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.

Why Grow Cover Crops?

Cover crops give gardeners powerful tools to grow the healthy soil that grows healthy and productive plants—and a thriving garden ecosystem. They’re grown at times when food crops aren’t growing or in open spaces between distantly spaced food crops to protect, build, and maintain healthy soil and provide habitat for garden beneficials. They act as a living mulch on otherwise bare soil, thwarting erosion, adding precious organic matter and nutrients, discouraging weeds, and building the larger garden ecosystem. In so doing, cover crops move a garden from a place where gardeners merely grow that which feeds themselves to one where gardeners grow that which feeds the land as well—their garden’s soil, their garden’s ecosystem, and even the larger ecosystem of which both are a part.

Underlying Premise

Mimic natural processes. Any time you can mimic natural processes in your garden, you attune it, and yourself, to the powerful systems that have worked for millennia. Bare soil is rarely found in nature. Instead, soil is constantly covered by living plants, their decaying pieces, or both. Why? Water and nutrient cycling, numerous soil building processes, and homes for an ecosystem’s denizens. Without plants and their decaying parts to protect it, bare soil washes away, and its soluble nutrients leach away deeper into the soil. Furthermore, we all know it’s the decaying fallen leaves, matted grasses and forbs, and decaying roots that build up beautiful rich topsoil in nature over time. Cover crops use the same principles to build up rich topsoil in our gardens. And their presence gives homes and food for species that benefit our gardens greatly. 

This is the underlying premise. Some of the details and intricacies of this eloquent natural process are outlined below.

Reasons

1. Protect soil from compaction and erosion
2. Loosen soil
3. Increase water and air infiltration
4. Increase water holding capacity
5. Increase water quality
6. Conserve soil moisture
7. Add organic matter and fertility
8. Provide home-grown mulch and compost material
9. Capture nutrients
10. Make nutrients more available and usable
11. Produce nutrients
12. Boost yields
13. Cheaper
14. No chemicals
15. No need for even typical organic fertilizers, foliar applications, and inputs
16. Provide homes, and possibly food, for beneficials
17. Protect soil from temperature extremes
18. Break disease and pest cycles
19. Prevent and discourage weeds
20. Give a solid next step

Protect soil from compaction and erosion. A single raindrop hits with such force it can splash and displace soil up to six feet from its point of impact. Like a parade of marching, melting BBs poured from above, the cumulative impacts of raindrops severely compact bare soil over time. They also erode it. Soil displaced by splashing is extremely susceptible to further erosion. Plus, each bit of increased speed of surface water flow causes a directly proportional increase in its ability to carry away soil. With little or nothing to slow water’s rush, bare soil is ripe for massive erosion. Worse, erosion in our gardens takes the topmost soil first, which is typically the soil with the highest nutrient and organic matter content. Cover crops work to thwart soil compaction and erosion by absorbing the direct impacts of raindrops, transferring fallen water to the soil in gentle trickles and drips, and providing stems, stubble, stalks, and roots to slow water’s flow. What’s more, the roots of cover crops mechanically hold the soil together in an interlocking web to further thwart erosion. 

Loosen soil. Cover crops go beyond protecting soil from compaction and actually actively loosen the soil. Because their roots nudge soil upward and they provide habitat for the earthworms, beetles, etc. that do the same, all cover crops loosen and foster the loosening of topsoil to some degree. Annual ryegrass and cereal rye are among the best at it. Deep-rooted cover crops, on the other hand, go beyond loosening topsoil and even loosen the subsoil, increasing soil aeration and drainage. Forage radishes—a cheaper-seeded, less-tasty version of the daikons many of us grow to eat—and alfalfa grow deep, thick roots that are superb at this, with alfalfa being a perennial option and forage radishes being an annual option. Although not nearly as thickly rooted, sorghum-sudangrass hybrids grow immense root systems that loosen topsoil and subsoil admirably, as well, especially if mowed when their stalks reach a height of three or four feet tall.

Increase water and air infiltration. Cover crops help increase air and water infiltration into the soil in two main ways: slowing water flow and fostering the creation of soil aggregates. Transferring water from their foliage to the soil in gentle trickles and drips and providing stubble and stems to slow water’s surface flow significantly slows water’s pace and not only decreases erosion but also allows for much greater water infiltration into the soil. 

