Often used in and around vegetable or flower gardens, companion planting is a natural way to create a healthy environment in your landscape. Employed by farmers for centuries as a low-cost, organic way to protect crops, companion planting utilizes the qualities of certain plants and the relationships between different plants to deter bad insects, attract helpful bugs, improve soil quality, enhance flower blooms and increase crop yields.
Deter Harmful Insects
Companion planting is most commonly employed in vegetable gardens to keep destructive pests away from the plants. Many different companion plants can deter insects from invading vegetable gardens by confusing or offending the pests with their strong odors. Some plants even trap or poison harmful insects. For example, cosmos will deter destructive insects; French marigolds' strong odor will confuse pests; and catnip repels flea beetles, aphids, squash bugs, Japanese beetles, ants, weevils and even mice. Garlic will repel beetles and aphids, while chrysanthemums kill bad soil nematodes. Comfrey will trap slugs, and four-o'clocks will attract Japanese beetles, which die after feeding on the poisonous foliage.
Attract Beneficial Insects
Another main reason for companion planting is to attract beneficial insects that feed on harmful bugs while leaving your plants untouched. Beneficial insects tend to be attracted to plants that bloom with many small flowers containing desirable nectar and pollen. They're also attracted to certain plants that provide hiding places or breeding grounds. The herbs anise, fennel, dill and coriander attract parasitic wasps, which feed on aphids and other harmful bugs. Yarrow also attracts parasitic wasps, as well as bees and hover flies. Borage will attract bees and parasitic wasps, deter tomato hornworms and cabbage loopers, as well as add trace minerals to the soil. Goldenrod attracts predatory beetles, parasitic wasps and ladybugs, which are notorious aphid-killers.
Create Symbiotic Garden Environments
Companion planting can also improve your plants' health and vegetable crop yields by creating a nutrient-rich, symbiotic garden. Some plants actually return certain nutrients into the soil and some provide shade or crowd out weeds for the plants around them. For instance, beans will enrich the soil with nitrogen by processing fixed nitrogen from the air and releasing it through the roots. Plant beans beside heavy nitrogen-feeding plants like carrots, corn, eggplant, celery, chards, potatoes and cucumbers. Plants like buckwheat and alfalfa are often planted as cover crops in the autumn and tilled into the soil in late winter to provide calcium, phosphorous, nitrogen, iron, magnesium and potassium. Alfalfa's long, tough taproots also break up hard clay soils. Native Americans planted "three sisters gardens," containing corn, beans and squash planted in circular hills. The beans would feed the corn and squash with nitrogen, the corn would provide a natural climbing support for the beans, while the squash plants would trail along the ground and naturally smother weeds.
Improve Edible Plants' Flavor
Companion planting is also used in vegetable gardens to improve the flavor of certain crops. For example, planting basil nearby will improve the flavor of tomatoes, at the same time repelling thrips. Chives will enhance the flavor of carrots and tomatoes. German chamomile improves the taste of cucumbers, onions and cabbages, while attracting parasitic wasps and returning to the soil nutrients like calcium, potassium and sulfur.