The Joys of Cover Cropping
The Miracle of Soil Fertility (Part 1) | Cover Crop Strategies and Species (Part 2)
Cover Cropping Strategies and Species (Part 2)
We all know that cover cropping will do marvelous things for the fertility of our garden soil. But how to pull it off? Of course, if we have the space for it, the easiest strategy is what I call the “dueling gardens” approach: Maintain two garden spaces. Put one of them in a cover crop for an entire year, while growing crops in the other. The following spring, switch.
But most gardeners cannot allow that much space to gardening. So we have to come up with strategies for frequent and effective cover cropping in the same spaces where we’ll be growing our harvest crops—often a fiendishly complex challenge. Gardeners who like jigsaw puzzles are going to love cover cropping. Perhaps a better analogy is chess, since we have to plan many moves ahead, in both time and space, using playing pieces (crop species) capable of different moves. Fortunately, we have even more “pieces,” each with an array of potential “moves,” among cover crop species than on a chess board. Repeat after me: This is going to be fun!
Starting the crop
Eliot Coleman advises: Give the same care to starting your cover crop as you do to planting harvest crops—and that is good advice. Prepare a good seedbed, scatter small-seeded types (the size of clover seeds up to small grains), rake in, tamp down to ensure good seed-to-soil contact, and water in well. With larger seeds like peas, make furrows, sow seeds in the furrows, cover and tamp to firm the soil. Whether making rows for larger seeds, or scattering smaller seeds, be generous with the seed, and aim for much closer spacing than for garden crops.
Whether starting small- or large-seed crops, I like to put down a thin mulch such as straw to conserve moisture, especially in that critical top quarter-inch in which small seeds germinate. Check the soil each day and water as needed to keep it moist. Once the seeds germinate, they put roots into the deeper levels of the soil rapidly—indeed, this reach for assured water supply takes precedence for the moment over growing leaves—and afterwards, frequent watering will not be needed.
That’s the ideal way. Inevitably you’re going to feel rushed by the season and want to get the cover crop in “on the cheap”—just scatter the seed and assume that, since no one “plants” and pampers seeds in the wild, they have a good chance of establishing. Don’t tell anybody, but that’s how I sometimes do it. In this case, though, it is especially important to use a light mulch to keep the seeds themselves—not just the soil surface—moist enough to germinate. It helps if you soak the seed overnight before sowing. As with many lazy-gardener shortcuts, such a strategy can mean spending more time on the project overall, since you’ll probably do more frequent waterings until the plants establish. But in some cases, it is worth the additional effort, spread out over a number of days, in lieu of putting off the planting of a cover. Do note that some species are easier to start using this “impatient ”strategy; and that it is more likely to succeed in the spring or fall, when soil moisture is naturally higher and temperatures less extreme. Starting cover crops in the summer is more challenging, and less forgiving of haphazard methods.
The turning of the great wheel
Cover crop species vary first of all with regard to their relative cold hardiness, and how well they handle drought, so crop choice will depend on the point in the rolling of the four seasons.
There are many cover crop species that are quite cold hardy, and thus are ideal in early spring, long before the last killing frost. Rye or peas, for example, can be planted as soon as the ground is thawed. All the small grains are cold hardy—oats can be sown as early as mid-winter. Many cruciferous crops like mustard and turnips can be sown early in the season.
Because there are so many cover crops appropriate to spring, keep in mind this strategy for doing a lot of cover cropping: Think about those beds which will have no early crops, but in which you plan to plant warm-weather crops like squash or tomatoes—maybe you’re even reserving beds specifically for fall crops. By all means, plant those beds to cover crops for the intervening part of the season, however long or short. For example, I can plant field peas (Pisum arvense) as soon as the ground is thawed—say around March 1, if not earlier. I don’t plan to set out my pepper and tomato starts, or plant the winter squashes, until mid-May. So I have well over two months to grow a cover of field peas in that bed before it is time to plant these warm-weather crops. What a wasted opportunity if I simply let those beds sit idle until the season is sufficiently advanced.
