Companion planting is based on the idea that certain plants can benefit others when planted nearby. Companion planting is simply the practice of planting two or more plant species in close proximity so that some cultural benefit (pest control, higher yield, etc.) is derived. Perhaps the best historical example of companion planting is the “Three Sisters” in which corn, beans and squash are planted together in a hill. Native Americans developed this system to provide food for a balanced diet from a single plot of land. Each of the crops is compatible with the others in some way. The tall corn stalks provide a support structure for the climbing beans. The beans do not compete strongly with the corn for nutrients since as legumes; they can supply their own nitrogen. Squash provides a dense ground cover that shades out many weeds which otherwise would compete with the corn and beans.
Most plants produce defensive chemicals that help to fend off both insect and disease pests. These chemicals may be insect poisons, feeding deterrents or have fungicidal properties. The roots of some French and African marigolds contain a substance which is toxic to certain types of nematodes. Nematodes are soil inhabiting microscopic roundworms that damage many species of plants. Certain nematodes can be eliminated from a site by growing a thick crop of marigolds for one season prior to planting the vegetable or fruit crop, or by inter-planting marigolds between crop rows.
Destructive insects often locate their food by smell. Many plants, especially culinary herbs, produce strong scents which may confuse insect pests looking for a host to feed on. Garden vegetable plants, such as garlic, onions, chives, and herbs such as catnip, horehound, wormwood, basil and mints, all produce scents which seem to repel insects or mask the scents which attract insects. A certain level of insect protection can be achieved by carefully inter-planting some of these as companions to vegetables.
Many insects are helpful because they eat or parasitize harmful insects. Most species of wasps and spiders are beneficial, as are ground beetles, praying mantids, lady bird beetles, pirate bugs and several species of flies. It is possible to attract beneficial insects by planting flowers near the garden. Dill, parsley, carrot, coriander, angelica, and parsnip feature flat topped clusters of small flowers that have strong fragrances. They also seem to attract large numbers of beneficial insects, particularly predatory wasps and flies. This characteristic makes them good candidates for companion planting.
Companion plants can increase garden yields and flower production. Mixing flowers and herbs with vegetable plantings creates better vegetables. The flowers produce nectar, attracting predator insects to protect the vegetables from harmful insect pests.
Here are combinations that folklore says are effective companions.
Chives planted at the base of roses repels aphids, garlic planted at the base of peach trees repels borers, basil planted among tomatoes may repel tomato hornworms; nasturtiums grown near squash may repel squash bugs; tomatoes planted among asparagus may repel asparagus beetles; and marigolds, mint, thyme or chamomile may repel cabbage moths. Garden borders planted with low-growing thyme or lavender may deter slugs and pennyroyal repels ants.
Many plants produce substances that are toxic to other plants. The best example of this phenomenon, called allelopathy, is a substance called juglone which is produced by black walnut trees. Juglone acts as a natural herbicide and prevents the growth of many different plants beneath or near black walnut trees. Additional common landscape trees with allelopathic properties include sugar maple, tree-of-heaven, hackberries, southern wax myrtle, American sycamore, cottonwood, black cherry, red oak, black locust, sassafras and American elm. Other plants having allelopathic effects include sunflowers, cucumbers, oats, alfalfa, rye and tobacco. When these crops are planted prior to other crops, weed pressure is reduced.
On a final note, grass plantings between rows of perennial crops such as fruit trees and blueberries are highly desirable companion plants. The grass alleys cool the soil, prevent erosion, improve water penetration, exclude weeds and harbor beneficial insects.
Donald D. Tapio is the Washington State University Extension regional specialist for Grays Harbor and Pacific counties. He can be reached at 482-2934 or firstname.lastname@example.org.