Cover Crops

Cover crops are traditionally thought of as plants used when the garden or field is empty, such as over winter and sometimes over summer.

But I like to broaden this definition to also include plants used during the growing season, inter-planted with food crops or even in ornamental beds where they’re sometimes called groundcovers or living mulch.

Crotalaria or “rattlepod” is a leguminous green manure often grown in the tropics. It’s great for fiber and building soil, but poisonous to large mammals.

They’re also referred to as green manures, generally when they’re going to be incorporated into the soil after a certain period of growth.

A good goal is to make sure your soil is always covered with plants, and cover crops help achieve this goal.

Some soil experts consider this more important than composting. It’s certainly more natural, and more feasible in large gardens and on farms where it’s difficult to make enough compost to cover everything, let alone distribute it.

The main reasons home gardeners use cover crops are to improve soil fertility, increase organic matter and control weeds and plant predators.

There are also many other uses, such as to prevent erosion, send roots deep to break up compaction, conserve moisture and increase water infiltration, attract insects and other animals, and even provide food for humans and animals. Let’s look at the three main reasons.

Cover crops improve soil fertility in a couple of ways.

The main benefit most people think of is the increase in nitrogen we can get from planting legumes, such as clover.

Many leguminous plants and a few others form symbiotic relationships with nitrogen-fixing bacteria such as Rhizobia and Bradyrhizobium.

The plant provides the bacteria with a home on its roots, as well as food. The bacteria give some nitrogen to the plant in return. Some of that nitrogen ends up in the soil when the plant is alive, and more when it eventually decays.

I’ve read that 22 lbs. of fixed nitrogen is as good as applying 220 lbs. of ammonium sulfate or 1,100 lbs. of sulfate of ammonia because the fixed nitrogen is efficiently used as it’s produced, whereas most of the chemical ammonia that is applied is leached and volatilized.

Fertility is also improved by many cover crop plants that send deep roots into the soil and harvest minerals that aren’t reached by other plants.

Rapeseed (brassica napus) and other members of the brassica family are often used as deep rooted cover crops, as are some grains.

Even if they don’t send deep roots, just being there is hugely beneficial because many of the nutrients would otherwise leach.

Cover crops can stop a lot of this leaching both by taking up some of the nutrients themselves and by taking up a lot of the water that would otherwise be heading back to the water table, bringing nutrients with it.

Nitrate nitrogen is a prime example of a leachable nutrient because it’s water soluble, so it doesn’t stay around for long in the soil when there’s a lot of water moving through.

While legumes fix the nitrogen, grasses and brassicas are much better than legumes at stopping it from leaching. Many other nutrients such as calcium and potassium can also leach without plants and organic matter to hang onto them.

The next big benefit is organic matter.

Any plant will increase the organic matter content of the soil simply by the dropping of leaves (including evergreens) and the constant growth and death of roots, but cover crops are often turned into the soil to give a huge influx of biomass to it.

Less talked about, but potentially more important, plants send a huge amount of carbon — as well as protein, amino acids and thousands of other substances — into the soil as exudates.

Grasses are especially adept at quickly getting big and building soil. For example, cereal rye adds large amounts of organic matter, sorghum-sudangrass sends deep roots to break up compaction, and annual ryegrass is great for stabilizing and drying out wet soil.

Sorghum, pictured here, can be grown as a forage crop, and is often hybridized with its relative sudangrass to make a deep-rooted, heat tolerant cover crop.

Along with the addition of plant biomass is the increase, or at least maintenance, of microbial biomass.

Without plants, many microbes will go to sleep, but if we keep plant cover on the soil they’ll keep working, even through winter, albeit more slowly.

This is especially important for mycorrhizal fungi, who need a host to stay active.

And then there is weed and pest control. Plants accomplish weed control by several mechanisms — competing for water and nutrients, shading out the soil, crowding out the soil below ground with their roots, and sending out chemicals to inhibit other plants from growing.

Cereal rye is an overwintering crop that controls weeds both physically and chemically by producing compounds that are toxic to many other plants. Sorghum-sudangrass does the same in the summer.

The chemicals can inhibit germination of your vegetable seeds — a process called allelopathy — so be sure to wait two to three weeks after incorporating the cover crop before seeding.

Plant predator control is achieved mainly by the cover crops inviting and hosting a diversity of microorganisms, nematodes and insects that keep the system more in balance, as well as producing their own antibacterial compounds.

Some people worry about the cover crop crowding out their other plants. It’s possible, but I like to have them touching so the beneficials climbing on the cover crop will move over to my other plants, too.