Cover crops, however, help increase water infiltration in another way: providing a host for making glomalin, the soil-gluing organic matter protein responsible for creating the soil aggregates that are the hallmarks of healthy soil. To get a better idea of how glomalin works, imagine soil that feels like a powdery dust as you work it with a hoe or rake. If you try to hold it in your hand, it just falls through your fingers like sand through an hourglass. Conversely, imagine a soil that crumbles into “grains” the size of rice as you work it with a hoe and sticks together in a clump when held it in your hand. The first soil feels like powdery dust because it lacks sufficient glomalin. The second soil sticks together because of glomalin. Lacking the glomalin glue that holds soil together, the particles in the dust-like soil just settle next to each other in the ground as gravity dictates, their only pore spaces for water and air the relatively small gaps between settled particles. The glomalin-filled, “rice-grain” soil particles, in contrast, are glued together, forming clumpy larger particles called aggregates. These aggregates naturally form correspondingly larger, oddly shaped spaces between them, which become a multitude of wonderful larger pores for increased water and air infiltration. If glomalin is so important, then, how do we get more of it?

Glomalin is produced by arbuscular mycorrhizal fungi. Mycorrhizal fungi create incredibly intricate filagreed networks of root-like hyphae throughout the soil and are especially adept at pulling minerals from the soil, which they trade with plants for carbohydrates in one of the soil’s most amazing and fundamental symbiotic relationships: The plants get minerals much more easily than they otherwise could in exchange for something the fungi cannot create on their own. Glomalin-producing mycorrhizal fungi, use glomalin to protect their hyphae, but it also works to hold soil particles together into aggregates. Since cover crops offer vastly extended amounts of time that roots are present in our gardens, they provide the mycorrhizal fungi a symbiotic partner on which to grow when our crops aren’t present, vastly increasing our soil’s glomalin and creating soil structure with aggregates that are the building blocks of healthy soil.

Increase water holding capacity. The large irregular pores that are present in soil with plenty of glomalin just hold more water. Plus, since organic matter acts like a sponge, holding almost its own weight in water, every increase in the soil’s organic matter equals a corresponding increase in the soil’s water-holding capacity. Even after pounding downpours of inches of rain in a matter of an hour that show significant signs of erosion almost everywhere I look, I’m always amazed at how little, if any, takes place in our healthiest, most-organic-matter-filled garden beds. Typically, there are no visible signs of runoff of any kind even though the soil has a mounded, “sloped” shape and sometimes only has a tiny layer of mulch. I credit ample organic matter, glomalin-created aggregates, loose soil, and mulch. 

Increase water quality. Stream waters laden with soil make life difficult for aquatic life. Runoff nutrients from farms, yards, and gardens create algae blooms that rob water and fish of oxygen as the algae die. Worse, runoff pesticides compromise and kill aquatic life. Decreased erosion and increased water infiltration not only protect our precious garden soil and make our soil more resilient against drought, but they also protect the water quality of our streams, rivers, ponds, and lakes and help resupply our groundwater, including our springs and aquifers.

Conserve soil moisture. Cover crops also help conserve soil moisture in two ways. First, as described above, organic matter increases soil’s water-holding capacity, and cover crops increase the amount of organic matter in the soil. Second, if left on the surface, either standing or prostrate, cover crops act as a mulch to protect the soil from drying out.

Add organic matter and fertility. Not only do cover crops preserve the precious organic matter in topsoil by keeping it from eroding away, they also add significant amounts of it to the soil. Organic matter is the core of healthy soil. As it’s slowly decomposed by soil life, organic matter provides garden plants with exactly the nutrients they need in forms perfect for their use. For each one percent of organic matter in the top six inches of soil, for example, 20 to 40 pounds of usable nitrogen are released by microbes each growing season. However, most soil today is already well below the five-percent ideal for healthy soil and maximum plant growth. Plus, soil microbes are constantly doing their job of eating away organic matter and releasing its components back to the soil in a perfect feeding of our plants. Therefore, we need a way to replenish and even build up our organic matter to truly healthy levels to maximize the health and productivity of our plants. This is especially the case if we till regularly, since tilling adds oxygen near the microbes, speeding their breakdown of organic matter. By adding their slowly decaying roots, stems, and leaves, cover crops are one way a gardener can add back the organic matter microbes are constantly eating and even add “extra” to increase soil’s organic matter percentage over time. Actually, aside from adding cured compost, cover crops are probably the best way to add organic matter to the soil, and, along with making one’s own cured compost from garden-grown crops and cover crops, one of the only options that is always home-grown.

