The purpose of companion planting is to provide assistance to plants by the close growth of other plants, be it attracting beneficial insects, naturally regulating pests, repelling harmful insects, providing nutrients, part of a biological pest control program, and in some cases simply a shaded microclimate or climbing support. Click on this link to Wikipedia for a more complete list.
Amaranth—use it to host beneficial ground beetles; grow with sweet corn.
Allium—use with vegetables (except peas and beans), fruit trees. Use for aphids, carrot flies, moles, tree borers, and weevils.
Anise—repels aphids and hosts predatory wasps.
Basil—use with tomatoes and asparagus plants for flies, aphids, mites and mosquitoes. Helpful with tomato hornworms and asparagus beetles.
Bay leaves—place a fresh leaf in a container of beans or grains to repel weevils and moths. Use for ladybug invasion by laying leaves around the house.
Bee Balm—use for attracting beneficial bugs and bees, plant with tomatoes to improve flavor and growth.
Beet—leaves contain large amounts magnesium, which will help improve the soil as they decompose. Grow with lettuce and onions.
Borage—use around tomatoes, strawberries, and fruit orchards; repels tomato worms. Will attract honeybees and adds calcium and potassium to the soil.
Catnip—use around eggplant to drive away flea beetles.
Celery—use near cauliflower, tomatoes, leek, and cabbage. Will repel white cabbage flies.
Chamomile—plant only a few around cabbage and onions for better flavor.
Chervil—improves the growth and flavor of radishes.
Chives—repels many fruit tree and tomato pests. Will increase flavor and growth of carrots.
Coriander—plant near all vegetables to repel aphids. Will attract bees.
Dill—plant around cabbage to improve growth. The flowers will attract bees.
Garlic—repels aphids, beetles, weevils, borers, and spider mites.
Geranium plants—repel cabbage worms and japanese beetles. Plant around grapes, roses, corn, and cabbage.
Henbit—most insects will stay away from henbit.
Horseradish—plant around potatoes to encourage growth. May deter colorado potato and blister beetles.
Hyssop—will deter cabbage moths if planted around cabbage and grapes. May improve growth of these plants.
Lamium—repels potato bugs.
Leek—use near carrots, celery, onions to improve the growth. Repels carrot flies.
Marigold—encourages growth if planted near tomatoes, potatoes, strawberries, beans, and roses. Repels many insects like mexican bean beetles and harmful nematodes in the ground.
Mint—improves flavor and growth of cabbage and tomatoes if planted near them.
Mustard—known as a trap crop to attract many insect pests. Plant near cabbage, cauliflower, radish, brussels sprouts, collards, turnips, and kohlrabi. If insects are attracted, destroy the plants before other crops are harmed.
Nasturtium—repels aphids, cucumber beetles, squash bugs, white flies, and borers near fruit trees. Mainly cucumber and squash will benefit.
Onion—repels cabbage moths, aphids, weevils, carrot flies, moles. Controls rust flies and some nematodes. Controls red spiders near tomatoes. May alter growth of peas and beans. Plant near beets,tomato,lettuce, strawberry, cabbage, broccoli, and cauliflower.
Oregano—plant near broccoli. Repels cabbage butterfly.
Parsley—repels carrot flies, rose, and asparagus beetles. Plant near asparagus, carrots, tomato, and roses.
Peanuts—will encourage growth of corn and squash.
Peas—if planted near corn, will provide extra nitrogen.
Pennyroyal—deters ants, aphids, ticks, fleas, and cabbage maggots. Brussels sprouts, broccoli, and cabbage will benefit.
Radish—plant around cucumbers to deter cucumber beetles.
Rosemary—repels cabbage moths, beetles, mosquitoes, and slugs. Plant near beans, carrots, and cabbage.
Rue—japanese beetles are deterred. Plants like roses and raspberries will benefit.
Sage—repels cabbage moths, carrot flies, and ticks. Carrots love it.
Soybeans—add nitrogen to the soil, so plant near corn. Repels chinch bugs and japanese beetles.
Spearmint—deters ants and aphids.
Summer Savory—will improve growth of green beans and deter bean beetles.
Tansy—deters ants, striped cucumber beetles, flies, japanese beetles, and squash bugs. Plant near cucumber, squash, roses, grapes raspberry, and blackberry.
Tomato—plant near roses to protect them from black spot.
Wormwood—deters black flea beetles, cabbage worm butterflies.
Companion Planting for Veggies, by Annette Welsford
Carrots Love Tomatoes by Louise Riotte