November 12, 2017

Cover Crop Considerations: Meeting Your Gardening Needs

Southwest Yard and Garden |  by Marisa Y. Thompson

Cover crop mix of Triticale, oats, and white
clover. Planted in the last week of October
at the Los Lunas Ag Science Center
(photo by M. Thompson, taken 11/9/17)

Question:
We want to prep our garden area for the winter. What cover crop species do you recommend?
-        Jeff G., Belen, Valencia County, NM
Answer:
All of the NMSU Extension researchers I invited to give advice emphasized that the first step when considering a cover crop is to identify your management priorities. Are you planting this to reduce soil erosion, improve soil structure, build organic matter, or suppress insect pests, weeds, and diseases? There are many benefits to cover crops, and your selection(s) will depend on your end goal. 
For single cover crop species, Dr. John Idowu, NMSU Extension Agronomy Specialist, offered, “We have had success with wheat, rye, and barley cover crops. Oats are also a good option, but they may winterkill in northern parts of NM, depending on the severity of winter. In our research, we have seen effective ground cover and weed suppression with the winter cereals. They also produce considerable biomass, which can improve the soil organic matter when the plants are terminated and returned back to the soil.”
Dr. Stephanie Walker, NMSU Extension Vegetable Specialist, pointed to the use of legumes, such as vetch and clover, which can increase the nitrogen content of the soil, especially if they too are terminated and worked back into the soil before flowering and producing seed of their own.
NMSU Extension Weed Specialist, Dr. Leslie Beck, explains how cover crops can help control weedy take-over: "The use of cover crops can be very effective in suppressing weed germination and population establishment. The more dense and healthy the cover crop, the more it is likely to outcompete small germinating weeds for space, water, nutrients, and light... survival of the fittest! However, it is important to try and establish these cover crops as quickly as possible, and ideally prior to the ideal soil temperature and moisture ranges that trigger weed seed germination in the fall and the spring. If the cover crop is trying to establish from seed at the same time as summer or winter annual weeds, most likely the weeds will be able to outcompete the cover crops. As a result, you probably won't get the levels of weed suppression that may have been one of the primary reasons for planting a cover crop in the first place." 
Cover crops with penetrating root systems, like some of the brassicas (mustards), will help improve the permeability of hard, crusted soil. Cover crops can also serve as a catch crop, meaning they take up and hold nutrients left in the soil following a vegetable crop. Cover crop mixes, where seed from legumes, brassicas, and/or cereals are mixed together, may also be used so that beneficial impacts of the different types of cover crops can be combined.
NMSU Sustainable Crop Production Extension Specialist, Dr. Kulbhushan Grover, says, “Hairy vetch and winter rye is a popular cover crop mix with farmers. There is also a cocktail (sold as ‘fall mix’) that includes hairy vetch and winter rye, plus field peas, ryegrass, and crimson clover.”
Dr. Walker warns, “Caution should be taken that cover crops do not become a weed issue themselves, especially in warmer parts of the state where many will easily overwinter. For optimum impact, particularly in cooler parts of the state, cover crops must be planted early enough in the fall so that there is enough time for plants to grow to a helpful size. Brassica cover crops reduce germination of weed seed, but may also hurt vegetable seed. The use of vegetable transplants should be considered following a brassica cover crop.”
NMSU Extension Plant Pathologist, Jason French, suggests, “It is important to know what disease problems have occurred in the past. You do not want to plant a cover crop that is going to exacerbate your disease problem. Knowledge of past problems or common problems in a certain area will help you make the best cover crop selection.”
French added, “When soil conditions are poor, soil-borne diseases can be particularly damaging. Soil compaction, inadequate drainage, low organic matter, and poor fertility can predispose plants to infection. Cover crops can be used to improve these soil conditions and decrease the incidence and severity of the disease problems. Cover crops can also suppress plant pathogenic fungi and nematodes. For example, certain brassica cover crops contain high levels of glucosinolates, which act as natural antimicrobials when tilled into the soil. Still another use for cover crops is the promotion of biodiversity in the soil. Many of these organisms will compete with plant pathogenic microorganisms for resources. This competition can, over time, reduce pathogen populations.” 
Many cover ‘croptions’ are available. Drs. Idowu and Grover published a helpful Extension Guide, titled “Principles of Cover Cropping for Arid and Semi-arid Farming Systems” (Guide A-150), that details the reasons for cover cropping, challenges in our environment, and species selection.
Cover crops are a great tool for controlling weedy species, reducing erosion, and enhancing soil health in large- and small-scale gardens. Thanks for sending in an excellent question for this week’s column.
Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook page (@NMDesertBlooms)
Please copy your County Extension Agent and indicate your county of residence when you submit your question!

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page at http://aces.nmsu.edu/pubs/_h/

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