This is a cultural system where two or more plants are grown in close proximity in order that they may provide some form of benefit to one or all of them. This could be to deter or act as a decoy for pests and diseases, or to improve growth and flavour. Sometimes it is the aromatic foliage or flowers of the companion plant which deters the pest or draws it away.
- The improvement in growth or flavour is probably due to the companion plant adding some of the micronutrients or the roots may support microorganisms such as mycorrhizal fungi which improve their uptake. In the case of legumes they fix nitrogen, and White Clover is included in some grass seed mixes, for this reason - it is also drought resistant so 'greens up' quicker than grass when used in lawns.
- Some plants exude protective chemicals called allelochemicals from their roots or foliage which can deter pests, eg. African Marigolds produce thiopene which repels nematodes so it makes a good companion plant for root crops which are attacked by nematodes. This is given the title Allelopathy. The chemicals can also inhibit germination of seeds; this occurs under the canopy of many plants, most notably in pine forests where the floor remains relatively clear of competing plants even where light can get through, the decaying debris from the trees releases the inhibiting compounds. Ethylene gas is produced and released from plant tissues and it has a hormonal effect by either promoting or inhibiting processes in plants such as germination of seeds or ripening of fruit.
- Tall plants can provide shelter for lower-growing ones and create a microclimate allowing them to thrive where they may not normally grow. This may also protect them from pests. This effect is used in the Permaculture growing technique and land which would not support a crop grown on its own, or where space is at a premium, can become productive, as several crops together provide mutual benefits.
- Beneficial insects can be attracted to a cropping area by planting something which they use as a food nearby, eg. adult Hover Flies feed on nectar, but their larvae feed on aphids so planting nectar-rich flowering plants will attract them and they will lay their eggs on plants where there are pests.
Marigolds and Garlic have a reputation for repelling a number of insect pests such as Aphids and Carrot Root Flies. The Mexican Marigold, Tagetes minuta or Muster-John-Henry, is an annual which grows to about 1.2 m (the species name refers to the flowers not the height of the plant) The roots have an insecticidal effect on nematodes and some effect on keeled slugs. The secretions responsible begin about 3 months after sowing and also affect the growth of Ground-elder (Aegopodium podagraria), Bindweed (Convolvulus arvensis), Couch grass (Agropyron repens), Lesser Celandine (Ranunculus ficaria ) and Ground Ivy (Glechoma hederacea).
The Marigold is grown as a half-hardy annual which can be planted out after the risk of frost has passed, but this would not usually give them a long enough growth period to flower and set seed in Britain, so it does not become a weed itself. Deadheading prolongs the growth for protection purposes.
In sub-Saharan Africa the maize and sorghum crops are greatly weakened by a parasitic weed called African Witchweed (Striga hermonthica) which attaches to the roots and extracts nourishment. Using a herbicide would kill the host plant as well. Fortunately there is a companion plant, the Cowpea (Desmodium uncinatum) which releases an allelopathic chemical that kills the weed seeds as they germinate.
The Cowpea is also a deterrent to a moth, the Spotted Stemborer (Chilo partellus ) which lays its eggs on the stems of maize and sorghum. The caterpillar which hatches from these eggs eats its way up the inside of the stem weakening or eventually killing the plant.
This deterrence is helped by using another companion or trap plant, around the perimeter of the crop which appears to be a better place for the moth to lay its eggs. Unfortunately for the pest the Napier Grass (Pennisetum purpureum) which is used, produces a gummy sap that kills the caterpillars. This growing method has been given the name "push-pull" as the Cowpea pushes the moth away while the Napier Grass pulls them towards it. This is a much more sustainable method of increasing crop yields than developing genetically modified plants to solve the problem.
Below is a list of crops, the plants that can be grown with them to provide some benefits such as improved yeild, and in the right hand column some that do not.
CROP COMPANION PLANTS INCOMPATIBLE WITH Asparagus Tomato, Parsley, Basil Beans Most Vegetables & Herbs Beans, Broad Potato, Cucumber, Maize, Strawberry, Celery, Summer Savory Onion Beans, Runner Maize, Summer Savory, Radish Onion, Beets, Kohlrabi, Sunflower Cabbage Family Aromatic Herbs, Celery, Beetroot, Onion Family, Chamomile, Spinach, Chard Dill, Strawberries, Runner Beans, Tomato Carrots Pea, Lettuce, Rosemary, Onion Family, Sage, Tomato Celery Onion & Cabbage Families, Tomato, Broad Beans, Nasturtium Dill Cucumber Beans, Maize, Pea, Sunflowers, Radish Potato, Aromatic Herbs Aubergine (Eggplant) Broad Beans, Marigold Lettuce Carrot, Radish, Strawberry, Cucumber Maize (Corn) Potato, Broad Beans, Pea, Pumpkin, Cucumber, Squash Tomato Melon Maize, Nasturtium, Radish, Onion Family Beetroot, Carrot, Lettuce, Cabbage Family Beans, Peas Parsley Tomato, Asparagus Peas Carrots, Radish, Turnip, Cucumber, Maize, Beans Onion Family, Gladiolus, Potato Potato Beans, Maize, Cabbage Family, Marigolds, Horseradish Pumpkin, Squash, Tomato, Cucumber, Sunflower Pumpkins Maize, Marigold Potato Radish Pea, Nasturtium, Lettuce, Cucumber Hyssop Spinach Strawberry, Cauliflower, Celery Squash Nasturtium, Maize, Marigold Potato Strawberry Broad Beans, Lettuce, Onion, Spinach Cabbage Tomato Basil, Onion Family, Nasturtium, Marigold, Asparagus, Carrot, Parsley, Cucumber, Mint Potato, Fennel, Cabbage Family Turnip Pea Potato
This is a table of herbs which can be used to repel insects and the plants they can be used to protect. The final column gives the plants for which they are bad companions and some other points of note.
