Once you’ve chosen cover crops that fit your vegetable rotation, management goals, and garden site (See Part III: Selecting Cover Crops for Vegetable Gardens), it’s time to plant! This article contains tips on sourcing seed, and planting and managing cover crops using hand tools.
As I outlined in Part I and II of this series, cover crops can serve many purposes in small-scale vegetable gardens, including soil quality improvement, nitrogen (N) fixation, weed suppression, and habitat for beneficial insects. To achieve maximum benefits from cover crops, it’s important to select appropriate species (or species mixtures) for each garden bed. In this article I’ll highlight promising annual cover crop species for different seasonal niches, management goals, and environmental conditions. Much of this information is based on preliminary results from two seasons of cover crop research in Brooklyn, NY community gardens.1
Vegetable gardeners are turning to cover crops to improve soil quality, add nitrogen (N) to the soil through legume N fixation, suppress weeds, and attract beneficial insects in their gardens. In this article I’ll introduce several groups of cover crops. Cover crop species can be broadly grouped into non-legumes (those that do not fix N, but take up and recycle nutrients left in the soil) and legumes (which fix N). Mixtures of non-legumes and legumes may offer the benefits of both types of cover crops.
Non-legume cover crops
Non-legume cover crops include species in several plant families:
Buckwheat (Fig. 1c, Polygonaceae) is a popular summer cover crop.
Fig. 1. Examples of non-legume cover crops used in vegetable gardens(Photo credits: M. Gregory)
Fig. 1a. Oats (Avena sativa) is a winter-kill cover crop in USDA Zones 7 and cooler. It is usually planted in late August, and dies with the first hard frosts.
Fig. 1b. Winter rye (Secale cereale) is a hardy over-wintering cover crop. It can be planted in September or October, and produces large amounts of biomass by May.
Fig. 1 c. Buckwheat (Fagopyrun esculentum) is a fast-growing summer cover crop, suitable for planting between spring and fall vegetable crops.
Benefits of non-legumes: 1, 2
Prevent erosion – Non-legumes establish and grow quickly, provide rapid soil cover, and have dense, fibrous root systems that hold soil in place.
Build soil organic matter – Non-legumes produce large amounts of biomass, which contributes to soil organic matter.3
Retain and recycle nutrients – Non-legumes take up nutrients left in the soil after vegetable harvest, which prevents them from being leached out of the garden during heavy spring rains.
Suppress weeds – With their vigorous growth and high biomass, non-legumes can successfully compete with weeds, even in fertile soils. Some non-legumes (winter rye, sorghum-sudangrass, and Brassicas) also release chemicals that inhibit weed germination and growth. Residues of grass cover crops also provide a weed-suppressive mulch that lasts much of the growing season.
Disease management — Some Brassicaceae cover crops also release chemical compounds that may help control soil-borne pathogens and parasites(e.g., fungi, nematodes) upon incorporation. Winter rapeseed (Brassica napus) greatly reduced Rhizoctonia damage and Verticillium wilt in potato crops.1, 2
Drawbacks and constraints of non-legumes:
Slow nutrient supply and/or N immobilization — Non-legumes have lots of carbon (C) relative to N during growth, which causes them to decompose slowly after mowing or incorporation. As a result, nutrients in non-legume residues may not be available to vegetable crops quickly. If non-legume residues are incorporated into the soil, they may actually immobilize (“tie up”) N for a few weeks as decomposer microbes take up soil N to balance the large amount of C in the plant residues they’re breaking down.1 For this reason, it’s best to wait several weeks after incorporating a non-legume before planting vegetable crops.
Legume cover crops
Legume cover crops include field peas (Fig. 2a) crimson clover (Fig. 2b), hairy vetch (Fig. 2c), and cowpeas. They provide many of the same benefits of non-legumes, with the additional benefits of nitrogen fixation and feeding pollinators.
Fig. 2. Examples of legume cover crops used in vegetable gardens (Photo credits: M. Gregory)
Fig. 2a.Field peas (Pisum sativum) can be planted as a winter-kill or early spring cover crop. It should only be planted in full sun, as this legume performs poorly in shaded areas.4
Fig. 2b.Crimson clover (Trifolium incarnatum) over-winters in Zones 7 and up, and can be used as a summer or winter-kill cover crop in cooler zones. Crimson clover is a high biomass producer and is quite shade-tolerant.4
Fig. 2c. Hairy vetch (Vicia villosa) is the hardiest legume, and will over-winter in even the northernmost parts of the US. It is an excellent legume for adding fixed N to the soil.4
Additional Benefits of legumes:
Figure 3. A bumblebee visits a crimson clover flower in a community garden. Photo credit: M. Gregory.
