Photo Credit: Scott Vlaun, courtesy Seeds of Change

Peas, like other members of the legume family, provide nitrogen to corn.

When you observe plants in an undisturbed natural setting, you may not realize that often they grow where they grow because they have a beneficial relationship with each other. It makes sense that when you follow through with this concept of mutual benefit in your own garden, you not only have flourishing plants, you also don’t need an arsenal of chemicals to promote these happy relations.

Companion planting, as this concept is known, has been practiced for centuries, although the reasons why it works haven’t always been well understood. The benefits of companion planting are varied but can be categorized by type. Here are some of the best-known methods:

• Fix nitrogen in the soil by planting certain nitrogen-releasing species. As many gardeners may know, nitrogen is one of the three essential elements for successful plant growth. One of the best sources of nitrogen is legumes, which includes beans, peas and clover. These plants draw nitrogen from the atmosphere for themselves, as well as their neighbors, through bacteria in the soil known as rhizobium. Beans are often planted with other crops – corn, for instance – to provide nitrogen and reduce the need for fertilizer. Clover is planted with grasses for the same reason.

Photo Credit: Linnea Thornton

The yellow-flowering mustard plant in this border is eaten by flea beetles, which are distracted from the turnips, brassicas and arugula in the nearby plot. The other border plants offer a strong sagebrush smell, discouraging other insects from entering the garden.

• Divert pests from a desirable crop through “trap cropping” – a method that requires you to either plant or let grow a species that will attract pests away from your desirable crop. For example, some organic gardeners plant Chinese Southern giant mustard (Brassica juncea var. crispifolia) in borders to divert flea beetles from their cole crops (broccoli, cabbage, mustard and collard greens). Another example is the ornamental four o’clock (Mirabilis spp.), which attracts Japanese beetles, then poisons them with a toxin in its foliage.
• Suppress pests with plants that release biochemicals. Marigolds are among the foremost examples of an attractive cultivated plant that helps repel or suppress pests by releasing a chemical deterrent. Nematodes (the unbeneficial kind) are among those plant-attacking insects repelled by thiophene, the chemical found in marigolds. This chemical is released in the soil from the plant’s roots. Marigolds (go for the aromatic ones) are also believed to deter a variety of other pests through a scent that’s obnoxious to many insects. Another example is basil, which can be planted with tomatoes to deter thrips.