Each spring, I grow legions of onions and shallots from seed, and my biggest challenge is keeping them weeded. Last year, I planted pinches of arugula between the short rows of shallots, and the leafy, fast-growing arugula smothered any weeds and showed remarkably little damage from flea beetles, which often plague it. The arugula was ready to harvest just when the shallots needed room to grow. In a eureka moment, I realized I had discovered a vegetable companion-planting partnership I could use year after year to make my garden healthier and more productive.
The idea of “companion planting” has been around for thousands of years, during which time it has become so besmirched with bad science and metaphysics that many gardeners aren’t sure what it means. The current definition goes something like this: Companion planting is the establishment of two or more species in close proximity so that some cultural benefit, such as pest control or increased yield, may be achieved.
Historically, North American and European gardeners have based many of their attempts with companion planting on widely published charts, which were mostly derived from funky chemistry experiments using plant extracts in the 1930s. But it turns out many of the plant partnerships listed in these “traditional” companion-planting charts don’t actually work well. Reaping the benefits of companion planting is possible, though, as long as you look to time-tested crop combinations. (See our chart of Proven Companion Planting Combos for spring, summer and fall.)
Companion Crop Combinations
In North America, Native American tribes from the Northeast to the Southwest developed highly specialized intercropping techniques to grow the “three sisters” — corn, pole beans and squash. (There is a fourth sister, sunflower, but she didn’t make it into the early stories.) Three sisters gardens vary in shape, size and planting style (raised mounds in the East and North, recessed waffle beds in the Southwest). The three (or four) sisters technique works because the crops cooperate rather than compete with each other for light and root space. The corn supports the bean vines, the squash shades out weeds, and the roots of the different plants get along nicely below ground.
But what about all of the other vegetables you want to grow? An experienced gardener from China wouldn’t be surprised by my success with onions and arugula, because intercropping of vegetables is the traditional way to garden in China’s most fertile regions. Plant associations that work well are shared among neighbors, as everyone has learned that intercropping is more efficient, reduces weed and pest pressure, and lowers the risk of crop failure. Chinese intercropping practices use a broad range of crops, which makes them easy to replicate in a diversified organic garden.
Agricultural researchers have noted that the paired plants in Chinese intercropping practices usually differ in height, maturation period and rooting habit. In addition, many gardeners in China grow the following intercropped vegetables back-to-back during the course of the growing season:
- Onions with leafy vegetables, followed by green beans and Chinese cabbage or spinach
- Potatoes with leafy vegetables, followed by green beans and Chinese cabbage or spinach
- Spring kale with radishes, followed by celery and tomatoes
- Spring spinach, followed by lima beans and tomatoes
- Double rows of corn alternated with single rows of peppers
- A double row of garlic with spinach down the center
- Strawberries with watermelon
Using Plants to Manage Pests and Weeds
Some gardeners use companion planting to deter all manner of garden pests, such as planting hedges of marigolds to deter rabbits or using rattail radishes to confuse squash vine borers. In our online Pest Control Survey, the gardeners who reported the most success with companion planting to discourage pests used a single technique: “growing tons of flowers,” with borage, calendula, dill, sweet alyssum, and herbs such as basil, garlic chives and oregano named most frequently.
Several scientific studies have confirmed these and other flowering plants help reduce pest problems, particularly if your garden is troubled by early-season aphids or other small sucking insects, which are primary food sources for hoverfly larvae. Hoverflies are active, early-season aphid predators (before aphid-eating ladybeetles take over later in the season). Scientists have found that hovering in midair requires so much energy that hoverflies tend to stick close to nectar sources, so if you lure them in with the right plants, they’re likely to stay all summer. Cilantro (coriander) and fennel flowers are strong hoverfly attractants, as are Greek oregano, sweet alyssum, and many other herbs and flowers.
You can also use companion planting to draw the attention of birds, which eat a wide variety of garden insects. In a recent study conducted at four organic farms in Florida, sunflowers interplanted with collard greens, kale, summer squash, tomatoes and five other vegetable crops doubled the number of insect-eating birds that visited the garden plots. The birds used the sunflowers as hunting perches, then hopped down to feed on cabbageworms, grasshoppers and other small insects, including flea beetles. Could there be an easier, more beautiful way to reduce pest problems before they start?
Finally, one of the finest benefits of finding plant associations that work well for your garden is using them to keep weeds under control. Slower-growing, upright plants (such as onions and parsnips) can be nursed along with companion crops of baby greens, which are much more rewarding to pull than weeds. In some situations, however, even weeds may become desirable companion plants. Several MOTHER EARTH NEWS readers have reported using redroot pigweed as a trap crop for cucumber beetles and Mexican bean beetles. In some locations, wild mustards can work as trap crops for flea beetles in spring.
Companion planting for pest control purposes deserves close scrutiny and the willingness to trust your own experience over what you read in books. Planting basil with tomatoes sounds good, but the pairing does little to deter tomato hornworms (its intended purpose), and the basil eventually suffers from light deprivation. The combination also fails below ground, where the two plants grow on the same schedule and therefore take up nutrients at the same time. Radishes and cilantro may be wiser choices as pals for your tomatoes, because each make a quick crop before the tomatoes need more space, and a few radish or cilantro plants can be left behind to attract beneficial insects as they flower.
Agricultural researchers use a simple formula called the land equivalent ratio (LER) to measure the overall success of intercropped plantings. The LER balances space and production costs with yields. For example, cauliflower is a slow-growing crop that needs a lot of space, so it has an LER of 1. But if you grow lettuce between cauliflower plants (as researchers did in a recent study in Pakistan), the LER per square foot will rise significantly. Space planted with a dominant crop (broccoli, cauliflower, tomatoes) and a companion crop (lettuce, radicchio or sorrel) will almost always have a higher LER than space with a crop grown by itself.
Some gardeners grow companion plants in adjacent rows, and sweet corn is a champ in this role because of its ability to provide filtered shade to neighboring plants. In China, crowder peas, green beans and peppers are all popular crops to sandwich between rows of corn. In many hot climates, corn grown along the south side of potatoes provides shade in hot weather, which helps keep the soil cool and moist while the potatoes are making their crop.
Working with companion plants requires experimenting in your own garden, using the crops your family likes best, and allowing for an increase in leafy greens because they are such versatile companion plants. (A couple of chickens or rabbits will gladly feast on whatever your family can’t use.) Gardeners sometimes say the plants in good intercropping associations “just seem happy” — a worthy goal for any organic gardener.
Share What You Know
Have you found companion-planting techniques that are effective year after year? Please share your stories of companion success (and failure) in our Vegetable Companion Planting Survey. We’ll use your input to develop a companion-planting database everyone can use to grow healthier, more productive gardens.
Contributing editor Barbara Pleasant gardens in southwest Virginia, where she grows vegetables, herbs, fruits, flowers and a few lucky chickens. Contact Barbara by visiting her website or finding her on Google+.