It’s also true that cover crops can attract some predators. For example, cereal rye, orchardgrass and crimson clover may attract armyworms.

This is one of the reasons I like to plant a combination of two or three cover crops, to create more diversity, decreasing the chance of one insect causing problems.

Purple vetch, a legume, is often planted together with cereal cover crops like wheat (pictured here), rye or oats.

Some cover crop benefits, such as organic matter or fertility improvement, are only substantially noticeable after a couple of years of cover cropping, while others such as weed control and insect attraction are noticeable right away.

I have my dad cover cropping his entire tree nursery with a mixture of grasses and legumes. It’s work to keep it mowed, but it’s slowly increasing organic matter, improving soil structure and fertility, controlling erosion and attracting insects and other small animals.

Cover crop selection is an involved decision for farmers and gardeners, who have to decide which of the above benefits are most important and select appropriate crops based on that, as well as choosing crops that are going to work in their climate conditions.

We gardeners can spend a lot of time making this decision, but for most of us, I believe it’s more important to just get something in the ground that will satisfy the main goals of increasing organic matter and soil fertility. We can also enjoy the by-products of this choice, including some weed control, and we can always experiment with some different crops to see what we like best.

That being said, I have a few tips to help with the decision. While all cover crops have some ability to increase soil fertility and organic matter content and to control weeds and plant predators, certain plants excel at certain tasks.

Legumes

Although there are several non-legume nitrogen-fixing plants, often working in partnership with a group of bacteria called Actinomycetes, most of our nitrogen-fixers are from the legume family, working with Rhizobium bacteria.

It’s usually a good idea to use a legume as a cover crop, such as vetch or clover. This is especially important when you’re going to be planting crops that are considered to have a high demand for nitrogen, such as corn or other grasses, but almost all vegetable crops benefit from a legume. In fact, it’s a great idea to grow one interspersed among your beds.

The root hairs of a legume search the soil for the right species of Rhizobium bacteria with which to partner.

The little pinkish bumps are rhizobium nodules on this legume root.

Certain strains of Rhizobia provide optimum nitrogen production for each legume. If the plant roots don’t find their ideal match, they will work with the other strains, but they won’t be able to produce as much nitrogen.

The root encircles the bacteria colony and creates a nodule as big as a kernel of corn where the bacteria live. The bacteria make an enzyme that converts gaseous nitrogen to ammonia and the plant uses that ammonia to build amino acids.

In return, the plant gives carbohydrates and other organic substances to the bacteria.

When the plant goes to seed, the nodules are discarded and the bacteria remain viable in the soil for three to five years waiting for the next legume to come along.

Unfortunately, the bacteria may not be numerous enough to provide optimum nitrogen fixation when that next legume is planted. Since we don’t know if we have the right bacteria in our soil and whether or not they are numerous enough, it’s a very good idea to add them in with the seed or buy seed that has already been inoculated.

Each legume needs a different inoculant. Ideally, the garden center will have the right one or a mixture. If the folks in the garden center don’t seem to know what you need, you may have to order it online.

The two main forms of inocula are solid and liquid, with solid being the most common. Either of them can be mixed with the seed before you plant or applied in the furrow directly with the seeds.

Even though they’re sometimes planted earlier than grasses in the fall in order to get them established before winter, legumes don’t do much until spring, when they put on their growth and fix most of their nitrogen.

Most of the nitrogen fixed by a legume isn’t released to other plants until the legume decomposes.

Even then, if the legume is allowed to produce seeds, much of the nitrogen will end up in the seed. This is great if you want the plant to reseed on its own, or if you want to leave it as a groundcover such as in an orchard, but it’s not great if you want that nitrogen to be available to other plants.

What we often do, therefore, is cut the crop down right in the early to mid-stages of blooming in the spring before we seed or transplant our vegetables. If you want to save seeds, you can keep just a few plants for this purpose, and cut the rest down.

If you have bare soil during the summer, legumes work great to fill that in, too.

Cowpea is a favorite leguminous summer cover crop.

Legumes also contribute to an increase in organic matter and weed control, although not as much as grasses.

Also, since they break down much faster than grasses when you knock them down, due to their relatively higher nitrogen content, they don’t provide a long-lasting mulch.

The positive side of that is that they do release their nutrients more quickly to plants than grasses.

If you’re planning a lawn area and are a patient person, you might plant a legume cover crop such as clover or trefoil for a year before installing the lawn, for all of the above benefits.

It’s no fun trying to maintain a lawn that was planted in poor soil. You can sow seed right into that clover and watch each year as the grass slowly outcompetes the legume — provided your soil is healthy.

