What Are Cover Crops?

Cover crops are crops like wheat, oats, rye, forage radishes, clovers, and vetches that are grown to blanket, or cover, and protect the soil when the soil would otherwise be bare during the garden’s offseason, between widely spaced food crops, between temporal gaps in food crops. As such, they are also an acknowledgment that there’s a better option than leaving our soil empty and bare for six-to-nine months of the year.

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Growing Cover Crops

The Reasons

Like mulch, cover crops, sown to cover bare soil in the garden, add significant amounts of protection for and organic matter to your soil. Unlike mulch, however, their benefits are always home-grown.

Aside from adding cured compost, a cover crop is probably the best way to add organic matter to your soil. Cut and then smothered and/or worked into the soil before any flowers or seeds form, the vegetation and roots decay, adding significant organic matter and nutrients. All of the macro- and micro-nutrients the cover crop has pulled into its tissues from the sun, air, and deep in the soil are left on the surface to decay into the soil or tilled into the top six inches of earth, leaving a boon for the plants that follow.

Cover crops, additionally, benefit the soil’s worms and microbes and act as weed and erosion control. Since they can’t endure the temperature extremes and variations found in bare soil during cold spells, many of our soil’s denizens—like earthworms—flee uncovered soil in the winter or perish. As their name implies, cover crops blanket the soil in the winter, their vegetation acting as an insulating layer, much like mulch, moderating the temperature and making it hospitable for earthworms and our other beneficial soil creatures. Our soil microbes additionally benefit from cover crops by feeding on decaying bits of vegetation and growing more bountiful because of the added organic matter. This blanketing layer of vegetation, furthermore, shields the soil from the erosive and compacting effects of rain, and its roots hold the deeper soil together, thus thwarting erosion. Similarly, cover crops compete with existing weeds and shade the soil, keeping most new weeds from sprouting.

For more on these benefits and more, click here.

The Methods

Just as it is with gardening in general, there’s too much variability in the uses of cover crops to prescribe one method. The tiered sections below, however, give starting points for gardeners at beginner, intermediate, and advanced stages of experience with cover crops. The final section offers resources that will help all gardeners expand and refine their use of cover crops.

Easy Starts

Many of us start using cover crops for the benefits they provide during our garden’s off-season—that time of year when it’s too cold or too hot for our main fruit and vegetable crops, typically from late summer or late fall through late winter or early spring for all but the southernmost states and the heart of the summer for the southernmost states. This gap in time between our main crops provides an excellent opportunity to add fertility and organic matter to our gardens through the use of cover crops during a time of year when our gardens might otherwise be bare. For the same reasons, however, cover crops can be used to fill mid-season gaps between fruit and vegetable crops, for example, between an early crop of greens and a fall crop of brassicas. Here are a couple of easy starting ideas for each.

Easy Off-Season Cover Crop for Tilled Ground

Annual ryegrass makes a great off-season cover crop for beginners who till their gardens in the spring. It works well for most areas, grows relatively quickly and easily, provides enough foliage for soil coverage and fibrous roots to thwart erosion, lasts all winter in most areas, mows and tills in easily in the spring, doesn’t tend to become a weed problem, and has a great mix of other general cover-crop benefits such as scavenging nitrogen, adding organic matter, and loosening soil. These attributes not only make it an excellent cover crop for beginners but also a mainstay for many long-time cover-croppers, since all of its benefits for beginners also make it very attractive for many veterans. If you’ve heard enough and just want to get started using this simple, excellent cover crop, skip to “Planting Instructions,” the “Planting Chart,” and “Killing the Cover Crop to Readying the Soil for Planting” below.

Easy Off-Season Cover Crop for Untilled Beds

If you plant in beds where tilling is impractical or undesirable, you’ll likely prefer a cover crop that winterkills in your planting zone. Winterkilling avoids the hassle of having to mow and turn in or smother the cover crop to kill it when it comes time to ready the bed for planting the next spring. For gardeners in planting zones 6 and under (and parts of zone 7), oats are an excellent cover crop for this purpose, providing some winter cover but breaking down enough over the winter that some quick mowing and raking will give a fairly clear planting bed. Depending on how much it has broken down over the winter, the raked-off residue may also provide some mulch for your planting beds. Note, however, that oats winterkill early in some colder zones that they may necessitate some additional winter mulch afterward to keep the soil fully protected. If you’ve heard enough and just want to get started using this simple, excellent cover crop for untilled beds, feel free to skip to “Planting Instructions” and the “Planting Chart” below to get started.