Nitrogen fixation – Legume cover crops add ‘new’ nitrogen (N) to the soil through N fixation, which occurs when N-fixing bacteria in legume roots take N from the air and convert it to a form the plant can use. When legume residues break down, this N is added to the soil for food crops.5
Build soil organic matter and soil quality – While legumes don’t usually produce as much biomass as non-legumes, they also help build soil organic matter.6, 7 Legumes are also excellent soil conditioners, because legume roots ooze sugars that stick soil particles together in larger crumbs, or aggregates.8, 9 This helps the particles fit together loosely, making for a soft, porous soil.
Attract beneficial insects – Many legume species provide resources for beneficial insects. Crimson clover provides pollen and nectar for native pollinator bees (Fig. 3), and both crimson clover and hairy vetch host predators such as lady beetles, which eat many pest insects.1
Drawbacks and constraints of legumes:1, 3
Slow growth, lower biomass — Legumes establish and grow more slowly than non-legumes, and usually produce lower biomass.
Less weed suppression — Legumes may not suppress weeds as effectively as non-legumes, particularly in soils with high N fertility. In Brooklyn gardens, legumes suppressed weeds in soils with low to moderate N fertility, but not in soils with high N fertility.4 Legume residues break down quickly, so weed control by legume mulch may be short-lived.
Seed cost — Legume seeds are more costly than non-legumes.
Cover crop mixtures
Mixtures of non-legumes and legumes often combine the benefits of both types of cover crops.
Benefits of nonlegume/legume mixtures:
Produce large biomass and suppress weeds effectively — In many cases, cover crop mixtures provide more complete soil cover, greater biomass production, and more effective weed suppression than plantings of just one species.1, 3 This is because mixtures of grasses and legumes use water, nutrients and sunlight very efficiently due to complementary root systems and growth habits. Grasses (like rye) also provide support for viny legumes (like hairy vetch), which allows the legume to access more light.
Increase N fixation — Planting legumes with grasses may enhance N fixation. Grasses out-compete legumes for soil N, forcing the legume to rely on N fixation. As long as the grass doesn’t suppress legume biomass (see below), this can increase the total amount of N fixed. Promising grass/legume mixtures for N fixation include rye/vetch and Japanese millet/cowpea.10
Optimize nutrient cycling and nutrient supply to crops — Mixtures provide the benefits of N ‘scavenging’ by non-legumes and N additions by legumes.1 At maturity, grass-legume mixtures often have an ideal C:N ratio of 25:1 – 30:1, which promotes a steady release of N for vegetable crop use as the cover crop plants decompose. N-rich legume residues prevent N tie-up that can occur when incorporating pure grass residues, while C-rich grass residues slow the breakdown of legume residues such that N is released at a rate that vegetable crops can use through the growing season.2, 11
Fig. 4. Examples of grass/legume mixtures used in vegetable gardens (Photo credits: M. Gregory)
Fig. 4a.Oats/ Field peas is a common winter-kill or early spring mixture. It should only be planted in full sun. Since oats may suppress field pea biomass and total N fixed,4 try seeding the field peas at a higher rate.
Fig. 4b. Rye/ hairy vetch is an excellent over-wintering mixture. The hairy vetch ‘climbs’ the rye, which allows the legume to access more light. In Brooklyn gardens, rye/vetch mixtures produced the highest biomass of any cover crop combination.4
Drawbacks and constraints of nonlegume/legume mixtures:
Reduced N fixation if nonlegume out-competes the legume – Mixing a non-legume with a legume may decrease the total amount of N fixed if the non-legume suppresses legume growth and biomass. This occurs in mixtures of: oats/field peas,4, 12 rye/crimson clover,4 and sorghum-sudangrass/cowpea.10 Seeding the legume at a higher rate may result in a more even distribution of nonlegume and legume biomass – gardeners can experiment to find the relative seeding rate that works best in your soil.