Provide home-grown mulch and compost material. Some cover crops allow you to grow your own mulch, depending on when you plant them and when the cover crop dies back or gets worked in. When you cut and gather oats, wheat, and rye stalks, for example, you have grown your own straw. If planted at the right time of year, annual rye will allow for several cuttings of hay/straw. Alfalfa will, similarly, give several crops of cuttings per year, providing hard-to-find weed-free hay for you to use as a mulch or a high-nitrogen compost boost, especially if added while it’s fresh.

Capture nutrients. Unless taken up by plants, water-soluble nutrients like nitrogen constantly leach through the soil with the trickle and flow of water, some of them eventually flowing beyond the rooting zone of most crops. During times when garden areas would otherwise be bare of vegetation and especially susceptible to nutrient leaching, cover crops absorb and hold these leaching nutrients in their tissues simply as a process of their own growth. Worked into the soil before a main crop or simply mowed and left to rot on the soil’s surface, the cover crop’s vegetation and roots decay in the garden bed, adding their scavenged and captured nutrients back to the level of the soil where they can be reached by the garden plants that follow them. 

When discussing nutrient leaching, nitrogen is often the nutrient of focus since its greater water solubility means greater potential for it to leach out of the rooting zone. Whereas legumes are generally relatively poor nitrogen scavengers since they make their own (see “Produce nutrients” below), grasses and forage radishes are nitrogen scavenging and capturing champions. Winter rye, capturing up to 100% of the available nitrogen in one study, and sorghum-sudangrass hybrids are among the best. Barley, capturing an average of 64% of the available nitrogen over several studies; winter wheat, scavenging nitrogen up to five feet deep in one study; annual ryegrass, scavenging between 43 and 60 pounds of nitrogen per acre in two different studies; and oats are also excellent nitrogen scavengers. One note of caution, however: Upon their mowing, crimping, flattening, tilling, or smothering, grasses tie up the nitrogen they scavenged from the soil in their tissues for a few weeks before they start to release it back to the soil as they decay. To account for this, plan to kill and/or work in grass-only cover crops a few weeks before planting your main crop, or plant grasses mixed with a legume (see “Produce nutrients” below).

Make nutrients more available and usable. Cover crops make certain nutrients more available to or usable by plants as well. Deeper-rooted cover crops, for example, pull up macro- and micro-nutrients from deeper in the soil—especially calcium and potassium. Buckwheat, among other cover crops, secretes an acid that makes phosphorus more soluble and, hence, into a more plant-usable form. The mycorrhizal fungi that grow with many cover crops extract phosphorus from the soil in an organic form that’s more readily used by plants and exchange it with the cover crops for carbohydrates. As cover crops tissues decay, all of these macro- and micro-nutrients are left in these usable forms in the rooting zone of garden plants, providing a boon for those that follow.

Produce nutrients. Some cover crops even take an extra step to actually create more nutrients. Legumes—clovers, peas, vetches, etc.—have nodules in their roots that house bacteria that convert extremely prevalent but unusable atmospheric nitrogen into a form usable by plants, in a process known as nitrogen fixation. Legumes are, thus, wonderful at adding nitrogen to the soil. While most of that which they fix is tied up in their own growth, it remains present in the soil after their demise, making it readily available for the garden plants that follow. This property of legumes makes them wonderful cover crops in mixes with grasses. The grasses scavenge and capture existing nitrogen before it leeches out of the soil; the legumes make more. Plus, the nitrogen in legumes cycles back into the soil faster than it does in grasses, making up for the nitrogen tied up for a few weeks in the grasses and giving the option of early planting after killing and/or incorporating the cover crop.

Boost yields. Boosting nutrient availability by capturing them, producing them, and keeping them in more-usable forms boosts yields. All of the nutrients cover crops scavenge, capture, and produce significantly boost soil fertility and, hence, crop health and productivity. Good soil grows good plants. Amazing soil grows amazing plants that are incredibly healthy and productive.

