitrogen scavenger and erosion thwarter. Can be difficult to establish quickly, but does well in a wide variety of soils. Available in both multi-cut and single-cut cultivars. Hardy to zone 4. Kill by tilling, smothering, or naturally after their second year in most areas (but can become a short-lived perennial in the northern U.S. and Canada).

White Clover – An excellent source of nitrogen, a very good weed suppressor and erosion thwarter, and a good soil builder. Can be a little slow to establish, but once it is, can be a very good weed suppressor. Has three different main cultivar types that get less able to withstand traffic as they increase in height. Allegedly great for under-seeding corn and cucurbits, but since it can be difficult to establish quickly, I’ve had somewhat limited success with this cover crop in this method. However, because of shorter cultivars matting nature and ability to withstand moderate traffic, is one of the best choices for a living mulch on paths and between rows of vegetables, berries, or fruit trees. However, it can be difficult to pull or hoe weeds that emerge within its dense mat. Perennial to zone 4. Kill by tilling or smothering.

Woolypod Vetch – An excellent soil builder, weed suppressor, and source of nitrogen, and a good nitrogen scavenger and erosion thwarter. More of a west coast cover crop that works well in orchards and vineyards but could work as a winter-killed cover in zone 5 (and possibly zone 6) and below or an early spring cover in most of the rest of the U.S. Grows faster than hairy vetch with a bit less of a climbing habit. Hardy to zone 7. Kill by tilling, smothering, or letting winter kill.

Medics & Sweetclovers – Because already a weed problem throughout much of the United States, would not recommend growing.

Brassicas

Brassicas grow quickly, produce a lot of biomass, scavenge nutrients, and release compounds that help manage pests, bacteria, fungi, and weeds. However, unless planted in late summer or fairly early fall, they can be difficult to get established well enough to gain their benefits before they’re winter-killed by four or five days of temperatures below 25º F (-5º C). Winter-killing in zone 6-7 and below and decaying relatively quickly over the winter, they leave the soil clear in the spring, which makes for easy planting but mostly eliminates their use as erosion control once they decay. As such, they might be best grown as a late summer and fall cover crop—at the same time one grows other brassicas (e.g., broccoli, cabbage, and cauliflower) for a fall harvest—and then heavily mulched for the winter with leaves, straw, etc. after they start dying back in the late fall or early winter.

Forage Radishes – An excellent weed suppressor and nitrogen scavenger, and a good soil builder. Also known as “tillage radishes” or “biodrills” for their ability to break up heavy soil.

Mustards – A very good weed suppressor and soil builder and a good nitrogen scavenger. Contain high amounts of pest-, bacteria-, and fungi-fighting compounds.

Rapeseed – A very good weed suppressor and nitrogen scavenger and a good soil builder. Has anti-parasitic nematode and weed-suppression properties.

Other Sources of Info

The following resources are great for expanding your cover crop horizons.

1) Cornell University has a wonderful online cover crop guide—covercrop.org. Since it’s specifically for New York, it will be inaccurate for some areas, but it’s simple and an excellent place to start. It will help you decide which crops are best for your soil, time of year for planting, and goals. It has complete planting instructions, including seeding rates, and suggestions for avoiding introducing invasive plants.

2) As part of their “farmer-driven, grassroots grants and education programs,” Sustainable Agriculture Research and Education (SARE) has a wonderful, 240-page cover crop guide for all regions of the United States. You can buy a print copy for $19.00 or download the PDF.

3) Your county extension office also likely has plenty of information on the best cover crops for your soil and area. Give your extension agent a call or send him or her an email asking about cover crops specific to your area and needs.