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Understanding the benefits and limitations of non-legumes, legumes, and mixtures is a great starting point for selecting cover crops to plant in your garden. For guidance on choosing specific cover crops based on your vegetable crop rotation, management goals, and soil and light conditions, see Part III: Selecting Cover Crops for Vegetable Gardens.
(3) Snapp, S. S., S. M. Swinton, R. Labarta, D. Mutch, J. R. Black, R. Leep, J. Nyiraneza, and K. O’Neil. 2005. Evaluating cover crops for benefits, costs and performance within cropping system niches. Agronomy Journal 97(1):322-332.
(4) Gregory, M. M., L. E. Drinkwater. In preparation. Developing cover cropping practices to improve soil quality, nutrient cycling, and weed suppression in urban community gardens.
(5) Drinkwater, L. E. 2011. It’s elemental: How legumes bridge the nitrogen gap. The Natural Farmer, Summer 2011, pp. B-1 – B-6. Northeast Organic Farming Association, Barre, MA: Accessed online at: http://www.nofa.org/tnf/Summer2011B.pdf, 6 December 2014.
(6) Sainju, U. M., B. P. Singh, and W. F. Whitehead. 2002. Long-term effects of tillage, cover crops, and nitrogen fertilization on organic carbon and nitrogen concentrations in sandy loam soils in Georgia, USA. Soil & Tillage Research 63(3-4):167-179.
(7) Kong, A. Y. Y., J. Six, D. C. Bryant, R. F. Denison, and C. van Kessel. 2005. The relationship between carbon input, aggregation, and soil organic carbon stabilization in sustainable cropping systems. Soil Science Society of America Journal 69(4):1078-1085.
(8) Puget, P., L. E. Drinkwater. 2001. Short-term dynamics of root- and shoot-derived carbon from a leguminous green manure. Soil Science Society of America Journal 65(3):771-779.
(9) Haynes, R. J., M. H. Beare. 1997. Influence of six crop species on aggregate stability and some labile organic matter fractions. Soil Biology & Biochemistry 29(11-12):1647-1653.
(10) Drinkwater, L. E. 2011. A holistic view: Leguminous cover crop management in organic farming systems. The Natural Farmer, Summer 2011, pp. B-20 – B-24. Northeast Organic Farming Association: Barre, MA. Accessed online at: http://www.nofa.org/tnf/Summer2011B.pdf, 6 December 2014.
(11) Teasdale, J. R., A. A. Abdul-Baki. 1998. Comparison of mixtures vs. monocultures of cover crops for fresh-market tomato production with and without herbicide. HortScience 33(7):1163-1166.
(12) Schipanski, M. E., L. E. Drinkwater. 2012. Nitrogen fixation in annual and perennial legume-grass mixtures across a fertility gradient. Plant Soil 357(1-2):147-159.
Figure 1. Rye and vetch cover crop in a community garden plot in May, just before it was cut down and mulched in preparation for planting vegetables. Photo credit: M. Gregory.
Cover crops are close-growing plants sown in rotation with food crops, or inter-seeded between food crops to cover bare ground. They are not harvested, but rather are planted to improve soil quality and provide other benefits for crop production and the environment. Before planting the next vegetable crop, most cover crops need to be cut down. The shoots can be chopped (or mowed) and left as mulch on the soil surface, or incorporated into the soil.
There is a large body of research supporting the use of cover crops on organic and sustainable farms.1 However, vegetable gardeners can successfully plant and manage cover crops with hand tools, and reap the benefits of this practice for their soil and crops.2
Why should I plant a crop that I’m not going to harvest?
Cover crops provide many benefits for future vegetable crop production, and for the garden agro-ecosystem as a whole. Incorporating cover crops in vegetable rotations may:
Increase soil organic matter levels, and therefore improve soil quality. As cover crop roots and shoots decompose, they build soil organic matter. This improves soil structure and water-holding capacity (Fig. 2), and increases slow-release nutrient reserves.3 Fresh cover crop residues also nourish beneficial soil fauna (bacteria, fungi, worms, etc.) that improve soil tilth and aeration, recycle plant and animal wastes, and release nutrients for crops to use.
