Different plants have natural elements in different parts (roots, leaves and flowers) that can either repel and/or attract insects, and can also help enrich the cultivation and flavor of other plants. Gardeners can plant a variety of flowers and herbs as companion plants to fit their certain needs. For example, companion plants can help to repel pests while still attracting beneficial insects.

Companion planting allows different plants to complement one another, as well as helps improve and balance the eco-system in your farm or garden, promoting environmentally-friendly symbiotic relationships.

References:

http://www.ghorganics.com/page2.html

http://www.seedsofchange.com/enewsletter/issue_55/companion_planting.aspx