Companion Planting

By:

Eve Pranis
National Garden Association Staff

Arranging crops so they complement each other in some way is known as companion planting. Plants that have different requirements — for nutrients, sunlight, and space, for instance — often make good garden buddies. Because they are unlikely to compete for resources, you can plant them close together to save space. What's more, some combinations can actually help one or more of the companions flourish. Consider the following factors:
Sunlight — Some plants are sun worshippers and others prefer or at least tolerate shady conditions. Tall crops, such as peas and corn, can shade lettuce and spinach from the midday sun, extending the harvest season of these cool-weather crops.
Nutrients and Water — All plants need water and nutrients from the soil to survive, but they don't all have the same needs nor do they meet them in the same way. Corn, for instance, is a nitrogen hog, but carrots require much less of that nutrient. Some plants, such as squash, have deep roots that can pull nutrients and water from greater depths than can onions, lettuce, and other shallow-rooted crops. A family of crops called legumes, which includes beans, peas, peanuts, lupine, and clover, can actually return more nitrogen to the soil than they consume.
Other Factors — Some plants make good partners because their life cycles, growth rates, or temperature preferences differ. For instance, you can plant fast-growing, cool-weather crops like lettuce, radishes, or cilantro early in the season alongside slower-growing, heat-loving tomatoes. You'll be able to harvest your early crops quickly, making room for the tomatoes to take over.
The Three Sisters growing system, long used by many Native American cultures, is one of the most familiar examples of companion planting. The 'sisters' (corn, beans, and squash) are planted together in such a way that they aid each others' success. Tall corn stalks provide support for pole beans to climb. Beans, through their symbiotic association with a type of root bacteria, fix nitrogen from the air into a form that plants (especially nitrogen-hungry corn) can use. And large, ground-hugging, prickly squash leaves shade out weeds and may deter critters.
In addition to promoting better growth through complementary habits, companion planting is also practiced to prevent pest problems. Specific crops (typically flowers and herbs) are believed to repel certain pests, lure them elsewhere, or confuse them. Others entice beneficial insects, birds, and spiders that have a penchant for dining on problem visitors. The more diversity and variety of plants in your garden the greater will be the number of beneficial organisms that keep a check on the populations of pests. Here are some examples:
Plants that Attract Beneficial Insects: Many insects, such as small parasitic wasps, green lacewings, and lady beetles, depend on certain pests for nourishment. When pests are in short supply, these do-gooders feed on pollen and nectar. Plants with tiny flowers, such as members of the carrot (umbel family) and daisy (composite or aster) families, are among their favorites. You can encourage the pest-control squad by mixing the following members of those two plant families with your vegetables: anise, dill, caraway, fennel, yarrow, sweet cicely, zinnia, cosmos, and marigold, or by leaving related weeds (e.g., queen Anne's lace and wild daisies) nearby. Other small-flowered herbs such as thyme and catnip also appeal to these garden helpers. If you devote a section of your vegetable garden to perennial flowers, try coneflower, aster, liatris, coreopsis, and black-eyed Susan. Fruit-bearing trees and shrubs also offer food and shelter to birds, many of which feed on garden pests.
Plants that Lure Pests Away: Some plants are simply so attractive to pests that you can use them to lure garden intruders away from your precious vegetable crops. These attractants are referred to as "trap crops." Once the trap crops are infested with pests, you can pull them out and discard them, pests and all. Try these: nasturtiums for aphids, radishes or nasturtiums for flea beetles, dill and lovage for tomato hornworms, eggplant for potato bugs.
Plants that Repel Pests: Gardeners have long held that some plants are repugnant to certain pests. Lots of pests find their way to favorite crops through their sense of smell. Herbs often exude strong fragrances (from their essential oils) that are believed to repel insects or confound them by disguising the smell of neighboring plants. These aromatic plants include tansy, mint, basil, thyme, and santolina. Gardeners often tuck garlic and onions between other vegetables for the same purpose. Here are some plants thought to repel specific pests:
• onions for carrot rust flies.
• tansy for colorado potato beetles.
• marigolds and basil for flea beetles on eggplant.
• marigolds for Mexican bean beetles.
• nasturtiums for squash bugs.
• marigolds, mint, or thyme for cabbage moths.