Grasses

Non-legumes used for cover cropping are often grasses.

They’re used when your main goals are creating a lot of organic matter, controlling weeds, and taking up nutrients — especially nitrogen — from the soil so that they don’t leach.

If you have a bed that was mostly planted with legumes such as peas and beans during the growing season, or even better, if you have a soil test showing a lot of nitrogen in the soil after harvest, it will probably be more beneficial to plant a grass cover crop.

This grass will do a better job than a legume of keeping a lot of that nitrogen rather than allowing it to leach, and also acts as an excellent weed control and huge organic matter builder.

While legumes do help build humus, grasses and other grains really excel at this job.

It’s partly because they grow fast and get big, but also because they’re composed of more cellulose and lignin, materials that contribute to humus. In contrast, an annual clover will break down more readily and become a much better short-term source of nitrogen and other nutrients for crops.

Grasses have a higher carbon to nitrogen ratio than legumes, so they break down more slowly, acting as a longer-term mulch, but this also means the microbes may need to steal nitrogen from the soil in order to break them down.

The implication of that is you don’t want to use too much grass in a garden that is nitrogen-deprived, or at least you want to turn it in earlier in spring because the carbon content increases as the plant grows.

The longer breakdown period of grasses also means the nutrients will not be as immediately available to the next crop.

Winter annual grasses are seeded in late summer or autumn. They grow before winter, go dormant during winter, and grow again in the spring before we usually cut them down.

If you live in a climate with cold winters, some winter cereal cover crops may be killed by frost, so they’ll provide mulch over winter and you won’t have to cut them down in spring.

Summer annual grasses are occasionally grown during the summer if you’ve harvested everything out of a garden bed and aren’t going to plant it again until fall.

Companion Plants and Polycultures

Ecological landscape designers often look to the native plant communities in their region to see which plants grow together naturally, in order to help determine suitable plant combinations for their designs.

This is a form of companion planting, but plants need not have evolved together in order to benefit each other.

Companion planting in a vegetable garden involves pairing plants that may come from different regions but nonetheless work well together.

Examples are green beans and strawberries, carrots and tomatoes, and lettuce and spinach. Sometimes the plants simply work well together because they take up different areas above or below the soil.

Sometimes one plant deters a predator of the other plant. Often, the benefits realized are not nearly as grand as some gardening books indicate, but usually, no harm is done.

A polyculture goes a step further. It involves planting many plants together to take advantage of various niches in the garden, much the way nature fosters this diversity.

Some will grow tall and provide shade, while others hug the ground. Some are ready for harvest early, while others take longer, even within the same food group, such as lettuces or tomatoes.

Some attract beneficial insects, while others repel plant predators. Some provide nitrogen for the soil, while others happily gobble it up.

Permaculture has embraced the polyculture philosophy by using multi-level plantings to take advantage of all the various opportunities in the garden. Another concept is the guild, where a central plant such as a fruit tree is surrounded by a group of plants that benefit the tree.

For more information on permaculture and polycultures, a great place to start is Gaia’s Garden: A Guide To Home-Scale Permaculture by Toby Hemenway.

I just wanted to point out here that plants provide fertility for each other. While legumes and grasses are the most common cover crops, especially over winter, ornamental and food gardens should be interplanted with plants that provide more benefits than just being beautiful.

Comfrey is a classic nutrient accumulator plant that grows tons of leaves to use as mulch or add to compost. But be careful – it spreads easily and is incredibly hard to remove unless you get a variety that’s bred to be less invasive.

Examples of beneficial plants to use are yarrow, fennel, lemon balm and clover, and there are hundreds of others in books such as Gaia’s Garden listed above.

Mixtures

Let’s get back to legumes and grasses as off-season cover crops.

Rather than choosing one, I almost always mix grasses and legumes in order to get the benefits of both. By planting them together and occasionally going as far as to plant several cultivars or varieties of each, there is a much greater chance that at least one or two of them will do well in your soil and climate conditions.

For example, if you have a soil with a lot of excess nitrogen, your legume might not grow as much but your grass might be very happy, whereas a soil that is low in nitrogen may favor the legume. It’s more complicated than that, but the idea is that one of them will be happier in your soil conditions.

If it’s a wet season, even within one species, some cultivars will do much better than others.

Same for dry, cold, windy weather, etc. Different plants also have different growth habits, so a mixture can better cover the soil, make full use of different levels of sunlight and even grow off each other. Vetch will climb up a tall grass for support to reach sunlight, which is one reason that it’s often planted together with rye.

