Companion planting means growing two or more different plants in close proximity for a specific purpose, such as better pest control or higher yields. In the past, many companion planting practices were based on folklore or bad science, but recent research into reducing pesticide use on farms has shed new light on how companion planting works.
Increased diversity is at the heart of companion planting. Incorporating plants that attract beneficial insects, deter pests, or invigorate the soil will turn up the volume on interactions between plants, insects, and the soil food web — and the result is boosted garden resilience. A successful combination often has multiple benefits. For example, when arugula and onions are grown together, the onions repel some flea beetles from the arugula, while the wide leaves of arugula smother weeds and shade the onion roots. A good companion planting scheme often has a ripple effect that will enhance the health of your vegetable garden.
The trick is finding plant combinations that work well, are easy to manage, and accomplish one or more of these goals:
• Reduce pests.
• Deliver better weed control.
• Enrich the soil.
• Provide shelter from weather stress.
You can choose companion plants that influence insect activities in two different ways: by masking the presence of crops the pests are attracted to, or by attracting predators that will attack the pests.
In the first scenario, flying insects search for their favorite host plants by picking up their clues from the air, with their antennae sensing the gaseous plumes given off by the plants. Then, they use their eyes and feet to further examine a promising host plant. When confusing companion plants, such as marigold or mint, are part of the mix, cabbage and onion root maggots are reduced because the mother flies get frustrated after landing on the wrong plants too many times.
When it comes to “masking” crop plants with confusing companions, onions are the star vegetables in several research studies. Bulb onions, green onions, garlic, and other alliums don’t look or smell like other vegetables, and their upright stature makes them easy to use in companion planting combinations.
With little mites, thrips, aphids, and whiteflies that blow in on the wind, there’s not much host-plant searching to interrupt. But you can alter these pests’ access to your garden with a tall hedge of plants that filters incoming winds. A companion planting of dill on the south side of your garden might sieve out spider mites before they reach your cucumbers, or a sunflower screen could create a windbreak that stalls incoming aphids. Both dill and sunflowers provide nectar and pollen for ladybugs, lacewings, and other general predators that will help clean up soft-bodied insects that make it through the screen.
You’ll need a different companion planting strategy after pests become established in the garden. By summer, companion plants that attract beneficial insects skyrocket in value. Dozens of studies have shown that having plenty of natural predators means fewer pests. Attracting those predators is often as easy as growing plenty of flowers with small, open blossoms. Think spring-planted arugula, cilantro, or mysterious mustards from mesclun mixes that bolt into bloom as days get longer.
The chart below suggests easy plants for attracting beneficial insects, many of which have multiple talents. For example, the large, spreading leaves of borage create a nice habitat for ground beetles. Borage leaves are too hairy for slugs and aren’t preferred by spider mites, but they do host a few aphid species of interest to lacewings and hover flies, which helps build up predator populations. Borage blossoms attract honeybees and wild bees, too, and the heavy bee traffic may make the air space near borage dangerous for cucumber beetles and other small flying insects.
Insectary plants are agreeable in garden beds because most of them produce pretty flowers. When viewed from above by the big, compound eyes of hover flies and lacewings, the nectar in the shallow florets of sweet alyssum, Queen Anne’s lace, and yarrow glisten like jewels. Larger wasps appreciate the comfortable footing provided by coreopsis, cosmos, and other daisy-shaped flowers.
Spiders prey on anything that moves. Low-growing companion plants, such as clovers, lettuces, or radishes, can become spider havens when grown among taller plants, such as broccoli or tomatoes.
Deliver Better Weed Control
The close spacing typical of most companion planting schemes naturally squeezes out weeds. With widely spaced vegetables, such as cabbage or tomatoes, you can use a fast-growing companion crop of lettuce or other leafy greens as a temporary ground cover in the open space.
Planting cilantro, lettuce, mustard, arugula, or other leafy greens between onions, cabbage, tomatoes, or peppers, or between rows of slow-growing carrots or beets, creates a weed-smothering green mulch that can be harvested before the larger plants need more space. If you allow a few of the interplanted greens to flower, they’ll then serve a stint as insectary plants.
