When purchasing seeds or planning your garden, consider companion planting for low-cost, eco-friendly soil amendments and pest-control. Check out Part 1 (getting started) and Part 2 (herbs).
Beans: All bean enrich the soil with nitrogen fixed form the air, improving the conditions for whatever crop you plant after the beans are finished. In general they are good company for carrots, celery, chards, corn, eggplant, peas, potatoes, brassicas, beets, radish, strawberry and cucumbers. Beans are great for heavy nitrogen users like corn and grain plants because the nitrogren used up by the corn and grains are replaced at the end of the season when the bean plants die back. Summer savory deters bean beetles and improves growth and flavor. Keep beans away from the alliums.
Beets: Good for adding minerals to the soil. The leaves are composed of 25% magnesium making them a valuable addition to the compost pile if you don't care to eat them. Beets are also beneficial to beans with the exception of runner beans. Runner or pole beans and beets stunt each other's growth. Companions for beets are lettuce, onions and brassicas. Beets and kohlrabi grow perfectly together. Beets are helped by garlic and mints. Garlic improves growth and flavor. Rather than planting invasive mints around beets use your mint clippings as a mulch.
Broccoli: Companions for broccoli are: Basil, Bush Beans, Cucumber, Dill, Garlic, Hyssop, Lettuce, Marigold, Mint, Nasturtium, Onion, Potato, Radish, Rosemary, Sage, Thyme and Tomato. Celery, onions and potatoes improve broccolis' flavor when planted near it. Broccoli loves plenty of calcium. Pairing it with plants that need little calcium is a good combination such as nasturtiums and beets. Put the nasturtiums right under the broccoli plants. Herbs such as rosemary, dill and sage help repel pests with their distinct aromas. Foes: Grapes, strawberries, mustards and rue.
Cabbage: Celery, dill, onions and potatoes are good companion plants. Celery improves growth and health. Clover interplanted with cabbage has been shown to reduce the native cabbage aphid and cabbageworm populations by interfering with the colonization of the pests and increasing the number of predatory ground beetles. Plant Chamomile with cabbage as it Improves growth and flavor. Cabbage does not get along with strawberries, tomatoes, peppers, eggplants, rue, grapes, lettuce and pole beans.
Carrots: Their pals are leaf lettuce, onions and tomatoes. Plant dill and parsnips away from carrots. Flax produces an oil that may protect root vegetables like carrots from some pests. One drawback with tomatoes and carrots: tomato plants can stunt the growth of your carrots but the carrots will still be of good flavor.
Chards: Companions: Bean, cabbage family, tomato, onion and roses. Don't overlook chard's value as an ornamental plant in flower beds or wherever you have room for it. Don't grow chard near cucurbits, melons, corn or herbs.
Corn: Amaranth, beans, cucumber, white geranium, lamb's quarters, melons, morning glory, parsley, peanuts, peas, potato, pumpkin, soybeans, squash and sunflower. A classic example is to grow climbing beans up corn while inter-planting pumpkins. The corn provides a natural trellis for the beans, pumpkins smother the weeds and helps corn roots retain moisture. Corn is a heavy feeder and the beans fix nitrogen from air into the soil however the beans do not feed the corn while it is growing. When the bean plants die back they return nitrogen to the soil that was used up by the corn. A win-win situation. Another interesting helper for corn is the weed Pig's Thistle which raises nutrients from the subsoil to where the corn can reach them. Keep corn away from celery and tomato plants by at least 20 feet.
Cucumber: Cucumbers are great to plant with corn and beans. The three plants like the same conditions: warmth, rich soil and plenty of moisture. Let the cucumbers grow up and over your corn plants. Cukes also do well with peas, beets, radishes and carrots. Radishes are a good deterrent against cucumber beetles. Dill planted with cucumbers helps by attracting beneficial predators. Nasturtium improves growth and flavor. Keep sage, potatoes and rue away from cucumbers.
Eggplant: Plant with amaranth, beans, peas, spinach, tarragon, thyme and marigold. Eggplant is a member of the nightshade family and does well with peppers as they like the same growing conditions.
Leeks: Use leeks near apple trees, carrots, celery and onions which will improve their growth. Leeks also repel carrot flies. Avoid planting near legumes.
Lettuce: Does well with beets, broccoli, bush beans, pole beans, carrots, cucumbers, onion, radish and strawberries. It grows happily in the shade under young sunflowers. Dill and lettuce are a perfect pair. Keep lettuce away from cabbage. Cabbage is a deterrent to the growth and flavor of lettuce.