Filling Mid-Season Gaps Anywhere

For beginners looking to fill midseason gaps between garden crops, buckwheat is a simple, short-window cover crop with many benefits and is easy to manage on both tilled and untilled ground. Buckwheat grows so quickly that it only needs a six-to-eight-week window to complete its lifecycle, providing an excellent short-term, midseason cover between crops, and can be beneficial even if mowed or worked in earlier in its lifecycle. Furthermore, sown at the correct rate, it grows dense stands that outcompete most weeds, providing much-needed midseason weed suppression. Buckwheat also provides good soil building, especially considering it’s doing so on an area of the garden that would otherwise be left bare. Plus, its flowers provide a nectar source for beneficial insects, attracting them to the garden. One caveat: It is somewhat important that you mow it before its seeds mature to prevent it from becoming a weed problem. However, simply mow it 7-10 days after it starts flowering, before any seeds harden, and you should be fine. The mowed residue additionally provides a moderate amount of organic matter that can be added to the compost pile or used as mulch on garden beds.

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Intermediate Options

As you become ready to start delving deeper into cover crops, you have plenty of options. First, consider adding a few simple mixes to your cover crop palette. Second, consider increasing the types of cover crops you use and even experimenting with others. Field peas, cowpeas, hairy vetch, woollypod vetch, crimson clover, berseem clover, red clover, and white clover are great nitrogen-fixing legumes that are worth a try if they fit your zone and uses. Similarly, cereal rye (one of the powerhouses of nitrogen scavenging), annual ryegrass, oats, winter wheat, and sorghum-sudangrass hybrids are great grasses. Forage radishes, likewise, provide a great mix of benefits. All are worth a try, depending on your zone, desires, soil’s needs, and whether you till or don’t.

Finding cover crops that best meet your needs—for example, winterkill (e.g. berseem clover, oats, sorghum-sudangrass hybrids) or are hardy (e.g. cereal rye, winter wheat, hairy vetch) for your zone to fit whether you till or use no-till, fit your planting schedule (e.g. early: oats; late: cereal rye), and provide for your unique needs (e.g. desire for soil building and lots of organic matter: sorghum-sudangrass hybrids)—is the key to using cover crops to maximize fertility and productivity in your garden. Always feel free, however, to go back to your tried-and-true methods. Sometimes simple and well-known are also best.

Intermediate Options for Off-Season Cover Crops In Untilled Beds

Mixes

A slightly more advanced approach than using oats alone as a winterkilled cover crop on untilled beds is to use a mix of oats and a clover that will winterkill in your zone. Berseem clover, for example, the least hardy of the annual clovers, is a good choice for gardeners in zones 7 and under, while crimson clover becomes an additional option for gardeners in zones 5 and under. In many zones, field peas are another option to add to the mix. Plant the oats at ⅔ their normal rate and the clovers or peas at ⅓ their normal rate. To get right to planting, skip to “Planting Instructions,” the “Planting Chart,” and “Killing the Cover Crop to Readying the Soil for Planting” below, where you will also find specific planting rates and winterkill zone information for each cover crop.

“Biodrills” & “Tillage Radishes”

For a great option that will break up compacted soil and winterkill in most zones, try forage radishes. A cheaper-seeded, less-tasty version of the daikons we eat, forage radishes provide excellent nitrogen scavenging and weed control and very good soil building and erosion control. Their real specialty, however, lies in breaking up compacted soil, doing so well at it that they’ve earned the nicknames “biodrills” and “tillage radishes.” Furthermore, forage radishes die once winter temperatures get below the mid-20s for several nights, with their foliage and the tops of the radishes slowly rotting over the winter, leaving you with a more easily cleared planting bed in the spring. For other options that will winterkill in your zone (albeit without the “biodrilling” effect), consult the “Planting Table” below.

Killing Non-Winterkilled Cover Crops

Additionally, for gardeners who use untilled beds, there’s a way to kill every non-winterkilled cover crop without having to till or even turn over the soil, giving bed gardeners access to the benefits of all of the non-winterkilled cover crops without the headache that can be involved in clearing them from untilled beds in the spring. While it’s possible to kill most winter-hardy cover crops with one well-timed mowing—usually between the first flowering and seed formation (some will require several mowings), most gardeners typically need to get their beds ready for spring planting long before most overwintering cover crops are even close to flowering. Luckily, there’s an easy answer: A combination of mowing and smothering kills every cover crop, no matter its stage of development. Plus, if timed correctly, it leaves the bed in great condition for planting.