There are dozens of cover crops, each with their own strengths and weaknesses and some more appropriate for certain regions, but there are a few that are used nearly universally and I recommend you start with them or find out what works in your area.

Once you have success with the basics, you can try others. You can even experiment with cover-cropping year round, in between your rows.

This is being done very successfully in many warm-climate countries where leaving fields fallow in only cover crops isn’t economically feasible, because the opportunity cost of not growing cash crops is too great.

Gardeners and farmers use all kind of plants as cover crops: dandelion, fava beans, fennel/dill, yarrow, and even parsley, but let’s start with the following: hairy vetch, clover, cereal rye and annual ryegrass.

Vetch

Hairy vetch (Vicia villosa) is the king of nitrogen production and that alone makes it worthwhile.

It doesn’t grow much in the fall but is a good survivor through winter (hardiness zones 4 and up). Come spring, hairy vetch can get big, contributing lots of organic matter and smothering weeds.

It doesn’t need particularly fertile soil and tolerates many soil conditions. A friend of mine has it growing on a steep slope as part of a mixture to help stabilize the soil.

Hairy vetch will often naturalize and grow wild, adding beauty and nitrogen to fallow fields and roadside ditches.

Vetch does need moisture to get going, but after that it can handle drought.

With sufficient moisture, it can be used in much of North America, but isn’t used much in the Great Plains, Southwest and Northwest. Even in the Southeast, clover is better. While it can definitely survive the winter, planting it with a grass will greatly improve its chances.

After you cut it down, you can seed vegetables right into it, although if you’ve let it grow for quite a while in the spring, you might take some excess to the compost pile.

Woollypod vetch (Vicia villosa ssp. dasycarpa) might be more appropriate in areas with little rain or irrigation. It’s used extensively in California, and up the West Coast.

Clovers (Legume)

Clovers don’t generally produce as much nitrogen as vetch, but they’ve been used for hundreds of years, so they must do something right. Bees sure like many varieties.

Here are just a couple of the most universal species. Your garden center should carry what’s right for your area. Towards the end of the summer, you can try seeding clover or another nitrogen-fixer into a bed before you harvest most of your vegetables.

Red clover (Trifolium pratense) is used mostly in the eastern half of North America as a dependable winter annual, biennial or short-lived perennial (zone 4 and up) that creates a modest amount of nitrogen. It’s not the fastest or biggest grower, but it’s one of the best when it comes to handling a wide range of soil and climate types.

White clover (Trifolium repens) is raised as a perennial to hardiness zone 4, although it’s used as a winter annual down south where drought and disease weaken it.

It’s the typical living mulch, used year-round in orchards and even vegetable gardens. If it were up to me, it would be used more in ornamental gardens instead of plants like ivy.

It stays fairly low at 6-12 inches, tolerates shade, cutting/mowing and foot traffic.

Yellow sweet clover (Melilotus officinalis) and white sweet clover (M. albus) can grow in most of North America. It’s not a huge nitrogen producer when used as a winter annual, because it’s mostly a biennial, doing much of its nitrogen production in year two.

It can, however, live on infertile soil and survive drought and winter, although it’s not a big fan of really wet soils. It has a taproot to break up compaction and creates a lot of biomass with its abundant growth.

Rye (Grass)

Cereal rye (Secale cereale) is my favorite grass for cover cropping. It’s an annual that grows well pretty much anywhere in North America other than the very Southwest.

Cereal rye.

It can handle infertile soil and can be planted fairly late in the fall and still do well. It prefers lighter soils, but grows in clay, and can tolerate both dry and very wet soils.

Rye is the absolute best when it comes to retaining soil nitrogen, makes a huge amount of organic matter, and is great at controlling weeds. It also controls predators and attracts beneficials.

It grows big and can get a high carbon to nitrogen ratio that can tie up carbon in the spring, so I combine it with a legume. I often use hairy vetch, which the rye protects over winter, at a 1:1 rye to vetch ratio by weight. You can also use it with clover at the same ratio.

An alternative for farmers and gardeners to deal with the carbon is to cut down the rye early in the spring, but in areas with high rainfall, your soil may get too wet after that and you may lose a lot of nitrogen.

Keeping the rye growing longer would suck up the spring rains.

A much better option for home and market gardeners who get a lot of spring rain would be to cut it down much later and just remove some of the clippings for our compost bins. Then we get the benefits of the grass staying in the soil longer — less nitrogen loss and drier soil — and we get to use all of that biomass.

