plant pals

Companion plants mutually benefit each other and produce tastier vegetables and higher yields. Here’s a look at plants that like to be together in the garden and ones that don’t.

By David Wann

When two people like each other, we call it “good chemistry.” When two plants thrive in each other’s company, gardeners call them “good companions.” No one knows exactly why this is true, but companion vegetables suffer fewer pest attacks, grow more vigorously and have better yields. Sometimes they even taste better, too.

While the hard science of companion planting is not complete, gardeners do have a long history of experience and observation to draw upon. Gardeners have been experimenting with companion planting since at least 300 B.C., when the Greek philosopher Theophrastus started recording his observations in a journal. A pupil of Aristotle, Theophrastus classified more than 500 Mediterranean plant species and described plant physiology and morphology in modern terms.

In the 1970s, ecologists Peter Atsatt and Dennis O’Dowd documented the ways in which physical, chemical and biological mechanisms combine to create a resilient plant community that impedes harmful insects’ senses of smell, sight and touch. They also recognized that interdependence among neighboring species takes place above and below the soil.

One study found that when cucumbers were planted in “monocultures” (without any diversity), they attracted 10 to 30 times more disease-carrying striped cucumber beetles than cucumber plots interplanted with good companions, like dill.

Dill is a good cucumber companion from seedling to pickle jar because it directly repels aphids and spider mites, and attracts beneficial insects like parasitic wasps and lacewings that control cucumber beetles and other pests. The herb also appears to stimulate a better flavor in cucumbers as the two plants mature.

Certainly, chemistry explains some of the mutual admiration among plant species. It’s well documented that when plants are attacked, some send out chemical “alerts” that stimulate surrounding plants to increase the natural toxicity in their leaves. The fact is garden plants have a vested interest in their neighbors’ health, as they’re all in it together—literally.

Sometimes, normal concentrations of plant toxins are enough to serve as pest deterrents. Solanine, an alkaloid found in tomato leaves and other members of the nightshade family, helps ward off cabbage loopers on neighboring broccoli and other cole crops, making tomatoes and broccoli good companions.

Sulfur compounds in onions seem to prevent certain pests from attacking tomatoes, while mint’s volatile compounds provide some protection for both tomatoes and cabbage. In addition to repelling bad bugs, mint attracts and harbors many beneficial insects because good bugs enjoy a nip of the blossom’s nectar and pollen after feasting on an entrée of bad bugs.

Nasturtium is another good addition to a diverse garden. It performs as a “trap crop,” attracting aphids that would otherwise prey on innocent garden crops. It also provides brightly colored blossoms—especially in nutrient-poor soil—that attract beneficial insects.

There are other reasons why plants prefer some neighbors over others. Crops that get along well don’t compete for nutrients and water. Deep-rooted squash, for example, is a good companion for shallow-rooted corn, and appreciates the late-summer shade the corn provides. Squash also likes to grow near onions, which help curb the appetite of squash bugs.

In general, heavy feeders like cabbage grow well with light feeders like garlic and beans. Many garden crops like to grow with beans and peas because of the industrious bacteria colonies living on their roots. These colonies pull nitrogen right out of the air and share it with the plants, to the benefit of both.

From Garden to Plate

Many companion-plant relationships continue after harvest: Dill and cucumbers, basil and tomatoes, lettuce and radishes, and beans and savory often go straight from the garden into the skillet, salad bowl or dinner plate.

But other plants truly don’t like each other. Cabbage and strawberries would rather not share the same plot. Tomatoes and corn may find common ground in a salad, but not in the garden. Dill and carrots are openly antagonistic, and onions and beans are not great friends, either.

In the upcoming garden season, I’ll be on the lookout for potato allies to help me discourage the Colorado potato beetle. I have an established patch of horseradish and I intend to test its reputation as a companion plant, as well as that of marigold and possibly tansy, although the latter can spread too quickly.

I’ll also start several herbs from seed in pots and disperse them throughout the garden. Mint, fennel, catnip, southernwood and borage will all find useful niches, and I’ll keep the wild chamomile on the edges of pathways. I’ll also interplant fragrant annuals like nicotiana and petunia, and nitrogen-fixers like lupine and volunteer clover.

I think of these strategies as “diversity by design,” which supplies pest control, fertility and beauty at the same time. They also help me achieve a primary goal: guiltless gardening. Rather than let crops become sitting ducks for bugs and hungry mammals, I’ll be a better companion to them, too, if I provide a measure of protection by growing plants together that like each other.

David Wann coordinates a community garden in Golden, where he tries to be a faithful companion to all his crops. He’s the author of The Zen of Gardening in the High and Arid West (from which this piece was excerpted) and teaches gardening at Denver Botanic Gardens.

Read insightful articles, find helpful home and garden tips and links to resources in every issue of Boulder County Home & Garden Magazine. Subscribe today!

© 2006 Brock Publishing Company
homes@brockpub.com