To smother non-winterkilled cover crops, first, mow them as close to the ground as possible. Then, cover them with cardboard or 8-10 layers of wetted newspapers followed by a four-to-six-inch layer of leaves or straw for four-to-eight weeks. This means you’ll want to start this process almost two months before you plan to plant the bed or that part of the bed. To prepare for early May garden plantings, for example, cut and smother the non-winterkilled cover crops in mid-to-late March. If you’re anxious to see how it’s working, after approximately four weeks, start checking an area of the bed every couple of weeks to see how much the stems and roots have broken down. Once the new growth that sprouted under the smothering cover is mostly decayed, and the roots are at least starting to decay, you can uncover it, but it’s better not to rush this stage if you don’t have to plant right away. If you’re planting warm-season fruits and vegetables, it’s a good idea, but not mandatory, to rake off the dead cover crop residue after uncovering the bed to give the soil a week in the sun uncovered just so it has a chance to warm a bit before you plant.

Clearing Winterkilled Cover Crops to Ready the Soil to Plant

The rootballs and short stubble left over in untilled beds from mowed winterkilled and smothered cover crops aren’t typically a problem for transplanted seedlings. Any that are in the way are easily removed and dug in or composted during transplanting, and the rest simply continue to decay and add nutrients and organic matter back to the soil over the growing season. However, they can be a nuisance when planting rows of seeds. Taking a cue from the transplanting strategy, though, can make things easier. While loosening the soil in what will be the row with a hand rake, use the hand rake, or a trowel if necessary, to remove any rootballs that are directly in the row (or possibly give your rows small diversions when it’s easier) and simply leave the cover crop tussocks in between your rows of cooler season crops to decay back into the soil over the season.

Intermediate Options for Off-Season Cover Crop for Tilled Ground

Mixes

A slightly more advanced approach than using annual ryegrass as a lone cover crop on tilled ground is to mix annual rye with a legume that also won’t winterkill in your planting zone. Crimson clover is a good choice for gardeners in zones 6 and up, while red clover becomes an additional option for gardeners in zones 4 and up. In zones 6 and up, field peas are another option to add to the mix. Plant the annual ryegrass at ⅔ its normal rate and the clovers or peas at ⅓ its normal rate. To get right to planting, skip to “Planting Instructions,” the “Planting Chart,” and “Killing the Cover Crop to Readying the Soil for Planting” below, where you will also find specific planting rates and winterkill zone information for each cover crop.

More Options

As a gardener who tills, not only do you have access to every cover crop available to gardeners who plant in untilled beds, but you also have several additional options. Winter-hardy cover crops—e.g. annual ryegrass, winter wheat, cereal rye (winter rye), barley, and especially white clover—become much easier to manage in any zone with tilling. Plus, all of the occasionally-hardy cover crops that don’t winterkill in your specific zone (see the “Planting Table” below) are also easier to manage. Choose from any mix you’d like to try, typically a grass or two and a legume or two, and try adding in forage radishes as you feel ready, noting that they seed in much better if they’re drilled rather than broadcast. Also note, however, that every time you till you’re adding more oxygen near the microbes in your soil, causing them to eat up more of your soil’s organic matter. Since it’s this process that releases the organic matter’s nutrients to your plants, you might see this as a good thing. It does, however, make its replacement ever more vital. In short, by tilling you have more options, but you also need them to keep your soil healthy.

Filling Mid-Season Gaps in Tilled Areas

A slightly more advanced option than using buckwheat alone to fill mid-season gaps between garden crops is using either cowpeas or sorghum-sudangrass hybrids, mixing cowpeas or sorghum-sudangrass hybrids with buckwheat, or mixing all three—with sorghum-sudangrass hybrid providing some support for the cowpeas. These three, warm-season cover crops need a six-to-ten-week window, with cowpeas and sorghum-sudangrass hybrids tending toward to higher end of that range. For gardeners growing in untilled beds, note that cowpeas and sorghum-sudangrass hybrids are difficult to kill mid-season without tilling unless one mows and smothers them as discussed above, and mowing and smothering midseason often presents its own problems. While cowpeas’ vegetative growth can at least be stopped with mowing, making it a possibility to fill midseason gaps in untilled beds, the time it takes to grow and then smother a sorghum-sudangrass hybrid likely makes it prohibitive in midseason untilled beds, unless it’s toward the end of the season, since it’ll be killed by the first frost. In fact, if any of the three persist longer than desired, it’s worth noting that they will winterkill.

