For hundreds of years before the arrival of Europeans, the Iroquois planted corn, beans and squash--called the three sisters--together in their fields. In this early form of what organic gardeners now call companion planting, the corn provided a climbing structure for the beans, which replenished nitrogen lost from the soil, and the large squash leaves shaded the ground, conserving water and controlling weeds. According to the National Sustainable Agriculture Information Service, companion planting places plants in proximity to each other based on the positive effects that they exert over each other. As "Rodale's Ultimate Encyclopedia of Organic Gardening" notes, companion planting often has its basis in tradition and folklore, though science is increasingly confirming its effectiveness and revealing the underlying biochemical mechanisms of its success.
Repelling pests is one of the most common uses of companion plants. Strong-scented plants, such as marigolds and mints, are especially effective, perhaps by masking the scent of plants desirable to pests. For example, planting marigolds in a flower garden may keep moth larvae from destroying your flowers, since moths won't land on a plant that they can't smell. Potted herbs or mulch made from scented herb leaves may also protect some plants from pests. Similarly, in trap cropping, gardeners and farmers plant companions that will attract pests from the main crop, such as planting collard to draw moths from cabbage.
Other companion plants have the opposite effect and attract insects--the beneficial kind. Certain species of insects like wasps and ladybugs eat pests, so attracting them to your garden is another chemical-free way to reduce pest infestations. "Rodale's Ultimate Encyclopedia of Organic Gardening" recommends planting varieties with numerous small flowers, such as coriander, calendulas or sunflowers, since these are easy-to-reach food sources for many beneficial insects.
As plants grow, they draw nitrogen from the soil and use it to build essential proteins. While nitrogen is overwhelming the most abundant gas in the Earth's atmosphere, it is not in a form that can be used by plants and requires "fixation" before it can be taken up by roots. Certain plants species--most notably legumes like clover and beans--have helpful bacteria in their root systems that fix atmospheric nitrogen, returning it to the soil for plants to use. Although the precise science was unknown to the Iroquois, nitrogen fixation explains the incorporation of beans into the three sisters, as beans fix nitrogen in the soil. The National Sustainable Agriculture Information Service recommends companion planting beans with any vegetable or herb to obtain the nutritive benefits of nitrogen fixation.
Efficient Use of Space
For the backyard gardener for whom space is a concern, companion planting allows for efficient use of even small plots of land. Combining tall, sun-loving plants with shade lovers that grow close to the ground lets those plants inhabit the same space. Other plants provide protection from harsh weather, wind and weed growth. Returning to the example of the three sisters, as the National Sustainable Agriculture Information Service points out, tall corn disorients squash bugs, while the prickly squash vines provide a physical barrier to raccoons try to get at the corn.
Planning for Companion Planting
Hundreds of years of folklore, recently bolstered by scientific investigation, provides advice on how to grow plants together. While most species exert, at worst, no influence on each other, and handful may have a negative influence, so planning is important. Organizations like the National Sustainable Agriculture Information Service maintain lists of plants that should be planted together and those that should not.