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.

Border planting

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.

Photo Credit: Linnea Thornton

Lettuce and sunflowers

Sunflowers provide shade for lettuce, which requires cooler conditions to flourish.

Photo Credit: Scott Vlaun, courtesy Seeds of Change

Corn and peas

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

Photo Credit: Scott Vlaun, courtesy Seeds of Change

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.
  • 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) 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), 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.
  • Suppress weeds and other unwanted undergrowth by shading them out with other plantings. Members of the gourd family (squash, cucumbers, pumpkins) are good examples of plants that can shade out weeds thanks to their low-spreading habit.
  • Protect plants by providing shade. Some plants, such as lettuce and spinach, need the cooler conditions offered by shade to flourish. Planting a taller species over them, like sunflowers, can help.
  • Provide a diverse or otherwise beneficial habitat for your plants. It just makes sense that the more diverse your plantings, the better chance your plants will have to prosper. Monocultures, or extensive plantings of one type of crop or plant, can be havens for a particular type of pest. Under natural conditions, you’ll seldom see anything resembling the monocultures that we humans create. Even mixing cultivars in a planting can sometimes help diminish pest attacks. Also, study lists of plants that should not be planted next to each other. For instance, though the alkaloids in the leaves of tomato plants are believed to deter black spot in roses (and thus tomatoes are good companions to rosebushes), the roots of tomato plants secrete a chemical that appears to inhibit the growth of apricot trees.
  • Create a beneficial habitat, or refugium, as it’s properly known. Some plants provide a desirable environment for beneficial insects, particularly predatory and parasitic species like ladybird beetles, lacewings, hover flies, mantises, robber flies and some types of flies and wasps, as well as noninsects like spiders and predatory mites.

All these examples illustrate the importance of thoughtful research when planning a garden or landscape. Plants, just like the humans who cultivate them, are sensitive to their neighbors, forming better relationships with some plants and finding others obnoxious (or even impossible) to live around. So consider planting a garden – whether flower- or veggie-filled, indoor or out – where you can maximize the benefits of this ancient practice of companion planting.