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Companion Planting

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.

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.
Cherry Laurel, Prunus laurocerasus, leaves contain cyanide and cutting or shredding the foliage can produce enough of the poison to be inhaled and be injurious, so it should always be done in the open. Elder Sambucus nigra also contains cyanide and folklore gives a warning that it is bad luck to bring it into the house or burn it, probably because it had been fatal when burnt in the open fires where the smoke filled the room before chimneys existed and it filtered out through the thatched roof.

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.

AsparagusTomato, Parsley, Basil  
BeansMost Vegetables & Herbs  
Beans, Broad Potato, Cucumber, Maize, Strawberry, Celery, Summer Savory Onion
Beans, Runner Maize, Summer Savory, Radish Onion, Beets, Kohlrabi, Sunflower
Cabbage FamilyAromatic Herbs, Celery, Beetroot, Onion Family, Chamomile, Spinach, ChardDill, 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
MelonMaize, Nasturtium, Radish,  
Onion Family Beetroot, Carrot, Lettuce, Cabbage FamilyBeans, 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
StrawberryBroad Beans, Lettuce, Onion, SpinachCabbage
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.

Allium (Onion Family)vegetables, fruit trees aphids, carrot flies, moles, tree borers, and weevilspeas and beans
Angelica (Angelica archangelica)    Dill
Basil (Ocimum basilicum)TomatoesFlies, Mosquitoes Rue
Bay leavesbeans or grains weevils and moths 
Borage (Borago officinalis) Tomatoes, Squash, StrawberriesTomato Worm 
Caper Spurge or Mole Plant (Euphorbia lathyris Moles and Micepoisonous, irritant milky sap
Caraway (Carum carvi)Good for loosening the soil. Avoid Dill
Castor Oil Plant (Ricinus communis).  Molesseeds extremely poisonous
Catnip (Nepeta cataria) AubergineFlea Beetle, Ants 
Chamomile (Chamaemilum nobile)Cabbage, Onion  
Coriander (Coriandrum sativum)all vegetablesAphidsattracts Bees
Chervil (Anthriscus cerefolium )Radish   
Chives (Allium schoenoprasum) CarrotsRoot Flies 
Dill (Anethum graveolens)Cabbage Carrots and Caraway
Fennel (Foeniculum vulgare)   Most plants
Feverfew (Chrysanthemum parthenium syn. Tanacetum parthenium) Rosesattracts Aphids awayseeds freely
Garlic Roses, Raspberries Aphids  
Henbit (Lamium amplexicaule)  General Insect Repellent  
Hyssop (Hyssopus officinalis ) Cabbage, GrapesCabbage White ButterfliesDislikes Radishes
Lavender (Lavandula angustifolia)   Moths - make sachet with southernwood, wormwood and rosemary, use in doors  
Marigolds (Tagetes sp.) most have aromatic foliageGood 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 Beetlesinvasive roots
Mustard (Brassica juncea)cabbage, cauliflower, radish, brussel sprouts, turnips, and kohlrabia trap crop to attract many insect pests 
Nasturtium (Tropaeolum majus)Radishes, Cabbage, Squashes and Pumpkims, fruit treesAphids, Squash Bugs, Striped Pumpkin Beetle 
Oregano(Origanum vulgare)BrassicasCabbage Butterflies 
Pennyroyal (Mentha pulegium ) RosesFlies, Mosquitoes, Fleas, others invasive roots
Petunia (Petunia hybrida)Beans  
Pot Marigold (Calendula officinalis)TomatoesTomato Worm, Asparagus Beetles, Whitefly 
Pyrethrums (Chrysanthemum cinaeriofolium) Dried flower heads make an insecticide. 
Rosemary (Rosmarinus officinalisCabbage, Beans, Carrots, Sage Cabbage Butterflies, Bean Beetle, Carrot Fly 
Rue (Ruta gaveolens) Roses and RaspberriesJapanese BeetlesDislikes Sweet Basil
Sage (Salvia officinalis)Rosemary, Cabbage, CarrotsCabbage Moth, Carrot Fly, Flea Beetle, SlugsDislikes Cucumbers
Southernwood (Artemesia abrotanum)CabbagesCabbage Butterflies 
Summer Savory (Satureia hortensis )BeansBean Beetles 
Tansy (Tanacetum vulgare)Fruit Trees, Roses, RaspberriesFlying Insects, Japanese Beetles, Striped Cucumber Beetles, Squash Bugs, Ants, Flies  
Thyme (Thymus vulgaris)CabbageCabbage 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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