Chapter 6

Cover Crops for Organic Farms

Managing Pests with Cover Crops

Cover crops not only play an important role in slowing erosion, improving soil structure, and providing fertility, but also play an increasingly important role in managing pests—insects, diseases, weeds, and nematodes. Cover crops, if properly selected and managed in an organic farming system, can be used to enhance the environment to control pests. Success depends on properly matching cover crop species with the cash crops and anticipated pest threats.

Cover Crops for Managing Insects

Cover crops can provide food and habitat for beneficial insects. Table 7.3 lists beneficial and pest insects that are attracted to or harbored by common cover crop types. It is important to note that an increase in beneficial insects does not always translate into a general decrease in insect pest species. Each cover crop provides different resources and habitats that may encourage some species and not others. The simplest strategy is to plant cover crops which favor more beneficial insects than insect pests. A high diversity of cover crop species can also help assure that there is habitat for predatory insects at all times.

Managing Cover Crops to Encourage Beneficial Insects

The success of cover crops in preserving or encouraging beneficial insects depends in large part on how the cover crop is going to be destroyed or suppressed so that crops can be planted. The frequency and intensity of disturbance is important to the stability of populations of pests and their natural enemies.

Cover Crops for Managing Diseases

The impact of cover crops on pathogens—agents in the soil, such as bacteria or viruses that cause disease—can be good, bad, or nonexistent. This impact varies broadly depending on individual circumstances and situations. A cover crop can act as a host for soil-borne pathogens, or it can serve as an effective form of biological control for other plant pathogens. In some cases, a cover crop can reduce populations of one insect pest but serve as a host that increases populations of other insect pests. Cover crops can help reduce disease problems in cash crops in the following ways:

Cover Crops for Managing Weeds

Cover crops suppress weeds by competing for light and nutrients or, in some cases, releasing compounds that inhibit the germination or growth of weeds through allelopathy. Weed suppression by cover crops varies by species, management (e.g., planting dates, planting densities, tillage, and residue management, etc.), existing weed populations, and weather conditions.

Choose the Right Cover Crop for the Climate and Season

Sow warm season, frost-tender cover crops like soybean, buckwheat, and millet after the spring frost-free date and at least six to eight weeks before the fall frost date to ensure rapid growth and good biomass production. Buckwheat is one of the easiest and quickest cover crops, used primarily to suppress weeds and improve soil tilth. It is a warm-weather crop that can be used between spring and fall crops. Cowpea, sorghum–sudangrass, and most millets require really warm soil—at least 65 to 70 degrees F (18.3 to 21.1°C).

Managing Cover Crops to Help Suppress Weeds

Numerous factors can influence the effectiveness of managing weeds using cover crops. Species selection and management should be planned carefully long before cash crop planting. Decisions should account for other farm-specific management goals, available equipment, production costs, and the dates when cover crops can be planted and terminated. Considerations include:

Cover Crop Residues

Cover crops can also serve as a “living mulch” to manage weeds. When the cover crop is killed, its thick residues remain on the surface and hinder weed growth by physically modifying the amount of natural light, soil temperature, and soil moisture that is necessary for weed seed germination. It’s important to note that suppressing weeds by smothering them becomes less effective as cover crop residues decompose.

Allelopathy

Some legume, cereal, and brassica cover crops release allelopathic compounds (plant-produced natural herbicides) that can reduce weed populations and/or suppress weed growth. Cereal rye is an overwintering crop that suppresses weeds both physically and chemically. If rye residue is left on the soil surface, it releases allelochemicals that inhibit seedling growth of many annual small-seeded broadleaf weeds, such as pigweed and lambsquarters.

Cover Crops for Managing Nematodes

Cover crops are used to manage nematodes because nematodes can move only very short distances on their own. Nematodes cannot migrate to another field if a cover crop is not a host to the nematodes. Instead, some of them may starve, which helps to manage their population. Fallow soil also helps to keep nematodes populations at lower levels, but it may lead to soil erosion and other problems. Several cover crops may be grown to minimize nematode damage to the following crops. Some cover crops (e.g., velvetbean, sesame, and rapeseed) produce nematicidal (killing) and nematistatic (suppressive) organic compounds.

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