Winter Cover Crops for Plant and Soil Health
Avoid lugging manure or compost to the garden or field and save money on fertilizer by using winter cover crops instead. A cover crop, also known as green manure, is easy to grow if the right crop is chosen, and it is planted and killed at the right time.
Winter cover crops will not take any space out of production, and should only be used in beds that will be planted with warm season crops (tomato, pepper, summer squash, etc.) so that spring crops won’t be delayed in planting. They should ideally be seeded between mid-August and early October, depending on location. If the cover crop is planted late, the seeding rate should be increased. Most crops need about 30 to 40 days to germinate and grow before a killing frost.
Hairy vetch (Vicia villosa) is a hardy, viney, moderately drought tolerant legume that fixes nitrogen. Vetch will survive winter temperatures to -20°F. It should be planted in early September at a rate of 1-2 lb/1000 ft2. The seed requires inoculation by Rhizobium bacteria, which comes as a dry peat-based powder and is shaken with the seeds just before sowing.
Vetch will begin to grow in the fall, with the majority of growth in the spring. The longer it is allowed to grow, the more nitrogen it adds to the soil. Hairy vetch residues decompose rapidly and release nitrogen more quickly than most other cover crops.
The downside of vetch is that it may be difficult to remove or till. Its heavy growth may clog a rototiller. Several low mowings or cutting the tops off at the crown with a sharp hoe or machete can also kill the vetch. It must be killed before it seeds or it will become weedy.
Austrian Winter Pea
Austrian winter pea (Pisum sativum subsp. arvense), also known as field pea or Canadian field pea, is an annual that is only hardy to 10°F, and may die in winter in northern Utah. This plant will establish quickly (planted at 2-4 lb/1000 ft2) in fall, and produce a significant amount of residue before winter. The stems will grow up and over other plants, so it is best to combine it with an upright winter grain. In spring, if the plants have not been winter-killed, pull them up when they are about 2 feet high, and compost or turn under.
Winter rye (Secale sp.) is an annual grain that is cold hardy and vigorous. Rye seeds are cheaper than legumes, and tolerate a later planting at 2.5 lb/1000 ft2. Rye germinates quickly, and grows longer in fall and earlier in spring than other grains.
A winter cover crop can:
Rye’s deep and fibrous rooting system helps to build soil organic matter and is excellent for breaking up soil particles and preventing compaction. It also has excellent weed preventing benefits. The decomposing mulch not only smothers weeds, but also chemically suppresses weed seeds through allelopathy. Vegetable seeds can be planted within 3 weeks, and transplants are not affected by the allelopathic chemicals.
In spring, rye should be killed via removal or top-kill herbicide when it is about 6-8” tall or killed with mowing at flowering. Do not let it go to seed or it will become a weed. To kill by mowing, wait until the anthers are extended, and pollen falls from the flower heads when shaken. The residues are high in nitrogen and can be left on the ground or composted.
Some gardeners will mix rye with a legume to get the properties from both crops. The grain will provide an upright support, allowing for better growth of the legume and easier killing by mowing in the spring. If the crops are to be mixed, a little trial and error may be necessary to optimize the seed ratio to prevent the grain from outcompeting the legume. In general, rye should be seeded at about 40% of full rate and the legume should be closer to 80% of the full rate.
Pacific Gold Mustard, Rapeseed, Forage Radish
Pacific gold mustard, rapeseed (canola), forage radish, and other brassicas have received much attention due to some species’ biofumigant properties that may suppress soil-borne pathogens and plant-parasitic nematodes. Results in research trials have been inconsistent, and more research is needed to clarify under what conditions the plants’ chemical compounds work. Most brassicas are probably best utilized in a long rotation to get the full effects, rather than as a winter cover crop.
Forage radish is one brassica that some are trying as a winter cover crop. It will die in northern Utah winters, but it grows quickly in the fall, “trapping” soil nutrients for release the following spring, thus preventing nutrient leaching and adding soil organic matter. Because the radish dies back, there is no worry about killing it in the spring, or waiting for it to decompose. In addition, its long taproot breaks through compacted earth, leaving the soil in friable condition and improving water and root penetration for the next crop.
Killing the Cover Crop
Although herbicides or tillage are often used to kill cover crops before planting the main crop, a well-timed mowing is just as effective. Backyard gardeners may instead choose to remove the plants entirely and compost them or use them as mulch. Another option for cover crop removal that some farmers have used is to pen chickens in the field to feed on the crop until it is gone. (Note that seeds of hairy vetch are poisonous to chickens.) Unless the plants are pulled out, wait 3 weeks before planting the vegetable seed or transplant crop to lessen problems of nitrogen immobilization and allelopathic effects.
There have been many research reports on the effects of cover crops on pests. Most pest management benefits have been documented from the use of long rotation cover crops rather than a seasonal winter rotation. Much information about pest management benefits from winter cover cropping is anecdotal, and often difficult to replicate.
In general, the primary pest benefit is against weeds. A heavy surface residue suppresses perennial weeds, and the allelopathic effect of some crops suppresses weed seeds. An indirect effect of winter cover crops is through the increased health of the soil (increased organic matter, nitrogen, and friability) leading to improved growth and vigor of crop plants.
Winter cover cropping is not for every farmer or field, but in the right situation, can be an important tool in nutrient management, and indirectly, pest management. First timers should start small to determine the crop that works best and to prevent possible pitfalls such as generating an unintended weed problem if the cover crop is allowed to go to seed.
For a video on cover crops, visit the Peaceful Valley Farm and Garden Supply website.
-Marion Murray, IPM Project Leader
Featured Picture of the Quarter
Watermelon Mosaic virus was fairly widespread in northern Utah this season, shown here on squash. Some growers had to plow their crops under because of the disease, which has no cure. Watermelon Mosaic virus is transmitted by aphids, and as the name implies, causes a mottling of foliage called mosaic. Fruit symptoms can range from subtle color change to severe deformation. For more on viruses and their vectors, see the article on page 8.
-Image by Claudia Nischwitz,
Extension Plant Pathologist