Companion Planting Ecogardening Factsheet #10, Winter 1994

Most people think of plants as very passive organisms. They grow almost unperceptively, and only once a year do they flower or produce edible products. However, plants are very active in ways that are not so obvious to the casual observer. For example, plants change the chemistry of the soil, and influence the types of microorganisms that grow there. They actively compete with other plants for space. Some will poison their neighbor's offspring to maintain a competitive advantage, while others change the environment in ways that benefit other species. Plants wage a constant battle with insects, relying heavily on chemical warfare.

Naturalists have known about these properties of plants for thousands of years. For example, about 2,000 years ago the Roman agriculturalist, Varro, declared "Large walnut trees close by, make the border of the farm sterile." Chemicals in oak leaves retard the development of insects that feed on them. Some insecticides are derived from plants; examples include rotenone, sabadilla and ryania. But not all effects of plants are deleterious on other organisms. Alfalfa and clover enrich the soil with nitrogen that they capture from the air. Certain trees move groundwater to the soil surface where shallow-rooted plants can grow even under droughty conditions. Groups of plants which grow well together are called "companions."

Â© Copyright, Department of Horticulture, Cornell University.