Companion planting is based on the idea that certain plants will benefit from one another when planted next or close to one another.

Companion planting is usually thought of as a small scale gardening practice, but it can easily be applied on larger-scale. It has been proven that by having a beneficial crop in a nearby field that lures certain insects away from a neighboring field that has the primary crop this can prove very beneficial. This action is known as trap cropping.

A small "Three Sisters" garden. The corn provides a trellis for the beans to climb. The beans give nitrogen to the corn and the squash sprawls along the ground, creating a micro climate that conserves water and suppresses weeds.

While companion planting has a long history, the benefits have not always been understood. Traditional recommendations for companion planting have been used for a long time, but recent tests are proving scientifically that it is a lot more beneficial than it was once thought to be.

Another way that companion planting can be very beneficial is to plant a crop like any legumes, in an field or area where it will add nitrogen into the soil, then it isn’t necessary to use any chemical fertilizers for the next crop. The African marigold, along with several other plants, are known to be well suited for companion planting, as they exude a chemical from their roots or leaf parts that can suppress or repel pests and protect plants near them.

Companion planting also exists in physical ways. For example, tall-growing, sun-loving plants may share space with a lower-growing, shade tolerant species of plant, resulting in higher overall yields from the land. This is called spatial interaction, and can also result in pest control benefits, for example, the presence of a prickly vine is said to discourage raccoons from trying to eat sweet corn.

Yet another form of companion planting is called ‘nurse cropping’, this is where tall or dense canopied plant is used to provide protection to a more vulnerable plant by providing shade or by being a windbreaker. Oats have been used to help establish alfalfa crops and other forages by replacing the more competitive weeds that would grow in their place. In many instances, nurse cropping is just another example of physical spatial interaction.

Companion planting offers benefits to certain plants by giving them pest control without needing to use chemicals, and can and often do cause a higher crop yield than they would have normally without being planted next to a suitable companion. Beneficial habitats, often called refugia, are another way you can use companion planting that has received a lot of attention recently. The benefit is when companion plants provide a solid environment for beneficial insects, and arthropods, especially those of the predatory and parasitic species that can help keep unwanted pest populations in check.

Here is a helpful table listing common companion plants courtesy of http://www.companionplanting.net

List of Companion Plants

Here follows a list of some of the plants that grow well together, for the vegetable garden.

Plant	Companions	Incompatible
Asparagus	Tomato, Parsley, Basil
Beans	Most Herbs & Vegetabes	Onion
Cabbage	Aromatic Herbs, Celery, Beets, Onion Family, Chamomile, Spinach, Chard	Strawberries, Tomato, Dill
Carrots	Peas, Lettuce, Onion, Sage, Tomato	Dill
Celery	Nasturtium, Onion, Cabbage, Tomato
Cucumber	Beans, Peas, Sunflower, Raddish	Aromatic Herbs, Potato
Lettuce	Carrot, Radish, Strawberry, Cucumber
Onions	Beets, Carrot, Lettuce, Cabbage	Beans, Peas
Parsley	Tomato, Asparagus
Peas	Carrots, Raddish, Turnip, Cucumber, Beans,	Onions, Potato
Potato	Beans, Cabbage, Horseraddish, Marigolds	Sunflower, Cucumber, Tomato
Raddish	Peas, Nasturtium, Lettuce, Cucumber	Hyssop
Spinach	Strawberry, Faba Bean
Tomato	Onion, Marigold, Asparagus, Carrot, Parsley, Cucumber	Cabbage, fennel, Potato
Turnip	Pea	Potato