This article is part of a four-part series about cover cropping in vegetable gardens. Stay tuned next week for Part II
- Part II: Types of Cover Crops — Non-Legumes, Legumes, and Mixtures (oh, my!)
- Part III: Selecting Cover Crops for Vegetable Gardens
- Part IV: Planting and Managing Cover Crops in Vegetable Gardens
What are cover crops, anyway?
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
- 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.
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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).
(1) Clark, A. 2007. Managing cover crops profitably, 3rd ed. Sustainable Agriculture Network, Beltsville, MD. Accessed online at: http://www.sare.org/Learning-Center/Books/Managing-Cover-Crops-Profitably-3rd-Edition, 7 December 2014.
(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.
(6) Schonbeck, M. 2011. How cover crops suppress weeds. Accessed online at: https://www.extension.org/pages/18524/how-cover-crops-suppress-weeds, 6 December 2014.
(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.