Companion planting is not a new concept. The benefits of growing certain plants together is part of gardening wisdom that has been passed down for centuries. Herbs are known for their abilities to repel insect pests. Traits besides pest control also make companion planting valuable to gardeners. Legumes, which naturally place nitrogen in the soil, are ideal to plant where they will be followed by heavy feeders such as corn. In contrast, some plants are not compatible with others. The interrelationships between plants makes companion planting an interesting aspect of gardening.
Onions, garlic, leeks and shallots are part of the same family of plants. All members of the odiferous onion family repel insects, and they may be interplanted with most other garden vegetables. Plant onions with tomatoes, cabbage, broccoli, celery and carrots. Onions are an effective slug repellent for strawberries and lettuce. Garlic is especially helpful to roses for repelling Japanese beetles.
Avoid planting members of the onion family with beans and peas.
Beans and Peas
Beans and peas are good companions with any herbs and vegetables except the onion family. They improve the soil by adding nitrogen.
Marigolds are notorious for their ability to prevent root knot nematodes from infecting tomato and pepper plant roots. Marigold roots give off a substance that drives away soil nematodes. The odor of marigolds comes from the entire plant, and it confuses and repels a host of insect pests above ground as well. Plant marigolds throughout the garden, and spread cut branches or flowers as mulch.
Mints attract pollinating insects when they bloom, which aids in fruit set in the garden. Three common mints included in this plant family are peppermint, spearmint and catnip. The scent of mint plants also deters pests, and they are especially good at keeping cabbage moths from cabbage, broccoli, cauliflower and Brussels sprouts.
Caution: Mints can be invasive.
Tomatoes and basil are natural companions, as well as other aromatic herbs like oregano, sage, thyme, chives and onions. Marigolds and borage both improve the growth of tomato plants, and they repel tomato hornworms. Borage attracts pollinating insects and aids in fruit production. Bee balm also aids in tomato growth, and it is thought to improve the flavor of the fruits.
Besides the lovely, edible flowers nasturtiums provide, the plants deter aphids and many squash plant pests. Plant nasturtiums under fruit trees and near roses where aphids may be a problem. Plant them with cucumbers and all squash.