Companion planting has been used for centuries to enhance vegetable gardens to help prevent pests, disease and save space. Native Americans used companion planting extensively and called the combination of corn, beans and squash "Three Sisters." This term was commonly used by the Iroquois of the northeastern United States. Modern science has discovered that beans add nitrogen to the soil, which helps to feed corn and squash; corn provides a sturdy stalk for beans to run up; and squash helps to shade the roots of the other two to keep in moisture. These are among a small sampling of the many benefits to companion planting.
Companion Planting for Pest Control
Planting collards near cabbage is a long-standing practice for farmers in the South. This is an attempt to keep the diamondback moths away from the cabbage since the moth is more attracted to the collards. Garden yellowrocket (Barbarea vulgaris), a common weed in the northeastern United States, was used in plantings with broccoli and cabbage to attract diamondback moths, according to Cornell University's department of entomology. The moths were attracted to the weed and laid their larvae among its leaves. These larvae could not be sustained and died. The yellowrocket is considered to be a good planting companion for these vegetables in areas where diamondback moths are a problem. Alfalfa can be used around strawberries to attract pests and keep them off of the strawberries. Chervil attracts slugs, dill and lovage help to protect tomatoes from hornworms, and horseradish attracts the Colorado potato beetle away from potatoes.
Companion Planting for Beneficial Insects
Beneficial insects such as praying mantis, ladybugs and parasitic wasps aid gardeners by preying on many of the insects that damage crops. Ladybugs lay their eggs near or on infested plants and adults, and their larvae feed on soft-bodied pests, mites, aphids, and insect eggs. Parasitic wasps are especially beneficial to gardeners with hornworm problems as the wasps lay they eggs on the backs of the worms. These eggs feed off of the hornworms, killing them by the time the eggs hatch. Farmers who set up their gardens to include sunflowers, parsley, carrots, mint and clover can attract beneficial insects to their gardens to help keep damage to their crops down. These are also helpful in attracting the pollinating insects that are so essential in vegetable production.
Companion Planting for Nutrient Enhancement
Beans, legumes and alfalfa are known for their nitrogen producing capabilities. Planting a crop of alfalfa and under-tilling it before planting a vegetable garden is a well-known method of providing natural nutrients to the soil. Planting peas, beans and clover alongside nitrogen-needy plants such as corn is beneficial to the corn, as these crops can pull nitrogen from the air and distribute it to their own benefit and to the benefit of surrounding plants as well.
Companion Planting for Space Saving
Miingling tall plants with low-growing plants can save space in a garden and help those with small spaces by producing more vegetables per square foot. Use the Native American "Three Sisters" approach for space saving corn, beans and squash. Plant tall, sun-loving plants such as sunflowers and corn alongside smaller plants, such as peas, cucumbers or newly started plants, that do not tolerate direct sun as well. The peas and cucumbers can use the sunflower stalks to begin their climb from the ground, while sunflowers attract bees for pollination and provide shade. Using this technique saves space and negates the need for trellising the peas or beans.
Companion Planting for Biochemical Release
Many plants release chemicals that deter pests or inhibit growth of other plants. Rye grass, cut and used as mulch, releases a chemical that helps to prevent weed growth. The black walnut suppresses the growth of many plants and should not be grown near vegetable gardens, and African Marigolds release thiopene which repels nematodes.