Seed rye in late summer to autumn in hardiness zones 3-7 or even as late as mid-winter in zones 8 and up. The seed prefers to get down into the soil one to two inches so some light cultivation will be beneficial, but broadcasting on top of the soil is possible.

While it depends greatly on your region and goals, a good time for most of us to cut it in the spring is when it’s 12-18 inches tall, before setting seed. Wait three to four weeks to seed vegetables and at least a week before planting transplants.

Annual Ryegrass (Grass)

Annual ryegrass (Lolium multiflorum) is not the same as cereal rye, but it’s another great crop to use for soil building, erosion prevention and weed control, across most of North America, other than the Southwest, Northwest and Great Plains.

It’s inexpensive and fast to establish, although it may not make it through the winter in the north.

Even there, if established early enough in the fall, it will produce some good biomass before winter and act as a great nurse crop to clover that you can plant at the same time at a 1:1 ratio by weight. A nurse plant is a plant that offers protection or assistance to another plant.

Annual ryegrass can also be used over the summer.

Ryegrass likes fertile soil, so I wouldn’t use it if you’re just starting on poor subsoil. It also needs a lot of moisture, and generally shouldn’t be left to grow between rows during the growing season unless you have plenty of water and fertility.

In zone 5 and cooler, seed at least six weeks before the first hard frost to ensure the grass gets established.

The annual ryegrass is often half the price of the perennial rye and much easier to cut down in the spring with a hoe. Be sure to do that before it goes to seed or it can become weedy. I always chop it down before it gets eight inches tall.

Annual ryegrass has much smaller seeds than cereal rye.

If you have a perennial garden, you might rather have a low-growing perennial such as white clover that will be there every year to protect and enrich the soil, and that won’t get so big as to need much maintenance.

How to Seed and Care for Cover Crops

Cover crops are just plants. They are cared for much the same as other plants.

If you’re using them in a vegetable garden in the fall, they should be seeded right after your main harvest time or even a week or two before your harvest.

Of course, many of us grow a mixture of vegetables that we continuously harvest, and in that case, we just plant the cover crop towards the end of the season. I like to plant fall cover crops a couple of months before frost to get them firmly established.

If possible, it’s best to get seeds into the soil rather than just broadcasting on the surface. Small-seed legumes go about 1/4 inch deep while larger seed legumes and small grains are planted 1 to 1 1/2 inches deep.

While generally not as successful, you can broadcast the seed for small-seed legumes on the soil at a higher rate and you should get germination if you keep the soil moist for the next week.

If you have a large area to seed, but broadcasting is inefficient and hand seeding is time consuming, you can try a small manual seeder to get the seeds into the soil.

You’ll probably be cutting down your cover crop before you seed or transplant.

Farmers mostly do this with herbicides, but you can just use a hoe or various other tools. It’s not a bad idea to leave some of the plants alive to produce seed, attract insects, and for many other benefits, but most of it will be cut down.

After that, it’s generally a good idea to wait two to three weeks before you sow seed and one week before you transplant, to reduce allelopathy and potential predators and allow some nitrogen release from the plants.

That being said, some cultures have kept soils going for thousands of years by skipping this downtime. If you use liquid fish and/or EM, you can spray that on the cover crop when you turn it in order to hasten decomposition.

We’ll look more at tilling another time, but I’ll mention here that you can lightly incorporate your cover crops into the top few inches of soil a couple of weeks before you plant.

I believe in disturbing the soil as little as possible through no-till or low-till practices, but it’s a good idea to lightly work a green manure into the top layer of soil to speed up the decomposition and decrease the amount of nitrogen lost to the air.

Residue from a grass/legume mix will have a higher carbon to nitrogen than the legume alone, slowing the release of nitrogen so it’s not as vulnerable to loss.

Summary

• Covers crops improve soil fertility, increase organic matter, control weeds and pests, prevent erosion, send deep roots to break up compaction, conserve moisture and increase water infiltration, attract insects and other animals, and even provide food for humans and animals.
• They can be used seasonally or on an ongoing basis through companion planting and polycultures.
• Legumes such as vetch and clover attract beneficial insects and partner with bacteria to convert gaseous nitrogen into a form plants can use, some of which makes its way into the soil when the crop is turned in.
• Grasses such as cereal rye and annual ryegrass retain nitrogen and other nutrients in the soil, soak up excess water, control weeds and can create a huge amount of organic matter to enrich the soil.
• A mixture of a legume and a grass contributes the benefits of each and is best seeded a couple of months before frost, incorporating into the top of the soil two to three weeks before seeding in the spring.