Melon: Companions: Corn, pumpkin, radish and squash. Other suggested helpers for melons are as follows: Marigold deters beetles, nasturtium deters bugs and beetles. Oregano provides general pest protection.
Onion: Planting chamomile and summer savory with onions improves their flavor. Other companions are carrot, leek, beets, kohlrabi, strawberries, brassicas, dill, lettuce and tomatoes. Intercropping onions and leeks with your carrots confuses the carrot and onion flies! Onions planted with strawberries help the berries fight disease. Keep onions away from peas and asparagus.
Peppers, Bell (Sweet Peppers): Plant peppers near tomatoes, parsley, basil, geraniums, marjoram, lovage, petunia and carrots. Onions make an excellent companion plant for peppers. They do quite well with okra as it shelters them and protects the brittle stems from wind. Don't plant them near fennel or kohlrabi. They should also not be grown near apricot trees because a fungus that the pepper is prone to can cause a lot of harm to the apricot tree. Peppers can double as ornamentals, so tuck some into flowerbeds and borders. Peppers can be harvested at any stage of growth, but their flavor doesn't fully develop until maturity.
Peppers, Hot: Chili peppers have root exudates that prevent root rot and other Fusarium diseases. Plant anywhere you have these problems. While you should always plant chili peppers close together, providing shelter from the sun with other plants will help keep them from drying out and provide more humidity. Tomato plants, green peppers, and okra are good protection for them. Teas made from hot peppers can be useful as insect sprays. Hot peppers like to be grouped with cucumbers, eggplant, escarole, tomato, okra, Swiss chard and squash. Herbs to plant near them include: basils, oregano, parsley and rosemary. Never put them next to any beans, broccoli, cabbage, cauliflower, Brussels sprouts or fennel.
Pumpkin: Pumpkin pals are corn, melon and squash. Marigold deters beetles. Nasturtium deters bugs, beetles. Oregano provides general pest protection. Again dill may help repel those frustrating squash bugs. See squash entry for more tips.
Radish: Companions for radishes are: radish, beet, bush beans, pole beans, carrots, chervil, cucumber, lettuce, melons, nasturtium, parsnip, peas, spinach and members of the squash family. Radishes may protect squash from squash borers. Anything that will help keep them away is worth a try. Radishes are a deterrent against cucumber beetles and rust flies. Chervil and nasturtium improve radish growth and flavor. Planting them around corn and letting them go to seed will also help fight corn borers. Chinese Daikon and Snow Belle radishes are favorites of flea beetles. Plant these at 6 to 12 inch intervals amongst broccoli. In one trial, this measurably reduced damage to broccoli. Radishes will lure leafminers away from spinach. The damage the leafminers do to radish leaves does not stop the radish roots from growing, a win-win situation. Keep radishes away from hyssop plants, cabbage, cauliflower, Brussels sprouts and turnips. Radishes are a good indicator of calcium levels in the soil. If your radish grows and only produces a stringy root you need calcium.
Spinach: Plant with peas and beans as they provide natural shade for the spinach. Gets along with cabbage, cauliflower, celery, eggplant, onion, peas, strawberries and fava bean. Plant spinach with squash. It's a good use of space because by the time squash plants start to get big the spinach is ready to bolt.
Squash: Companions: Beans, corn, cucumbers, icicle radishes, melon, mint, onions and pumpkin. Helpers: Borage deters worms, improves growth and flavor. Marigolds deters beetle. Nasturtium deters squash bugs and beetles. Oregano provides general pest protection. Dill may repel the squash bug that will kill your squash vines. Generously scatter the dill leaves on your squash plants. Keep squash away from potatoes.
Tomatoes: Tomato allies are many: asparagus, basil, bean, carrots, celery, chive, cucumber, garlic, head lettuce, marigold, mint, nasturtium, onion, parsley, pea, pepper, marigold, pot marigold and sow thistle. One drawback with tomatoes and carrots: tomato plants can stunt the growth of your carrots but the carrots will still be of good flavor. Basil repels flies and mosquitoes, improves growth and flavor. Bee balm, chives and mint improve health and flavor. Borage deters tomato worm, improves growth and flavor. Dill, until mature, improves growth and health, mature dill retards tomato growth. Enemies: corn and tomato are attacked by the same worm. Kohlrabi stunts tomato growth. Keep potatoes and tomatoes apart as they both can get early and late blight contaminating each other. Keep apricot, dill, fennel, cabbage and cauliflower away from them. Don't plant them under walnut trees as they will get walnut wilt: a disease that attacks tomatoes growing underneath these trees.