Companion Planting

Companion planting is thought by its practitioners to assist in the growth of one or both plants involved in the association. Possible mechanisms include attracting beneficial insects, repelling pests, or providing nutrients such as by fixing nitrogen, shade, or support. Companion plantings can be part of a biological pest control program.

Although a large number of companion plant associations have been proposed, only a few have been subjected to scientific testing. Thus where a table column for example states Helps or Helped by, this is to be read as meaning that traditional companion planting involves putting the named plants in that column into an association with the plant named at the left of the row, with the intention of causing the one plant to help or be helped by the other.

Mechanisms that have been scientifically verified include using strongly aromatic plants to deter pests; using companions to hide crops from pests; providing plants as nurseries for beneficial insects including predators and parasitoids; trap cropping; and allelopathy, where a plant inhibits the growth of other species.

List of Companion plantings traditionally used for vegetables

(From Wikipedia)

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Common name	Scientific name	Helps	Helped by	Attracts	Repels/+distracts	Avoid	Comments
Alliums	Allium	Fruit trees, nightshades (tomatoes, capsicum peppers, potatoes), brassicas, carrots	Carrots, tomatoes, carrots and African spider plants (Cleome gynandra) together, marigolds (Tagetes spp.), mints	Thrips	slugs (see Garlic), -aphids, carrot fly, -cabbage loopers, -cabbage maggots, -cabbage worms, -Japanese beetles	Beans, peas	Alliums is a family of plants which includes onions and garlic.
Asparagus	Asparagus officinalis	Tomatoes, parsley	Aster family flowers, dill, coriander, tomatoes, parsley, basil, comfrey, marigolds, nasturtiums			Onion, garlic, potatoes
Beans, bush beans	Phaseolus vulgaris	Cucumber, strawberries	Celery, strawberries, grains			soybeans, dry beans, alfalfa	"Lettuce, potato, tomato, other legumes, crucifers, or cucurbits increase sclerotinia" in the soil and should be avoided before and after snap beans. See the entry for "Legumes" for more info
Pole beans	Phaseolus vulgaris		Radishes, Corn			brassicas, kohlrabi	the stalk of the corn provides a pole for the beans to grow on, which then gives nitrogen to the soil of the corn. Beans and corn are (with squash) traditional "Three Sisters" plants. As for Radishes, see the entry for "Legumes".
Fava beans	Vicia faba		Strawberries, Celery				See the entry for "Legumes" for more info
Beets	Beta vulgaris	bush beans, cabbage, lettuce, kohlrabi, onions, brassicas, passion fruit	Bush beans, onions, kohlrabi, catnip, garlic, mint			Runner or pole beans	Good for adding minerals to the soil through composting leaves which have up to 25% magnesium. Runner or pole beans and beets stunt each other's growth.
Brassicas	Brassica	onions,	Beets, spinach, chard, Aromatic plants or plants with many blossoms, such as celery, chamomile, and marigolds. Dill, sage, peas, peppermint, spearmint, rosemary, rye-grass, garlic, onions and potatoes. geraniums, alliums, nasturtium, borage, hyssop, tomatoes, thyme, wormwood, southernwood, beans, clover		Wireworms	Mustards, nightshades (tomatoes, peppers, etc.), pole beans, strawberries	Brassicas are a family of plants which includes broccoli, Brussels sprouts, cabbage, cauliflower, Chinese cabbage, kohlrabi, radish, and turnip. Thyme, nasturtiums, and onion showed good resistance to cabbage worm, weevil and cabbage looper.
Broccoli	Brassica oleracea	Lettuce	Beets, dill, onions, tomato, turnip, clover				Broccoli as a main crop intercropped with lettuce was shown to be more profitable than either crop alone. Turnip acts as a trap crop. See brassicas entry for more info
Brussels sprouts	Brassica oleracea		Sage, thyme, clover
Cabbage	Brassica oleracea / Brassica chinensis	Beans, celery	Beans, clover, calendula/pot marigold, chamomile, larkspur, nasturtiums, dill, coriander, hyssop, onions, beets, mint, rosemary, sage, thyme	Snails and slugs		Grapes	See brassicas entry for more info. If using clover as an intercrop it should be sown after cabbage transplant so as not to affect crop yield. Nasturtiums repel cabbage moths.
