Companion planting for vegetables is a time-honored practice of home gardeners and small-scale agriculturalists alike. In addition to providing a measure of pest and disease protection, a thoughtful garden plan that places compatible plants close to one another is thought to promote beneficial growth of the plant itself. Other helpful plants are referred to as "allies," while vegetable plants that can inhibit each other's growth are called "enemies."
Historic Companion Planting
One classic vegetable combination planting was referred to by Native Americans as the "three sisters": the interplanting of corn, beans and squash. Beans, a legume, provide the nitrogen in the soil required by corn, which in turn acts as a pole on which the beans can climb. Squash growing underneath creates a living mulch by casting shade, which inhibits weed growth and provides cool roots for all three plants.
Other Common Companions
Some companion plantings are well known, such as the notion that basil improves the flavor of tomatoes when planted nearby, and that carrots play nice with onions and lettuce. Other good combinations include radishes with beans, carrots, cucumbers, lettuce or peas; potatoes with cabbage; and onions with beets, chard, peppers or strawberries.
Onions, garlic, chives and their relatives are known to stunt members of the legume family, or beans and peas. Sage also inhibits the growth of cucumbers, and cabbage and strawberries should never be grown together. Some plants are bad companions simply because they share a common pest or disease. Tomatoes and corn are both attacked by the hornworm, and all members of the Solanaceae family---tomatoes, potatoes and peppers---are subject to infection by the bacteria that causes blight. Proper spacing of these plants can help prevent problems from becoming widespread.
Many flowers and herbs provide protection from notorious garden pests like cutworms, cabbage moths, beetles and aphids. Marigold and nasturtium are two plants that are frequently recommended for inclusion into the vegetable garden, as they are reputed to deter beetles. Thyme, oregano, chives, chervil and borage are all flowering plants that are recommended as pest deterrents, as well as being tasty and useful herbs.
Flowering plants in the garden also provide the additional benefit of attracting beneficial insects, such as wasps, mantises, lacewings and ladybugs, all of which prey on a huge variety of garden pests.
How to Plant Companion Vegetables
Spacing of vegetables and their companions depends heavily on the mature size of the plant. For instance, a tomato plant can become a substantial shrub measuring several feet across. Sun-loving basil should be planted far enough away to still receive adequate light when the tomato has grown out, though lettuce thrives in the shade created by a mature tomato plant.
A hexagonal planting plan is another way to space out plants so that each has plenty of room to leaf out, while still being near to its neighbor. Each plant is installed the same distance from the next, with a standard distance for smaller plants being around 6 inches.
Many university agricultural extension offices provide free charts and information sheets on companion plants and allies to help the home gardener organize his or her planting plan.