While gardeners know that planting marigolds throughout a vegetable garden helps confuse and repel pests, it's not always easy to remember which vegetables work well together, or why. Every grower seems to have a perfect method for putting compatible vegetables together, but a few tried and true pairings or groupings have stood the test of time.
Three Sisters: Corn, Pole Beans and Squash
The oldest and best-known companion planting method, the "three sisters" practice--invented by Native Americans--relies on the interplanting of pole beans, corn and squash or pumpkins. Corn stalks anchor the arrangement, allowing beans to use them as a trellis, while vining squash grows between the bean-corn plantings.
Beans, like all legumes, act as "green manure" by adding nitrogen to the soil. Squashes (including pumpkins, butternut squash, acorn squash and vining summer squashes) need plenty of that nitrogen in order to grow and set fruit. The pole beans further help corn by supporting it during times of high wind. The corn, of course, provides a trellis for the pole beans, allowing the beans to ripen without rotting on the ground. The squash, meanwhile, aids the other two plants with its plentiful foliage, which acts as living mulch to suppress weeds and retain moisture.
Radish, Cabbage, Parsnip, Beans and Lettuce
"Gaia's Garden," a book exploring sustainable agriculture techniques, profiles Welsh-born farmer Ianto Evans, who developed several vegetable garden designs based on the "polyculture" technique--growing vegetables and herbs that help one another in close proximity. Evans broadcasts radish, parsnip and lettuce seeds in a single bed, and starts cabbage plants indoors. He also adds dill and calendula to the vegetable bed.
In Evans' companion planting method, the radishes, which grow quickly, provide shade and weed suppression for the slower-germinating parsnips and lettuce. Within a few weeks, Evans removes some of the mature radishes and replaces them with the cabbage seedlings, which help anchor the soil and prevent erosion during the growing season. As some of the earlier lettuce varieties mature, Evans harvests them and replaces them with bush bean plants, which add nitrogen to the soil. The calendula and dill, meanwhile, repel predatory insects, while the dill further attracts parasitic wasps, which attack cabbage predators.
Carrots and Onions
On a much simpler scale, try interplanting carrots and onions. Onions apparently disguise the smell of carrots from the white fly, which routinely attacks carrots, according to the National Sustainable Agriculture Information Service (NSAIS).
Radishes and Zucchini
Radishes, a sort of multi-purpose "scarecrow" crop, remains a favorite of many squash growers desperate to deter the squash vine borer, which attacks, zucchini, patty pan squash, winter squash and pumpkins. Susan Riotte, author of "Carrots Love Tomatoes," recommends planting a few radishes in a circle around each squash plant, harvesting some and letting others go to seed to continue protecting the squash from vine borers.
Collards and Cabbages
This pairing, while not mutually beneficial, does show promising results for deterring the harmful cabbage moth, according to the NSAIS. In this case, collards act as "trap crops" to distract the moths from eating the cabbages. Of course, gardeners can always chop up and eat any surviving collard greens, but the point here is to protect the more vulnerable cabbages.
Daikons and Broccoli
Another trap crop pairing involves using daikon radishes to distract black flea beetles from munching on broccoli crops. The beetles turn to the leaves of the radishes instead. In this case, the surviving daikon roots may be harvested along with the healthy broccoli.
Tomatoes and Asparagus (or Carrots)
Tomato leaves contain a powerful substance called solanine, which repels the pests of certain other vegetables, including the asparagus beetle. Riotte plants rows of tomatoes to alternate with the perennial asparagus rows. As her book title suggests, she's also found carrots to benefit from close proximity to tomatoes.