Cheaper. Because cover crops can reduce or entirely eliminate added fertilizers, they give gardeners a way to retain and add nutrients to their gardens without buying soil amendments of any kind, organic or chemical. What’s more, gardeners can also typically seed an entire garden for a few dollars or less. For both of these reasons, cover crops are less expensive fertilizers than purchased granular or liquid fertilizers of any kind, organic or chemical. Plus, cover crops offer their fertility in a way that is deep and lasting instead of that which is gone with the dissolution of a pellet.

No chemicals. With all of this, cover crops give gardeners a way to achieve tremendous yields—and grow amazing, healthy plants—without a single chemical of any kind. Although I grew up gardening this way—my Dad once told me, “It wasn’t called anything back then. No one used the term ‘organic.’ We just knew we wanted you kids eating healthy food without a bunch of chemicals put on it.”—when I was old enough I started a quest to see if the anti-organic naysayers were as biased, short-sighted, and off base as I’d guessed. In short, I wanted to see if I could grow garden crops without chemicals but still get yields at the highest of levels, competing with yields of conventional agriculture and gardening. Now, I’d never turn back. This way of growing didn’t just compete. It blew the doors off of conventional chemical gardening and agriculture. And it made me come to search for a deeper understanding of why. What were the keys that made this way of growing result in such markedly healthier and incredibly productive plants? Some of this insight is reflected in this post, chapter, website, and book. However, for the organic grower and also those just wanting a more life-sustaining approach for themselves, their children, and the rest of the life that surrounds us, cover crops are clearly part of a way of growing that offers a way to accomplish all of that which chemical additions accomplish and more that chemicals will never be able to offer.

No need for even typical organic fertilizers, foliar applications, and inputs. Cover crops give gardeners a way to add fertility to their soil without chemical, or even organic, fertilizers. Along with the droppings and bodies of all that roams, they are, in fact, the original and most profoundly beneficial way to add this fertility. Truth be told, back when I was testing whether or not this way of growing would compete with conventional agriculture and gardening, I also wanted to see if I could have that level of success even without the standard organic foliar applications and soil amendments. Cover crops, cured compost, gentle soil loosening, companion planting, great garden locations with plenty of sunlight, climates with adequate rainfall, and staying off the growing areas were my only real tools. It turns out that this way of gardening is amazingly productive. When our tomato plants were eight feet tall and three feet or more wide with more fruit on even just a few than we’d expect on a dozen—and all of our other garden plants were similarly, massively productive and healthy—we knew this was a way of gardening that out produces any conventional system.

It also became clear, however, that it also outproduces any organic system that uses the same paradigms as conventional chemical agriculture and gardening but simply replaces the chemical inputs with organic ones. It takes more than dumping “amendments” on the ground and insect killers on plants, organic or not, to work with a system larger than yourself. This way of growing gives us a way to garden that constantly heals the soil instead of robbing it or even just barely replacing what we use, that fosters diversity in the soil and surrounding ecosystem instead of destroying it, and that honors natural systems and cements our place in working with them instead of ignoring them. It builds life, diversity, resilience, health, and growth, like natural systems always have and always should. To get those results, we’d take a little less production, but we don’t need to. Our gardens are, instead, more productive than we ever could have imagined.

Provide homes, and possibly food, for beneficials. Where do the insects and arachnids, birds and worms, and millipedes and microbes go when there’s nothing to eat and nowhere to hide? They leave or die. Cover crops provide the habitat—the nooks and crannies to hide, hunt, sleep, and eat—that these wonderful beneficials need to survive and stay, so they’re there to be the constant protectors of your next crop and builders of your soil. Flowering cover crops, like buckwheat, even provide nectar sources that attract beneficial insects, like the predatory wasps, lacewings, and hoverflies that eat pests directly or lay eggs for their young to eat them. Be careful, however, not to let most cover crops flower for long enough to set seed (usually a week or), so you don’t create an unwanted weed problem. Still, cover crops can add to an otherwise dynamic approach to attract and retain beneficials that also involves longer-lasting nectar sources and shelter.