Many of the cold hardy cover crops do not do well when the heat of summer comes on; but fortunately, there are alternatives which will germinate in the warmer and drier soil, thrive in the hot sun, and tolerate drought: a number of species of beans and millets, a sorghum/sudangrass hybrid, and more. An especially useful summer cover is the easy-to-start, astoundingly fast-growing buckwheat (thirty days from seed to flower in my garden).
Frosty weather returns in the fall, so we rely again on our friends the cold hardy cover crop species. All the small grains are especially useful as fall covers. Clovers give a lush cover if planted early enough. In earlier times, farmers grew fall crucifers like rape, kale, and turnips, onto which they turned pigs ready for fattening in preparation for fall slaughter.
Perhaps the most critical time to insure protection of the soil with cover crops is during the winter, when bare soil would be sorely abused by wind and rain.
There is a catch, however: As we get deeper into fall, the decreasing day length imposes limits to growth that are more critical than the lower temperatures. Some species are more useful than others for making a good start as we move toward the greater darkness of winter. Clovers, for example, might well make a tight cover if we sow at the end of summer or early fall; but may either remain dormant over winter, or fail to make a dense, soil-protecting cover, if sowed too late.
So in beds that were harvested well into the fall, we need to think of the queens of the fall cover crops: rye and vetch (often planted together). They start in colder soil than perhaps any other candidates, and will establish a protective cover over the winter soil. When the longer days of late winter arrive, they draw from the reserves stored in the roots, and come on with a rush of green.
Undersown cover crops
If you “just don’t have the room” for cover crops in the garden, look for possibilities for growing cover crops right in the same space as harvest crops. Any crop with a small “footprint” on the bed—trellised tomatoes and pole beans for example—are excellent candidates. Simply sow the cover—before, simultaneously with the main crop, or later, depending on the nature of the cover crop used—to enjoy all the benefits of a cover crop on the bed, while simultaneously growing the main crop to harvest. Dutch white clover is the cover I use most often in this application: It comes up really fast and makes a dense cover, therefore suppresses weeds and retains moisture; is low-growing, so doesn’t interfere with working or harvesting the bed, and sets nitrogen in the soil for the whole season—what more could you ask of a hard-working garden ally? (An alternative “hole-in-the-cover” strategy: In a bed with an established cover like Dutch white clover, you could simply cut planting holes for transplants such as trellised tomatoes, or a narrow band of cleared soil for direct-sown crops like pole beans down the center of the bed, and keep the existing cover in place.)
A crop such as corn or sorghum may have a small footprint on the bed, but it casts a lot of shade. A useful undersown cover for such crops should therefore be shade tolerant. I’ve grown cowpeas below corn and sorghum. The cowpeas do not grow as lush as in full sun, but they do add to soil organic matter and fix nitrogen.
It’s possible to start a following cover crop even before harvest of the current crop. Imagine a crop of spring broccoli whose harvest will be complete in two weeks. Sow the cover crop under the broccoli plants. Germination time will vary by species and point in the season, but during the germination phase, the reduced sunlight under the broccoli will not be a limiting factor—indeed, the enhanced moisture and cooler temperatures will assist germination. When the broccoli plants are removed, the newly emerged cover crop plants will surge. Cover crops like clover, some species of which can tolerate shade, could be sown even earlier in relation to completion of the broccoli harvest.
Winter-killed cover crops
One of my favorite cover-cropping tricks is growing a thick, abundant mulch in place. A good example is my strategy with asparagus, which is both a heavy feeder and benefits from a weed-suppressing, moisture-retaining mulch in spring. Around the third week of September I cut down and remove all the asparagus fronds. I don’t do so any earlier, or the asparagus would be stimulated to put up a lot of new shoots, which would deplete reserves needed for next spring. But I don’t want to wait any later, because my cover crops have to have time to come on strong and lush.
I then sow in the beds a mix of oats and field peas (which remember will set nitrogen in the beds for the heavy-feeding asparagus). Both these species are quite cold-hardy, and sail through the heavy frosts of fall, growing knee-high in a tangled mass of green. But when the ground actually freezes (to a depth of half a foot or so in my area), these plants reliably winter-kill—that is, they lie down in a thick mulch that continues to protect the soil for the remainder of winter, and helps retain soil moisture and suppress weeds come spring.