Carrots	Daucus carota	Tomatoes, alliums, beans, leeks, lettuce, onions, passion fruit	Lettuce, alliums (chives, leeks, onions, shallots, etc.), rosemary, wormwood, sage, beans, flax	Assassin bug, lacewing, parasitic wasp, yellow jacket and other predatory wasps	Leek moth, onion fly	Dill, parsnip, radish	Tomatoes grow better with carrots, but may stunt the carrots' growth. Beans provide the nitrogen carrots need more than some other vegetables. Aromatic companion plants repel carrot fly. Alliums inter-planted with carrots confuse onion and carrot flies. For the beneficial insect-attracting properties of carrots to work, they need to be allowed to flower; Otherwise, use wild carrot, Queen Anne's Lace, for the same effect.
Cauliflower	Brassica oleracea	Beans, celery, spinach, peas	Mixture of Chinese cabbage, marigolds, rape, and sunflower. Spinach, peas				See brassicas for more info. See peas regarding their mutualism with cauliflower.
Celery	Apium graveolens	Bush beans, brassicas, cucumber	Cosmos, daisies, snapdragons, leeks, tomatoes, cauliflower, cabbage, bush beans		Whiteflies	Corn, aster flowers	Aster flowers, can transmit the aster yellows disease
Chard	Beta vulgaris ssp. cicla	Brassicas, passion fruit
Corn / Maize	Zea mays	Beans, cucurbits, soybeans, tomatoes	Sunflowers, dill, legumes (beans, peas, soybeans etc.), peanuts, cucurbits, clover, amaranth, white geranium, pigweed, lamb's quarters, morning glory, parsley, and potato, field mustard			Tomato, celery	Provides beans with a trellis, is protected from predators and dryness by cucurbits, in the three sisters technique
Cucumber	Cucumis sativus	Beans, kohlrabi, lettuce	Kohlrabi, nasturtiums, radishes, marigolds, sunflowers, peas, beans, chamomile, beets, carrots, dill, onions, garlic, amaranth (Amaranthus cruentus), celery, Malabar spinach	Beneficial for ground beetles	Raccoons, ants	Potato, aromatic herbs	Sow 2 or 3 radish seeds in with cucumbers to repel cucumber beetles. One study showed a 75% reduction in cucumber beetles with the concurrent seeding of amaranth. Various sprays from lettuce, asparagus, Malabar spinach, and celery were found to reduce whiteflies. See cucurbits entry for more info
Cucurbits	Cucurbitaceae	Corn	Corn, grain sorghum				Cucurbits are a family of plants that includes melons, cucumbers, gourds, pumpkins, and squash
Eggplant or Aubergine	Solanum melongena	Beans, peppers, tomatoes, passion fruit	Marigolds, catnip, redroot pigweed, green beans, tarragon, mints, thyme				Marigolds will deter nematodes.
Kohlrabi	Brassica oleracea v. gongylodes	Onion, beets, aromatic plants, cucumbers	Beets, cucumbers
Leek	Allium ampeloprasum v. porrum	Carrots, celery, onions, tomato, passion fruit	Carrots, clover			Swiss chard	See Alliums entry for more info
Legumes	Phaseolus and Vicia	Beets, lettuce, okra, potato, cabbage, carrots, chards, eggplant, peas, tomatoes, brassicas, corn, cucumbers, grapes	Summer savory, beets, cucumbers, borage, cabbage, carrots, cauliflower, corn, larkspur, lovage, marigolds, mustards, radish, potato, peppermint, rosemary, lettuce, squash, lacy phacelia	Snails and slugs	Colorado potato beetle	Alliums, gladiolas	Hosts nitrogen-fixing bacteria, a good fertiliser for some plants, too much for others. Rosemary and peppermint extracts are used in organic sprays for beans. Summer savory and potatoes repel bean beetles.
Lettuce	Lactuca sativa	Beets, beans, okra, onions, radish, broccoli, carrots, passion fruit	Radish, beets, dill, kohlrabi, onions, beans, carrots, cucumbers, strawberries, broccoli, thyme, nasturtiums, alyssum, cilantro	Slugs and snails		Celery, cabbage, cress, parsley	Broccoli, when intercropped with lettuce, was shown to be more profitable than either crop alone.
Mustard	Sinapis alba	Beans, cabbage, cauliflower, fruit trees, grapes, radish, brussels sprouts, turnips			Various pests	AVOID	See Brassicas entry.
Nightshades	Solanaceae		Carrots, alliums, mints (basil, oregano, etc.)			Beans, black walnuts, corn, fennel, dill, brassicas	Nightshades are a family of plants which include tomatoes, tobacco, chili peppers (including bell peppers), potatoes, eggplant, and others
Okra	Abelmoschus esculentus	Sweet potato, tomatoes, peppers	Beans, lettuce, squash, sweet potato, peppers				Okra and sweet potato are mutually beneficial when planted simultaneously.
Onion	Allium cepa	Beets, brassicas, cabbage, broccoli, carrots, lettuce, cucumbers, peppers, passion fruit, strawberries	Carrots, beets, brassicas, dill, lettuce, strawberries, summer savory, chamomile, pansy			Lentils, peas, beans	See Alliums entry for more info
