Like humans and animals, plants love harmonious companionship! Many people don't realize that certain plants "don't like" each other, or might not grow or produce properly if seated next to a plant they "dislike". Likewise, there are plants who thrive near or are complemented by others.
"Natural plant associations" are groups of plants that occur and live together in nature, mutually complementing each other. This could mean that a shade loving plant lives happily underneath a taller sun-lover, or that a deep rooted plant resides contentedly alongside a shallow-rooted friend. These plants don't usually flower and fruit at the same times and often mature at different rates to maximize their light source.
Herbs may enhance or hinder the growth of their neighbors. For example, dandelion exhales ethylene gas, a hormone that promotes the premature ripening of fruits and fruiting of plants, inhibiting neighboring plant growth. Some herbs repel harmful insects by emitting aromas from their leaves or roots or aid fruit and vegetable plants by attracting otherwise harmful pest insects to themselves. Aromatic herbs like chamomile, chives, lavender, marjoram, parsley, sage, tarragon, thyme and yarrow enhance neighboring growth, repel pest insects, and attract butterflies and bees to your garden!
The active constituents and essential oil content can be increased in herbs by planting stinging nettle and yarrow in your herb garden. Peppermint's essential oil is almost doubled when grown with stinging nettle! I think that is just amazing.
The following is a list of companion herbs and short descriptions of their uses. When I say a plant "likes" another, it simply means it helps or is beneficial to its growth.
Alfalfa: Protects shallow rooting plants and is one of the few plants that actually grows well with Dandelion.
Anise: Seeds germinate and grow better alongside Coriander.
Basil: Really doesn't like to grow near Rue! Likes tomatoes, peppers, oregano and asparagus.
Borage: Attracts lovely pollinating bees. Likes just about all plants; specifically aids strawberry, tomato and cucurbit growth.
Chamomile: Increases essential oil content of Peppermint when grown nearby. Likes Basil, onions, cucumbers and cabbage.
Celery (the word "herb" includes all plant species): Mutually beneficial grown with tomatoes.
Chervil: Helps broccoli, lettuces and radishes.
Chives: Great under apple trees. It prevents apple scab! Also likes carrots, tomatoes, broccoli and cabbage.
Cilantro: Helps spinach.
Coriander: Hinders fennel seed formation. Attracts bees.
Dandelion: Exhales ethylene gas, inhibiting neighboring plant growth.
Dill: Mature dill inhibits carrots. Likes cabbage, broccoli, corn, lettuces and asparagus.
Fennel: Inhibits dwarf beans and tomatoes.
French Marigold: Roots excrete a substance which kills nematodes. Likes tomatoes.
Garlic: Great companion for roses. Controls potato and tomato blight. Garlic spray treats brown rot of stone fruit like peaches, plums, nectarines, apricots and cherries. Along with onions and shallots inhibits growth of peas and beans. Also likes cucumbers, peas, lettuces and celery.
Hyssop: Inhibits radish growth. Likes cabbage and increases grape yields when planted near grapevines.
Lavender: Attracts beneficial insects and aids general plant growth. Helps repel the clothes moth, whose larvae feed on wool, feathers, fur, hair, leather, lint, dust, paper, and occasionally cotton, linen, silk, and synthetic fibers.
Lemon Balm: An all around beneficial herb, and is said to promote milk flow in cows when planted in their pasture/meadows! I've also read that if you rub Lemon Balm inside a new bee hive, the swarm will never leave. Fascinating! I plan on trying this out for myself this fall when we put out some new hives. I do know that our honey bees love the lemon balm that grows in my garden!
Marjoram: Likes peppers.
Mint: Repels cabbage butterfly caterpillars.
Nasturtium: Helps to keep broccoli free from aphids. Great planted under apple trees to keep away woolly aphids.
Parsley: Great for roses and tomatoes.
Pennyroyal: Repels ants and protects against mosquitoes.
Peppermint: Protects cabbage from the white cabbage butterfly.
Rosemary: Repels the carrot fly. Rosemary and Sage have a stimulating affect on each other. Also likes beans.
Roses: Aided by Garlic and Parsley.
Rue: Inhibits most neighboring plants, especially Basil. But it repels houseflies, so I keep some close to my house!
Sage: Also protects cabbages from the white cabbage butterfly and tends to make cabbages more tender.
Summer Savory: Beneficial to onions and green beans.
Spearmint: Repels ants and controls aphids on vegetables.
Stinging Nettle: Very beneficial to neighboring plants by increasing essential oil yields and medicinal constituent contents. Just keep the little ones out of your nettle patch. It bites!
Tansy: Repels ants and flies. Tansy is also perfectly gorgeous.
Tarragon: Generally beneficial for the whole garden.
Thyme: Repels the cabbage root fly.
Valerian: Attracts one of my favorite garden visitors, the earthworm!
Winter Savory: General insect repellent.
Wormwood: I haven't personally had any trouble with it, but it's known to inhibit the growth of neighboring plants. It also repels moths and helps protect cabbages from the .. you guessed it .. white cabbage butterfly.
Yarrow: Increases the aromatic quality of all herbs and enhances growth of neighboring plants.
This isn't all of them, by any stretch, but it's a good starting point. There are lots of books out there on companion planting and Amazon.com is a great place to find good, used (a.k.a. cheap) books, and they have plenty to choose from on this subject. The best way to utilize companion planting is to start with a general guide and experiment in your own garden. This method is invaluable for the organic garden and for defense against the 300+ insects in the world that are now resistant to an increasingly wide array of pesticides. More on natural pest control later...
Happy planting! :D