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Advanced Options

As you become more comfortable over the years, you’ll likely start delving even deeper into the cover crop options and literature (See “Resources to Advance on Your Own” at the end of this article.). As you’ve noticed by now, your options are as varied as your imagination.

Home-Grown Garden Fertility

Home-grown garden fertility is the goal of all cover crops. Used to cover the soil during gaps between main crops and tilled in a few weeks before planting or cut and left in place, cover crops add their fertility to the succeeding crop and soil. However, cover crops can also be grown year-round, becoming part of the planting rotation and supplying the garden with even more added solid fertility and a wonderful boon of seed-free organic matter that’s great for making your own compost and applying mulch.

If great gardeners really grow soil—and let amazing plants be the byproducts that well-grown soil—then one way to think of a garden is not just as an area in which grow the fruits and vegetables you eat (that which feeds you) but also an area in which to grow that which feeds your garden’s soil. While cover crops wonderfully provide this opportunity for homegrown fertility during times when garden spaces would otherwise be devoid of fruit and vegetable crops, another, more-advanced option is to specifically reserve a fraction of the garden to grow cover crops year-round, even during the main growing season, with the specific intent of that fraction of the garden providing an extra boost to growing the garden’s own increased fertility. Typically, such a gardener will rotate ¼, ⅓, or even ½ of the garden’s growing area through year-round cover crops that become part of a garden’s regular crop rotation. These cover crops are specifically chosen to 1) provide lots of organic matter for compost and/or mulch and 2) build soil fertility and tilth through nitrogen fixation, deep root penetration, soil loosening, and large biomass/organic matter contribution from decaying roots. Finally, one can achieve all of this using a succession of annuals or a single perennial.

An Example with Annuals

Just one example of an approach using annuals is to grow a sorghum-sudangrass hybrid, perhaps mixed with buckwheat and cowpeas, to provide lots of biomass and nitrogen fixation over the summer, followed by winter wheat or cereal rye mixed with a clover or winter (field) peas and possibly forage radishes over the winter. Sorghum-sudangrass hybrid produces massive volumes of top growth during the summer, providing vital organic matter for mulch and compost, and has deep roots that both loosen and stabilize the soil and supply it with lots of organic matter when the plants die after a frost. Winter wheat and cereal rye provide the same over the winter, if maybe a bit less in volume, with the cereal rye providing additional allelopathic effects on weeds the next spring if some of it is left as mulch. The legumes—cowpeas, field peas, and clovers—fix nitrogen in their roots that is left in the rooting zone for use by the next crop. Still, there are as many variations and combinations as you can figure.

An Example with Perennials

One example of using a perennial for this approach is growing alfalfa. Alfalfa is one of the powerhouses in this regard, providing three or four high-volume, high-nutrient cuttings per year for use in compost and mulch and very deep, large taproots that loosen the soil deeply, fix a large amount of nitrogen, and leave a great deal of organic matter in the soil. Not having to replant regularly is an additional benefit. After two or three years, till in the alfalfa, leaving a boon of healthy, nitrogen-rich soil for the following crops. Done on a “three-patch” method, where one patch is sown and one patch tilled each year, alfalfa can just become part of the garden’s crop rotation.

Undersowing (Overseeding) Covers

A little more advanced approach than waiting to plant your cover crops is to undersow (or overseed) them. This is a great technique if you don’t have enough time to establish your preferred cover crop after harvest (perhaps because of an early frost or late harvest), or if you want to provide some cover on the bare soil under certain crops during the growing season. For example, you might broadcast a winter cover around your late-maturing brassicas, long before they’re harvested, providing the winter cover with the headstart it needs before your first frost. Similarly, you might broadcast a summer cover, such as white (Dutch) clover, under and around crops with open soil, such as cucurbits or corn, to provide them with their own living mulch during the summer. If you do this, be sure to give the corn or cucurbits about a two-week headstart. Any less and the cover crop might interfere with the main crop. Any more and the main crop tends to shade out the summer cover before it’s established.