Parsnip	Pastinaca sativa	Fruit trees		A variety of predatory insects			The flowers of the parsnip plant left to seed will attract a variety of predatory insects to the garden, they are particularly helpful when left under fruit trees, the predators attacking codling moth and light brown apple moth.
Peas	Pisum sativum	Turnip, cauliflower, garlic	Turnip, cauliflower, garlic, mints		Colorado potato beetle		Peas when intercropped with turnips, cauliflower, or garlic showed mutual suppression of growth; however, their profit per land area used was increased.
Peppers	Solanaceae, Capsicum	Okra	Beans, tomatoes, okra, geraniums, petunias, sunflowers, onions crimson clover, basil, field mustard			Beans, kale (cabbage, Brussels sprouts, etc.)	Pepper plants like high humidity, which can be helped along by planting with some kind of dense-leaf or ground-cover companion, like marjoram and basil; pepper plants grown together, or with tomatoes, can shelter the fruit from excess sunlight, and raise the humidity level. Sunflowers, when in bloom at the right time, shelter beneficial insects, lowering thrips populations.
Potato	Solanum tuberosum	Brassicas, beans, corn, peas, passion fruit	Horseradish, beans, dead nettle, marigolds, peas, onion, garlic, thyme, clover		Mexican bean beetle	Atriplex, carrot, cucumber, pumpkin, raspberries, squash, sunflower, tomato	Horseradish increases the disease resistance of potatoes. It repels the potato bug. Garlic is more effective than fungicides on late potato blight. Peas were shown to reduce the density of Colorado potato beetles.
Pumpkin	Cucurbita pepo	Corn, (in trad. Three Sisters partnership) beans	Buckwheat, Jimson weed, catnip, oregano, tansy, radishes, nasturtiums	spiders, ground beetles		Potatoes	Radishes can be used as a trap crop against flea beetles, cucurbita can be used in the Three Sisters technique. Nasturtiums repel squash bugs.
Radish	Raphanus sativus	Squash, eggplant, cucumber, lettuce, peas, beans, pole beans	Chervil, lettuce, nasturtiums		flea beetles, cucumber beetles	Grapes	Radishes can be used as a trap crop against flea beetles. Radishes grown with lettuce taste better.
Soybean	Glycine max		Corn, sunflower				A mixture of corn, mungbean, and sunflower was found to rid soybeans of aphids.
Spinach	Spinacia oleracea	Brassicas, passion fruit	Strawberries, peas, beans				The peas and beans provide natural shade for the spinach. See cauliflower notes regarding mutualism with spinach.
Squash	Cucurbita spp.	corn, beans, okra	Beans, buckwheat, borage, catnip, tansy, radishes, marigolds, nasturtiums	Spiders, ground beetles			Radishes can be used as a trap crop against flea beetles, cucurbita can be used in the three sisters technique. Marigolds and nasturtiums repel squash bugs. Marigolds repel cucumber beetles.
Sweet potato	Ipomoea batatas	Okra	Okra				Okra and sweet potato are mutually beneficial when planted simultaneously
Tomatoes	Solanum lycopersicum	Celery, roses, peppers, asparagus	Asparagus, basil, beans, bee balm (Monarda), oregano, parsley, marigold, alliums, garlic, leeks, celery, geraniums, petunias, nasturtium, borage, coriander, chives, corn, dill, mustard, fenugreek, barley, carrots, eggplant, marigold, mints, okra, sage, thyme, "flower strips", cucumbers, squash		Asparagus beetle	Black walnut, alfalfa, corn, fennel, chili peppers, peas, dill, potatoes, beetroot, brassicas, rosemary	Black walnuts inhibit tomato growth, in fact they are negative allelopathic to all other nightshade plants (chili pepper, potato, tobacco, petunia) as well, because it produces a chemical called juglone. Dill attracts tomato hornworm. Growing tomatoes with Basil does not appear to enhance tomato flavour but studies have shown that growing them around 10 inches apart can increase the yield of tomatoes by about 20%. One study shows that growing chili peppers near tomatoes in greenhouses increases tomato whitefly on the tomatoes. Cucumbers and squash can be used as living mulch, or green mulch, around tomato plants. The large leaves of these vining plants can help with soil moisture retention
Turnips and rutabagas	Brassica rapa and Brassica napobrassica	Peas, broccoli	Hairy vetch, peas			hedge mustard, knotweed	Turnips act as a trap crop for broccoli. See peas regarding their mutualism with turnips.