A further refinement of this strategy is to save space in other spring-crop beds by planting lettuces, chicories, spinach, raab, and other early greens along the edges of the asparagus beds. These crops as well benefit from the mulch and the enhanced fertility. After the asparagus harvest is complete, the growing fronds shade heat-sensitive lettuces for continued production into the summer season.
Mix ’em up
As already hinted above (peas and oats for a winter-killed “mulch grown in place,” rye and vetch for covers late in the season), it is often good strategy to grow a mix of species to make a cover. The shade-tolerant cowpeas, for example, can be grown with millet, to help replenish nitrogen taken up by the millet. Small grains and other grasses can be sown in mixtures with legumes like beans, peas, and clovers. Crucifers like rape, turnip, mustard, and forage radish make excellent mixed covers.
Sometimes one partner in the mix is grown as a “nurse” for the other. Oats can nurse clovers in the spring: The fast-growing oats help the shade-tolerant clover get a good start through moisture enhancement and weed suppression, then die back after the hot weather comes on, leaving the field to the clover.
I used buckwheat to nurse a planting of alfalfa which I sowed in late summer. The buckwheat kept the soil more moist and suppressed weeds to help the slower-growing alfalfa establish. The buckwheat died with the first touch of fall frost, leaving the perennial, cold hardy alfalfa to grow a dense cover over the soil before going dormant for the winter.
Preparing the way for heavy feeders
We have likely been told that we should add purchased “fertilizers” as a side-dressing around heavy feeders like corn, squash, and alliums (onions and garlic). But why not grow our extra fertility right in place, even before planting? Whenever possible, I precede heavy feeders with a leguminous cover crop. Cowpeas, for example, are an excellent cover to grow in preparation for fall-planted garlic and shallots, or brassicas like cabbage and broccoli. I’ve had my best successes establishing new beds of strawberries if I grow a winter-killed cover like cowpeas or field peas in the bed the previous season.
Preventing too big a “flush” of nutrients
Remember what was said above about the quick action by bacteria to break down fresh green material that is turned into the soil. Sometimes the resulting “flush” of nutrients can exceed the capacity of the various players in the soil food web to absorb, with the danger that—as with highly soluble chemical fertilizers—the more “mobile” compounds will leave the system (through leaching to groundwater or volatilizing to the atmosphere) before they can be taken up into the bodies of soil community members and saved long-term in the soil bank. The possibility of such leaching could be another argument against turning a cover crop into the soil, as with a power tiller. Release of nutrients will be slower if the cover crop plants are on top of the soil, as a mulch, or under a mulch.
If you have to deal with a situation where you feel a flush of nutrients (large amounts of fresh green material or manures incorporated into the soil) creates the danger of leaching and potential nutrient loss, plant a “catch crop” (rye planted as an over-winter crop is a good example) that aggressively “scavenges” or “sops up” excess nutrients, preventing their loss from the soil and retaining them for future availability when the catch crop itself dies.
The Cast of Characters
There is a tremendous range of cover crop species, each with its own gifts and services to offer, its unique ways of fitting into the puzzle. Don’t get stuck in a rut with your cover cropping, using just the few species and strategies you’ve become comfortable with. Diversity is the key to natural gardening and ecological health—use the greatest range of plant families possible for the broadest range of agricultural and ecological benefits.
We’ve already met some of the cover crop allies we might use, in the discussion of cover crop strategies. Let’s consider the possible cast of characters in more detail.