HERB COMPANION FOR PESTS REPELLED INCOMPATIBILITIES Allium (Onion Family) vegetables, fruit trees aphids, carrot flies, moles, tree borers, and weevils peas and beans Angelica (Angelica archangelica) Dill Basil (Ocimum basilicum) Tomatoes Flies, Mosquitoes Rue Bay leaves beans or grains weevils and moths Borage (Borago officinalis) Tomatoes, Squash, Strawberries Tomato Worm Caper Spurge or Mole Plant (Euphorbia lathyris Moles and Mice poisonous, irritant milky sap Caraway (Carum carvi) Good for loosening the soil. Avoid Dill Castor Oil Plant (Ricinus communis). Moles seeds extremely poisonous Catnip (Nepeta cataria) Aubergine Flea Beetle, Ants Chamomile (Chamaemilum nobile) Cabbage, Onion Coriander (Coriandrum sativum) all vegetables Aphids attracts Bees Chervil (Anthriscus cerefolium ) Radish Chives (Allium schoenoprasum) Carrots Root Flies Dill (Anethum graveolens) Cabbage Carrots and Caraway Fennel (Foeniculum vulgare) Most plants Feverfew (Chrysanthemum parthenium syn. Tanacetum parthenium) Roses attracts Aphids away seeds freely Garlic Roses, Raspberries Aphids Henbit (Lamium amplexicaule) General Insect Repellent Hyssop (Hyssopus officinalis ) Cabbage, Grapes Cabbage White Butterflies Dislikes Radishes Lavender (Lavandula angustifolia) Moths - make sachet with southernwood, wormwood and rosemary, use in doors Marigolds (Tagetes sp.) most have aromatic foliage Good companion to most plants Nematodes, Aphids and others Mexican marigold (Tagetes minuta), Muster-John-Henry Good insect repellant, inhibits ground-elder and some other weeds Mint (Mentha sp. ) Cabbage, Tomatoes Cabbage White Butterflies, Aphids, Flea Beetles invasive roots Mustard (Brassica juncea) cabbage, cauliflower, radish, brussel sprouts, turnips, and kohlrabi a trap crop to attract many insect pests Nasturtium (Tropaeolum majus) Radishes, Cabbage, Squashes and Pumpkims, fruit trees Aphids, Squash Bugs, Striped Pumpkin Beetle Oregano(Origanum vulgare) Brassicas Cabbage Butterflies Pennyroyal (Mentha pulegium ) Roses Flies, Mosquitoes, Fleas, others invasive roots Petunia (Petunia hybrida) Beans Pot Marigold (Calendula officinalis) Tomatoes Tomato Worm, Asparagus Beetles, Whitefly Pyrethrums (Chrysanthemum cinaeriofolium) Dried flower heads make an insecticide. Rosemary (Rosmarinus officinalis Cabbage, Beans, Carrots, Sage Cabbage Butterflies, Bean Beetle, Carrot Fly Rue (Ruta gaveolens) Roses and Raspberries Japanese Beetles Dislikes Sweet Basil Sage (Salvia officinalis) Rosemary, Cabbage, Carrots Cabbage Moth, Carrot Fly, Flea Beetle, Slugs Dislikes Cucumbers Southernwood (Artemesia abrotanum) Cabbages Cabbage Butterflies Summer Savory (Satureia hortensis ) Beans Bean Beetles Tansy (Tanacetum vulgare) Fruit Trees, Roses, Raspberries Flying Insects, Japanese Beetles, Striped Cucumber Beetles, Squash Bugs, Ants, Flies Thyme (Thymus vulgaris) Cabbage Cabbage Worm Wormwood (Artemisia absinthium) Moths, Slugs, Carrot Fly. Plant at boundaries to keep animals out of the garden. Fresh material will stunt growth of young plants, use dried sprigs Yarrow (Achillea millefolium ) Plant near aromatic herbs to enhance production of essential oils. Attracts Hover Flies and their larvae prey on Aphids Companion Planting.
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