Due to recent efforts by Cooperative Extension, the Natural Resources Conservation Service (NRCS) and many private groups, the phrase “cover crop” has become familiar to most vegetable farmers. It is now generally accepted that cover crops should be a part of any sustainable agriculture operation. For example, there is much information about how cover crops improve soil health and reduce the need for costly fertilizers. However, the focus here is on a lesser known role that cover crops play in the agro/ecosystem: pest management. Careful selection of cover crop species can help to fight pesky insects, diseases and weeds.
Following is a simple breakdown of which crops can be used in the spring and summer to battle uninvited guests. These include pigweeds (weeds often fed to pigs), nematodes (roundworms) and cabbage worms (a type of worm whose larvae eat cabbage and similar plants).
Weed Snuffers: Getting rid of weeds calls for a fast growing cover crop. It will quickly shade the soil and out-compete weeds. If a field is left fallow during the summer, a heat-loving grass, such as sorghum-Sudan grass or pearl millet, will choke out all but the toughest perennial (a plant with a life cycle of two or more years) weeds. The same crops can be seeded between plastic-covered raised beds. The cover crop should be mowed every so often to create a turf that feeds the soil while curbing weeds; it also creates a mud-free work area between rows. Sorghum-Sudan grass releases a chemical from its roots that acts like a pre-emergent herbicide; it prevents weeds from growing near it. A low-growing cover crop, such as buckwheat, can be used in the same manner along side vine crops that will not allow a mower between rows. Buckwheat will suppress weeds and diseases without competing too much with squash or melons.
Disease Eradicators: Cover crops help to fight plant disease in a few ways. First of all, cover crops with deep penetrating root systems help to improve drainage; this makes it hard for many diseases to thrive. Cover crops planted alongside cash crops help to protect plants from the damage caused by sun and wind. Less damage means there are fewer wounds where pathogens (agents capable of causing disease) can enter. Also, cover crops can help to reduce the splashing of soil onto cash crops, which is also a common route for infection. Lastly, some cover crops in the mustard family contain chemicals that are toxic to nematodes, diseases and even to small weed seeds. Some good examples of these bio fumigant (using natural aspects of a plant to reduce the number of weeds) cover crops are daikon (large, long, hard) radishes and mustards like ‘Pacific Gold’. With some careful management, these crops can work much like methyl bromide (a synthetic soil fumigant), with none of the environmental concerns.
Insect Annihilators: The relationship between cover crops and insects is a bit more complex than the relationship between cover crops and other pests. Unfortunately, there is no known cover crop that repels hornworms (certain caterpillars) or is toxic to cucumber beetles. But cover crops can help to kill bugs indirectly by providing resources for the predators that eat them. Many beneficial insects, both predatory and parasitic (feeding off a host), feed on nectar and pollen as adults; however, the less mobile larvae (insects in the juvenile stage of insect development) gorge themselves on aphids and caterpillars. Therefore, the key to attracting good bugs is having floral resources available at all times. Summer annual legumes, such as cowpeas and sunn (an Indian herb with strong fibers such as hemp), produce nectar both in their flowers and at spots along the stems called extrafloral nectaries (a gland that releases nectar). Fast-flowering buckwheat can be planted any time after the last spring frost; it can begin producing nectar and pollen in as little as 30 days. Many clovers, such as crimson and red clover, can support large numbers of predatory insects when they are in bloom. Borders and hedgerows are great out-of-the-way areas than can be used to plant flowering cover crops for beneficial insects without giving up field space.
So, the next time you are shopping for cover crops, get creative and think beyond the soil. Consider what you can do for the total farm ecology by cutting back on pesticides through the innovative use of cover crops that are compatible with vegetable production.
NOTE: This year, several of the cover crop options described above will be showcased by the Lincoln University Cooperative Extension (LUCE) IPM Program at the university farm field day at the Alan T. Busby Farm and late August for the Vegetable/IPM Festival at the George Washington Carver Farm).
(By Jacob Wilson, IPM Extension Technician)