Companion gardening is the planting of two or more plants in close proximity for the benefit of one or all of them.  Benefits provided by the companion plant may include:

1. Giving off chemicals that can deter pests.
2. Improving growth due to the companion plant adding something beneficial to the soil.
3. Providing shade for lower-growing plants.
4. Attracting beneficial insects needed for pollination or for controlling harmful garden pests.
5. Luring pests away from other plants.  In some cases the luring plant actually destroys the pests.  (3, page 1) 

An example of a plant giving off a chemical to deter pests is the marigold.  It produces thiopene that repels nematodes.  Therefore, the marigold makes a good companion plant for crops such as eggplants, parsley, and tomatoes—all which are subject to damage from nematodes.    The mustard plant releases thiopene in soil.  Thiopene inhibits the emergence of the cyst nematode that causes root rot in collards and Brussels sprouts. (1, page 395)

Chemical reactions may take place between plants.  The common dandelion was held in higher esteem with earlier gardeners as a companion plant in orchards and vegetable patches.  The dandelion not only attracts pollinating insects but it gives off ethylene, a gas that encourages fruit set and ripening. (1, page 107)  Legumes fix nitrogen in soil and therefore are a good companion plant for corn, spinach, and lettuce.Plants that shade the soil might keep their neighbor crop from becoming parched and woody.  Chervil makes sense as a companion for radish.  Nicotiana (flowering tobacco) and cleome (spider flower) give lettuce the light shade it needs to grow later in the planting season. (1, page 107)Alyssum flowers attract hoverflies whose larvae devour aphids.  (6, p. 13)  Planting alyssum near broccoli, cabbage,  Brussel sprouts, lettuce, and other plants commonly injured by aphids.While some companion plants protect their neighbors by repelling pests, others sacrifice themselves, luring pests away from other plants.  In some cases, the trapping plant destroys the pests as well.  As they feed, they get sick or lose their ability to lay eggs.  Eighteenth-century growers recommended radishes as a “trap” for cabbage maggots.  Oregano can be planted near cucumbers to repel cucumber beetles.An interplanted garden—one with many different kinds of plants chosen for sheer variety—can be healthier than a single-cropped one.  Since different species flower and fruit at different times, there tends to be less competition for nutrients and moisture.  Furthermore, the different odors, colors, and textures a heavily interplanted garden has seem to confuse pests, so it is more difficult for them to find their favorite food. A number of controlled experiments show that some common garden herbs and vegetables do indeed influence the health and growth of their neighbors.  How many plants exert this power and to what extent remains to be discovered.

Books:1.  Kowalchik, Claire, and William H. Hylton (eds.), Rodale’s Illustrated Encyclopedia of   Herbs, Rodale Press, Emmaus, 1987.2.  Oster, Doug, and Jessica Walliser, Grow Organic, St. Lynn’s Press, Pittsburgh, 2007.

Internet:3.  Dgsgardening.btinternet.co.uk/companion.htm4.  En.wikipedia.org/wiki/List_of_companion_plants5.  Ext.nodak.edu/county/cass/horticulture/vegetables/companion.htm6.  Ghorganics.com7.  Organicgardening.com (enter Companion Planting in the “search” box)