Plants have an effect on the environment around them. Their sense of survival gives them the ability to change the chemistry of the soil, produce scents that discourage insects, develop ways to store water for hydration, or give off toxins to prevent competition. Understanding how each plant works, then combining plantings in the garden to take advantage of those abilities is what companion planting is about.
Cover cropping is the most important companion plant to have in the home garden. Cover crops are deep-rooted crops that increase potassium and nitrogen in the soil. The large roots loosen the soil, making a good place to plant root crops, such as carrots or onions. Cover crops are usually mowed or cut before they seed. The plant material is then tilled into the soil, where it decomposes and becomes a green manure in the soil. Good selections for spring cover crops are buckwheat or winter rye. Late summer or fall cover crops are alfalfa and clover.
Some plants produce a natural herbicide that discourages other plants from growing. Sunflowers, oats, alfalfa and rye are examples of plants that produce this natural herbicide. Try planting an oat cover crop in the spring. Once the oat crop is tilled under, use the space for something that is difficult to weed through, such as carrots. Other plants, such as squash, work well when planted around corn. Squash does not have the natural herbicide tendency, but it does have large leaves that provide too much cover and shade for weeds to grow.
There are plants that give off a chemical to detract insects. For example, French marigolds contain a substance that is toxic to soil-living nematodes. Some insects use a sense of smell for their food. Gardeners can confuse this sense by planting culinary herbs that a have a strong scent to them. There are herbs with scents that repel insects. Intersperse plants such as basil, garlic, onions, tansies or wormwoods throughout the garden to repel insects.
Plant flowers or crops to attract beneficial insects to the garden. Beneficial insects include ladybugs, spiders, wasps, ground beetles and praying mantis. They will eat insects that damage a garden. Several of the plants that attract beneficial insects are already in a culinary garden, such as dill, parsley, carrot, coriander and parsnip. Scented flowers that are planted near the garden can also help attract beneficial insects.
Companion planting has both a science and a folklore base. There are some combinations that are common. Basil planted with tomatoes can repel tomato hornworms. Thyme planted along the border of the garden can deter slugs. Grow clover between corn rows to replace the nitrogen the corn takes out of the soil. Garlic is supposed to repel the aphids that damage roses. Nasturtium vines attract beneficial insects and work well when planted with cucumbers, as both need a trellis on which to climb. Folklore also says that nasturtium repels squash and cucumber beetles. Plant radish with spinach, as the radish will attract the leaf miners that damage spinach crops.