Legumes: beans and peas
This whole group of cover crops will set nitrogen in the soil and can be allowed to grow to maturity for seeds that feed livestock and/or people. The main limiting factor when choosing among them is “fit” with the season. Generally speaking, the beans are more suited to warmer weather; peas, to cooler. Exceptions are fava beans (Vicia faba), which can be planted as early in the season as peas; and field pea (Pisum arvense), which, unlike its cousin the garden pea (Pisum sativum), can be grown throughout the growing season. The common garden bean—Phaseolus vulgaris, in all its variants—is probably not suitable for cover cropping (though does set nitrogen in the soil, to the advantage of following crops). Soybeans are a possible summer cover crop (if you plant seeds that are not genetically modified). My preferred alternative to soybean for summer planting is the cowpea group (Vignus unguiculata subsp.), all of which yield lush, fast-growing cover crops that start easily and thrive in the hot weather and do well with drought, set lots of nitrogen in the soil, and can provide an abundant crop of black-eyed peas or crowder peas (among other local names) for both people and livestock such as chickens and pigs.
Legumes: clovers and alfalfa
Clovers come in many “flavors”—annuals, biennials, and perennials—suited to a variety of climates, soil types, and cropping strategies. Check with your seed source or county Extension Agent for advice about which will fit your needs. Clovers and alfalfa take longer to become established than many other cover crops, so you should plan to devote a piece of ground to these crops for a longer part of the growing season. If you can afford the space, dedicating an entire bed to one of these crops for a whole year is a good strategy. Do remember their potential as undersown covers, especially a low-growing species like Dutch white clover. These are among our most valuable cover crop choices.
All of the small grain grasses—wheat, rye, oats, barley—provide excellent service as cover crops. All are cold hardy, so fit with either fall or spring cover cropping schemes. (Note that some—wheat is a good example for my area—may not be suitable for planting in both spring and fall for harvesting grain, but all can be used both spring and fall as a cover crop.) All are annuals, so will die back after maturing their seeds, and thus do not necessarily have to be killed with additional labor from us. All are excellent candidates for providing self-harvested feeds to livestock, in addition to their roles in protecting and feeding the soil. Biomass production in the root zone can be quite large, with excellent enhancement of soil texture. Large amounts of root also mean faster accumulation of carbon (humus) in the soil. Less common grass species are also valuable combination cover/grain crops. There are millets of several species that serve the same roles as the more common agricultural grains, especially in difficult production environments. They grow well in summer, produce large amounts of biomass for the soil, and if grown to maturity produce nutritious seeds (with a protein content about the same as wheat) to feed people and livestock. Various millets, for example, would be good cover crops to mature and then turn in chickens to harvest all that free food, and to till in the cover crop.
Other grasses are useful as cover crops. When choosing species, be careful to avoid perennials such as fescue and perennial rye—while appropriate for permanent covers like pastures, they can be difficult to kill in more temporary cover cropping schemes. I have found “annual rye” or “Italian rye” (Lolium multiflorum) an especially useful grass cover—it is easy to grow, palatable to livestock, and suppresses weeds well. (Do note, though, that it can itself become a “weed” for following crops if allowed to set seeds.) This coming season I will be planting a sorghum/sudangrass hybrid (Sorghum x drummondii) for the first time. This is an excellent grass for summer (when most other grasses slow down a lot). Growing five to seven feet tall, it produces a tremendous amount of biomass, both above and below the soil; and will regrow after cutting, so can be used for mulches, hay, and warm-weather livestock forage. My ducks and geese love it.
Some of the common crucifers grown for people food also serve well as cover crops. Turnips, mustards, kale, and rape have a “cleansing effect” on the soil, ridding it of fungal diseases and parasites, such as root knot nematode. All are easy, fast-growing crops, and make excellent green forage for poultry, pigs, and other livestock. Grow them spring or fall—none of them do well as a hot-weather crop.
Buckwheat (Fagopyrum esculentum) is one of the most useful of all summer covers. As the “instant cover crop,” it is probably the best choice as a “filler” between harvest of a spring crop and planting of a fall crop. Though it does not provide as much biomass as many alternatives, its rapid rate of growth and dense canopy suppress weeds well. The stems are hollow and fragile, making this crop an easy one to sweep aside in preparation for other plantings, either with a rake or even by hand. If I send in the chickens to do the job, the buckwheat seems to melt away over the course of a couple of days. Poultry love the seeds, whether green or mature. Buckwheat is extremely sensitive to cold, dying back with the first touch of frost. But we can actually use that fact to advantage, as in the use of buckwheat as a fall nurse crop described above. A somewhat more cold hardy variant is tartary buckwheat (F. tartaricum), with mature seeds of higher protein content than common buckwheat. The seeds of both species have a long history of use as food for both people and livestock.