Timing No-Till Cycles

Another even-more advanced approach is timing the natural cycles of dying and reseeding to work with your existing crops. For example, if planted in the fall, winter wheat, cereal rye, annual ryegrass, hairy vetch, and crimson clover will die back naturally in the late spring or early summer and can reseed themselves to regrow the next fall. Hairy vetch and crimson clover are often used this way under corn, for example, and could be used similarly in combination with many vegetable crops. Winter wheat and cereal rye can be used this way for summer-planted fall vegetables and can provide an additional grain harvest, reseeding “themselves” with a slightly messy harvest. If used in combination with a nitrogen-fixing cover like hairy vetch or crimson clover, it might be possible to use them before later-planted, nitrogen-demanding corn, or even cucurbits, in some warmer regions. Annual ryegrass can be used to self-seed in orchards and the like and will withstand the mowing it will likely need. Each of these will take some adjustment, practice, flexibility, and possible ingenuity to meet your particular needs—that’s why they’re more-advanced options—but they also minimize the work needed to clear cover crops before planting main crops and to reseed them after the main crop has finished.

One major caveat: Gardeners are typically cautioned to time their sowings, mowings, incorporating, and winterkilling of cover crops to avoid their reseeding and becoming a weed problem. This approach obviously throws that caution to the wind. If there’s any chance you’d be creating a weed problem for yourself or for your neighbors or, worse, creating an invasive species issue, avoid this approach.

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Planting Instructions

Specific planting instructions should be provided by the seed company that sold you each cover crop. However, these general instructions should work for most. First, clear the area of residual garden plants (unless doing an overseed as described above), mulch, and weeds. Second, rake the soil with a bow rake to work up its surface. Third, broadcast seeds at the rate prescribed in the table below. Fourth, rake the seeds into the soil with the bow rake. To not clump the seed, this is best done with short, chopping strokes. Fifth, cover with a light mulch of straw. Finally, make sure they’re watered for 15 minutes a day for the first few weeks after planting, just to make sure the soil surrounding the seeds remains moist until they have large enough root systems to reach deeper moist soil (typically when they’re around six inches tall, but it depends on how dry your soil is).

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Planting Chart

Cover Crop	Winterkill	Mow Kill	lbs/Acre✝	lbs/1,000 ft.2✝	lbs/100 ft.2✝
alfalfa	depends on variety & zone		15-25	.34-.57	.03-.06
annual alfalfa	x (non-hardy cultivars)
annual ryegrass	possible zone 5 & ↓		20-30	.46-.69	.05-.07
barley		x* @ mid-late bloom	80-140	1.84-3.21	.18-.32
brassicas	depends on variety & zone	x	10-20	.23-.46	.02-.05
buckwheat	x	x* 7-10 days after flowering	70-90	1.61-2.07	.16-.21
clover, berseem	x (zone 7 & ↓, if below 20° F for several days)		15-20	.34-.46	.03-.05
clover, crimson	x (zone 5 & ↓)		20-30	.46-.69	.05-.07
clover, red	x (zone 3 & ↓)		10-12	.23-.28	.02-.03
clover, white			7-14	.16-.32	.02-.03
forage radish	x (4-5 nights below 23° F)		10-20	.23-.46	.02-.05
mustards	x (4-5 nights below 25° F)		10-15	.23-.34	.02-.03
oats	x (zone 6 & ↓)	x* @ soft-dough stage	110-140	2.53-3.21	.25-.32
peas, Austrian winter (field)	x (zone 5 & ↓, slimy until broken down)	x* after full bloom	90-140	2.07-3.21	.21-.32
peas, cow	x		70-120	1.61-2.75	.16-.28
rye, cereal		x* after sheds pollen	85-250 (later = heavier)	1.95-5.74	.20-.57
sorghum-Sudan grass hybrid	x (1st hard frost)		30-50	.69-1.15	.07-.11
vetch, woolypod	x (zone 5 & ↓)		30-60	.69-1.38	.07-.14
vetch, hairy	x (zone 3 & ↓)		25-40	.57-.92	.06-.09
wheat, winter		x* @ soft-dough stage	70-140 (later = heavier)	1.61-3.21	.16-.32

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Killing the Cover Crop to Ready the Soil for Planting

In the spring, use the smothering approach described in “Killing Non-Winterkilled Cover Crops” above or till the cover crop into your soil a few weeks before planting. If the cover crop is tall, mow it first, even several times and even finally with a push mower to chop it up well, and till these clippings in, as well.

One note of caution, be sure not to let crops like crimson clover, cowpeas, or buckwheat go to seed, or you’ll have just introduced a weedy headache for yourself and possibly invasive exotic to your area. Avoiding this is often easily done, however, by following common planting times for off-season covers (so their growth slows over the winter) and mowing them just before or just after they start to flower.

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Resources to Advance on Your Own

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.