The weed-suppressing abilities of vigorous vining squash is one feature that makes the ancient Three Sisters garden work. In this famous three-way companion planting plan, beans run up the corn, and the ground around the upright crops is shrouded in vines.
In similar fashion, you can allow pumpkins or vining squash to run under corn or upright blueberries or raspberries and they’ll do a good job of keeping weeds down in late summer.
Enrich the Soil
Gardeners often report that carrots and snap beans grown with marigolds and other companion plants taste better. A Polish study found that carrots intercropped with marigolds or calendula were sweeter than those grown without companions, which was likely due to healthier conditions below ground, in the root zone.
Marigolds exude root chemicals with bactericidal, insecticidal, and fungicidal effects, making it easier for their neighbors to enjoy robust root health. In addition, some marigolds release natural nematode repellents into the soil, making them good companions for tomatoes, potatoes, peppers, okra, and other nematode-susceptible crops. Marigolds’ gifts to the soil continue after the plants are dead. In one study, potato yields rose between 8 and 14 percent when they were planted after marigolds, probably due to beneficial residual microflora on the rotting marigold roots.
Legumes — peas, beans, and clovers — fix atmospheric nitrogen and store it in root nodules for their own use, and benefit plant neighbors by ramping up activity levels in the rhizosphere. This is why bush snap beans and potatoes make such good bed partners, and why legumes in general are important players in Three Sisters gardens and other classic companion planting combinations. By invigorating the rhizosphere, legumes used as companion plants enhance the ability of the crop plants to feed themselves, often with additional benefits. In a study in which broccoli was planted with a companion crop of crimson clover, the clover kept the soil cool and moist and provided habitat for predatory spiders, which reduced pest problems. The most common clover in North America, Dutch white clover (Trifolium repens), can be allowed to grow as a companion provided it doesn’t leave the main crop with inadequate water.
Provide Shelter from Weather Stress
One of the most versatile planting methods for home gardeners is to use companion plants to shelter crops from too much sun, wind, or flooding rains. Gardens laid out in rows are a perfect setting for placing selected plants in adjacent strips — say, peppers on the shady side of sweet corn. In most climates, peppers that get an afternoon break from summer sun produce more perfect fruit.
Several other vegetable pairings exploit the plants’ physical compatibility — such as growing celery, lettuce, or chard between tall caged tomatoes, or planting pole beans in gaps in a corn row. Whenever you have a tall, stable plant, you can usually match it with a low-growing veggie or vine that adapts to filtered shade.
Buckwheat is best known as a summer cover crop, but it also makes a good companion plant to sow alongside broad potato rows after the potatoes emerge. The upright buckwheat plants filter bright sun, which helps to retain crucial soil moisture for the potatoes. Buckwheat blossoms also attract beneficial insects, and the buckwheat “fence” will deter flea beetles, leaf hoppers, and other common potato pests.
“Nurse crops” are companion plants that’ve been selected and sited to provide shade, stabilize the soil, or offer wind protection for younger, weaker plants. A common method involves sowing radishes between rows of carrots or beets. The radishes sprout quickly and stabilize the site for carrots and beets, which take much longer to germinate. After a month or so, the radishes will be pulled to allow the other vegetables room to grow.
Cereal grains, such as wheat or oats, make interesting nurse crops, too. In one study, strawberries planted with a nurse crop of wheat were protected from weather damage and pests. When the soil warms in summer, buckwheat, bush beans, or crowder peas make fine nurse crops for summer-planted cabbage. Fast-growing oats are an ideal nurse crop for cool-season vegetables planted in August, or for a mixture of winter cover crops.
Every vegetable garden is a unique natural system that you can try to enrich, enhance, and balance for your own benefit. Companion planting increases the diversity of plants while accomplishing specific goals that enhance plant health — and a diverse garden is a happy garden.
Barbara Pleasant grows a large food garden in Floyd, Virginia. She has authored a number of books on vegetable gardening, including her newest title, Homegrown Pantry — Selecting the Best Varieties and Planting the Perfect Amounts for What You Want to Eat Year-Round.