Don’t forget roots as soil-improving cover crops. Any root crop—the deeper it grows into the soil the better—has a loosening effect on the soil. But leaving the roots to decay in the soil after the plants die has an even more dramatic effect on soil texture. I highly recommend oilseed or forage radish (Raphanus sativus var. oleiferus), a variant of garden variety daikon. Seeds are much cheaper, and the long white roots are edible, if not quite as refined as garden daikon. The long taproot grows into the subsoil depths to “mine” them of minerals to put into service in the more shallow layers of the soil profile. The roots are also reputed to help control nematodes, weeds, and soil diseases during decomposition. The large roots of mangels (or mangolds or mangel-wurzels, a variant of common garden beet, Beta vulgaris)—up to ten pounds or more—as well loosen the soil while producing large leaves for harvest even during growth for feeding poultry and livestock. The roots store easily and make good winter feed for cattle, pigs, goats, and chickens.
Expanding the circle
There are many other species we can put into service to protect the soil, temporarily or permanently—the main limit on our strategies being the limits of our imagination. For example, I plant comfrey under my nut and fruit trees as a permanent cover that is superior to grass. I also plant comfrey along a fence line with an adjacent property, to help prevent incursion by vigorous vines like honeysuckle and poison ivy. Do note that a comfrey planting should be considered permanent: With thick fleshy roots that go eight to ten feet into the deep subsoil, comfrey will not depart the plot you’ve chosen to plant it in without a serious fight. Be assured, however, that it is neither dispersive (scattering seeds far and wide, like dandelion) nor invasive (spreading by runners or stolons, like witch grass)—if you plant comfreys of the correct type: Some comfreys do set seeds. Superior garden clones, however, no longer have the ability to set viable seed. (They do flower, however, and so help support pollinators, especially small bumblebees.) My preferred source for live comfrey starts is Richter’s Herbs in Canada, offering two of the Bocking clones, #4 and #14. Start with half a dozen plugs of either or both. Once established, comfrey is child’s play to propagate for more extensive plantings.
Marigolds help control parasitic nematodes. I plant them thickly as a ground level cover at the feet of trellised tomatoes. Fenugreek (Trigonella foenum-graecum) is well known as a spice, but is also useful as a nitrogen-fixer (it’s in the Fabacae or bean family), soil-cleanser, and source of forage (for both pollinators and livestock).
In my opinion, even a cover of weeds is superior to bare soil. Though we’re conditioned to think of “weeds” as “the bad plants” collectively, we should wonder whether any species with the potential to flourish as part of our backyard ecology is worthless, is “the enemy.” Might a “weed” be best defined as a plant we haven’t yet developed an alliance with? Many of the plants we call “weeds” spring up almost instantaneously in parts of the ecology that are disturbed. Their work is to cover the exposed soil and protect the life in it—they are healers.
Some “weeds”—wild chicory comes to mind—make palatable and nutritious green fodder for livestock. (My geese love it.) Others are aggressive scavengers of soil minerals, with various species “specializing” in particular minerals. As such, they can enhance available minerals for following crops. Especially deep-rooted “weeds” like dandelion and stinging nettle are so efficient at bringing minerals up from the depths that they are classed as “dynamic accumulators,” with much to offer in a soil fertility program. As a bonus, though the latter two plants are almost universally cursed as super-weeds par excellence, both are so nutritious as “spring tonic” cooked greens that herbalists sometimes classify them as “medicinal,” sometimes as “superfood.” (Yet another bonus: Nettle’s emphatic sting can be used to relieve arthritis pain, as are bee stings.)31
We all love pulling that carrot, startling against the dark soil, or picking that perfectly ripe tomato, gleaming in the sunlight. But I love growing cover crops as well. They pull me into the greater Garden around me—the complex interplay of all species possible in my immediate ecology, and especially of those at work beneath my boots. To grow cover crops is to enter intimately into Creation itself, to engage the soil in an act of love.