Provide nitrogen for future food crops through legume nitrogen fixation. Cover crops in the legume family (e.g., beans, peas, clovers, and vetches) add “new” nitrogen (N) to the soil. Legumes host N-fixing bacteria in bumps on their roots, also called nodules (Fig. 3). These bacteria take N from the air and convert it to a form the legume can use . When the plant decomposes, the fixed N also becomes part of the soil organic matter. Eventually, this N is released by microbes for crop uptake.4
Improve nutrient retention and recycling. Over-wintering cover crops take up extra nutrients at the end of the growing season, which would otherwise be lost to leaching (when nutrients dissolve in rainwater and drain below the root zone, making the nutrients unavailable for plants). Over-wintering grasses like rye reduce N leaching by about 70% compared to bare soil.5
Suppress weeds. Growing cover crops reduce weed growth through competition (e.g., for space, light, moisture, and nutrients) and allelopathy (releasing chemicals that inhibit other plants). After , the cover crop mulch can prevent weed seedling emergence through the growing season.6
Attract beneficial insects. Cover crops often provide important resources (such as nectar and pollen and over-wintering habitat) for beneficial insects, including pollinator bees and natural enemies of insect pests like ladybugs and lacewing.1
Increase or maintain crop yields with less inputs. Well-managed cover crops can improve vegetable crop yields, or reduce the amount of fertilizer needed to obtain good yields.7-10
Figure 2. Demonstration illustrating the effect of soil organic matter (SOM) on water-holding capacity. Photo credit: Megan Gregory
On the left is soil from an urban garden that received a rye/vetch cover crop for more than five years, and therefore has high SOM.
On the right is soil from a garden that never received cover crops, and has lower SOM.
This photo was taken 30 minutes after pouring equal amounts of water through the soils. The high-OM soil held most of the water, while much water drained through the low-OM soil. Since both soils were of similar texture, the difference in water-holding capacity can be attributed to the SOM.
Figure 3. Nodules on the roots of legume cover crops: crimson clover (left) and hairy vetch (right). The nodules host nitrogen-fixing bacteria in the genus Rhizobia, which convert atmospheric nitrogen into plant-available forms. Photo credits: M. Gregory.
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Vegetable gardeners have a number of cover crop options suited to different seasonal niches, management goals, and environmental conditions. To learn about the main groups of cover crops and how to select cover crops for your garden, see Part II (Types of Cover Crops) and Part III (Selecting Cover Crops).
(2) Gregory, M. M. and L. E. Drinkwater. In preparation. Developing cover cropping practices to improve soil quality, nutrient cycling, and weed suppression in urban community gardens.
(3) Snapp, S. S., S. M. Swinton, R. Labarta, D. Mutch, J. R. Black, R. Leep, J. Nyiraneza, and K. O’Neil. 2005. Evaluating cover crops for benefits, costs and performance within cropping system niches. Agronomy Journal 97(1):322-332.
(4) Drinkwater, L. E. 2011. It’s elemental: How legumes bridge the nitrogen gap. The Natural Farmer, Summer 2011, pp. B-1 – B-6. Northeast Organic Farming Association, Barre, MA. Accessed online at: http://www.nofa.org/tnf/Summer2011B.pdf, 6 December 2014.
(5) Tonitto, C., M. B. David, and L. E. Drinkwater. 2006. Replacing bare fallows with cover crops in fertilizer-intensive cropping systems: A meta-analysis of crop yield and N dynamics. Agriculture Ecosystems & Environment 112(1):58-72.
(7) Abdul-Baki, A. A., J. R. Teasdale, R. Korcak, D. J. Chitwood, and R. N. Huettel. 1996. Fresh-market tomato production in a low-input alternative system using cover-crop mulch. HortScience 31(1):65-69.
(8) Abdul-Baki, A. A., J. R. Stommel, A. E. Watada, J. R. Teasdale, and R. D. Morse. 1996. Hairy vetch mulch favorably impacts yield of processing tomatoes. HortScience 31(3):338-340.
(9) Abdul-Baki, A. A., J. R. Teasdale, R. W. Goth, and K. G. Haynes. 2002. Marketable yields of fresh-market tomatoes grown in plastic and hairy vetch mulches. HortScience 37(6):878-881.
(10) Abdul-Baki, A. A., J. R. Teasdale. 1997. Snap bean production in conventional tillage and in no-till hairy vetch mulch. HortScience 32(7):1191-1193.