Protect soil from temperature extremes. Bare soil experiences temperature extremes in winter and summer—and even daily fluctuations—that are inhospitable for the multitude of life within it. The microbes, millipedes, sowbugs, pillbugs, stag beetles, earthworms and a multitude of other beneficial beings in our soil just don’t do as well in extreme temperatures or with large daily fluctuations. Being unable to endure the temperature extremes and variations found in bare soil during winter cold spells, for example, earthworms are especially vulnerable and either flee uncovered soil in winter or die. As their name implies, cover crops, in contrast, blanket the soil, their vegetation acting as an insulating layer for the soil that serves as a living mulch, moderating the temperature and making it hospitable for earthworms and the rest of the wonderfully beneficial life our soil contains.

Break disease and pest cycles. As long as they’re from a different plant family than the garden crops that will follow, cover crops also serve much like a crop rotation, breaking pest and disease cycles simply by not being susceptible to the same pests and diseases. Sometimes, you can see this in action. As the pests cling to or scurry away from garden plants as you pull them at the end of their season, you can see your pests’ lifecycles being short-circuited—especially if you crush those you find and leave the garden debris in a sealed garbage bag in the sun for a few days. Where will those who survive go? Your cover crop of buckwheat following early broccoli won’t provide any habit for cabbage worms, just like your cover crops of oats and berseem clover, annual rye and crimson clover, or forage radishes won’t provide any habitat for squash bugs and cucumber beetles.

Prevent and discourage weeds. One of the greatest innovations of plant species we consider weeds is their ability to produce seeds that stay dormant, and hence viable, until conditions are just right. Cover crops prevent and discourage weeds by providing shaded, cooler, vegetation-filled soil that prevents many weed seeds from sprouting in the first place. They also compete vigorously with any weeds that find a way to sprout, severely stunting them.

Give a solid next step. Finally, mentally, cover crops also give gardeners a solid next step, which is sometimes the impetus we need. We all sometimes let the remaining stumps, stalks, and leaves of garden plants stay in the garden for weeks after the part of the plant we eat has been harvested. In many ways, we’re working against ourselves, since we’re giving homes for that crop’s pests to complete their lifecycles. It, therefore, begs the question: Why would we do this? It’s hard to say, but I’d guess that sometimes it’s a sense of denial, not wanting to admit that a favorite season has faded. Possibly other times we think, “It’s better than nothing growing there,” and we’re right. 

On the other hand, if we know that we want to get cover crops going as soon as possible after another crop to get all of their benefits growing in our gardens, we’re more motivated to send our plant debris to the compost pile to become our future soil and get that section of the garden growing in something that adds even more to the garden’s health—especially since we know that our new set of cover crops will provide much better homes for our garden’s beneficial arthropods (e.g. insects, arachnids, sow bugs, and centipedes), annelids (earthworms), nematodes, and microorganisms that build our soil. One has to but walk both long-bare soil and soil in a cover crop to see the much greater amount of life present amongst the cover crops. The soil after one season in a cover crop feels springy and bouncy and is already darker with organic matter. Spiders scurry. Insects roam and buzz about. Earthworm burrows abound. The soil after one season of being bare, conversely, is already noticeably lighter in color with its loss of organic matter and seems well on its way to being comparatively devoid of life altogether.

Conclusion

Above are several reasons to grow cover crops. Yet, these are not their only benefits. Without any further scientific research, for instance, one could add, “Build soil health without killing the microbiota of the soil,” and go on to describe how chemical fertilizers leave salts in the soil that eventually kill soil microbes. One could, similarly, add that fungicides and pesticides kill soil microbes while cover crops foster them. However, the inner workings of soil and all of its biota are among the least understood aspects of science. The life-sustaining link between mycorrhizal fungi, soil, and plants was only relatively recently “discovered,” as just one example. What’s more, because they are part of mimicking a complex natural system that is hard for us to fully comprehend in its full complexity, there will always be more reasons to “discover.”

The lesson here seems to be that natural systems are both elegant and complex—and worth mimicking. Their inner workings may only be slowly teased apart by assiduous students over time, if all of them ever are. Still, observing their effects on the dynamism of life, we can mimic them long before we’ve overturned every minute rock of understanding and gain their lasting benefits for a system that is greater than us. We can put their powerful lessons to work in our gardens, and the complex inner workings of a natural system benefit our garden just as they’ve benefited the larger ecosystems for millennia.

For more on HOW to grow cover crops, check out this post.