Forget whatever you’ve heard about which plants ‘love’ which other plants. This isn’t Match.com for vegetables: There’s a science to it.
Before pushing that cart of vegetable seedlings to the checkout line, consider a stop in the seed aisle. On offer: potential organic pest control.
A packet of radish seeds could help fight the flea beetles on your new tomato transplants, and nasturtiums sown among your zucchini may limit the damage done by squash bugs.
Those are just two of the many strategic pairings suggested in Jessica Walliser’s book “Plant Partners: Science-Based Companion Planting Strategies for the Vegetable Garden,” which takes a new look at a popular subject that has long relied on folklore and conjecture rather than research.
She knows that what you’ve heard about companion planting was probably which crop “loves” which other one, like some kind of Match.com for vegetables. Whatever you’ve heard, it probably didn’t include “tomatoes love radishes” or “zucchini love nasturtiums” — or more specifically, that certain insect pests of tomatoes and summer squash don’t love those things and can be thrown off course by them.
Ms. Walliser — a horticulturist, self-described “science nerd” and the author of two other books on garden insects — wanted to know which pairings would help control pests and improve pollination, providing those and other ecosystem services to desired plants. Not satisfied with anecdotal recommendations, she turned to the scientific literature.
Admittedly, other than a few studies at botanic gardens and university extension facilities that mimicked the smaller scale of home garden beds, most of the literature she found was derived from research in agricultural settings. Nevertheless, the insights represented a leap forward from folklore, so she dug in.
If one idea unites the partnership possibilities Ms. Walliser discovered, it is this: Whether in farm fields, home gardens or the natural landscape, diversity is a powerful tool. Monoculture — too much of any one thing — always leaves us more vulnerable to loss.
Rather than replicating the rigid, old-style rows of a farm field in your vegetable garden, Ms. Walliser recommends a modern, vibrant jumble of vegetables, fruits, herbs and flowers.
And the goal, she writes, should be creating a habitat that supports beneficial insects, which means avoiding pesticides and other chemicals that kill indiscriminately.
Pest Control Plus
Gardeners have often looked to companion plants for pest control, as well-matched partners can attract natural enemies like syrphid flies and parasitic wasps to act as biological-control agents. But smart pairings can minimize weeds and improve soil, too.
A well-chosen, properly timed cover crop — like winter rye, oats, field peas, and crimson or white clover — acts as living mulch, limiting weeds. And some (including rye and oats) provide not just physical barriers, but are allelopathic, containing natural weed-suppressing chemicals. After the active growth phase, their remains, if left behind, provide extended control.
Cover crops can help build better soil, too, when they’re turned under — that’s why they are referred to as green manure. Legumes, in particular, including the clovers and peas, fix nitrogen in the soil as they grow, and adjacent plants may benefit.
Other well-chosen plant partners can attract pollinators, whose work improves the yields of insect-pollinated crops. In cucurbits, those additional pollinators could reduce the frustration of shrunken zucchini or cucumbers that didn’t size up to maturity because of insufficient pollination — or plants didn’t bear fruit at all.
What About Those Marigolds?
Sometimes the problem with traditional companion recommendations is not that they’re ineffective — it’s just that the why and how have been misunderstood. For example, the smell of marigolds doesn’t function as a repellent for every unwanted pest, as was inferred from oft-repeated conventional wisdom.
“The idea that ‘their scents send pests packing’ isn’t necessarily how it works,” Ms. Walliser said, citing the two marigold-vegetable partnerships that have been well studied: against onion root maggot flies and cabbage root maggot flies. (A 2019 paper from Newcastle University in England credits French marigolds as an effective tool against greenhouse whitefly infestations in hothouse tomatoes, too.)
Rather than repelling the flies, the marigolds’ volatile organic compounds may serve to mask the scent of the plant the insect is seeking, interfering with egg laying.
“They can’t home in on their host plants, because of the marigolds,” said Ms. Walliser, who is quick to add: “I’m not telling you not to plant marigolds — but by understanding better what’s at work, you can make more informed pairings.”
Setting a Botanical Trap
A particular insect may seek to lay eggs on a vegetable, or use it as food, or both. To disrupt such host-seeking behaviors, Ms. Walliser suggests a scaled-down version of trap cropping, a tactic used for centuries in farming.
Well-placed plants, grown at a distance from insects’ desired ones, become a decoy — a sacrificial offering. That’s how those radishes work if planted among young tomatoes: The flea beetles have at the radish foliage, not the tomato leaves. A radish trap crop can also protect young eggplants and peppers. Pak choi works as a decoy, too, but if you use radishes instead, you can salvage the roots to eat.
Where to plant the trap crop? That depends on which insect you’re trying to lure away. If it’s highly mobile (like the Colorado potato beetle or squash bugs) or it’s the offspring of something highly mobile (like imported cabbage worms, the larvae of the cabbage white butterfly), then plant the trap crop on the perimeter of your garden, several feet from the crop you’re protecting.
But if you’re fighting tiny pests with limited mobility, like aphids, mites, flea beetles and whiteflies, Ms. Walliser recommends planting the trap crop very close, in alternate rows with the crop you want to protect.
Sow or transplant the trap crops a couple of weeks earlier than the crops that need protection, so they get the pests’ attention, at a mass of about 10 to 20 percent of the total area of the vegetable you’re protecting.
Herbal Remedies, Vegetable-Garden Style
Who doesn’t want herbs like basil in the vegetable garden?
“I was so pleased to find out that the power of basil was pointed out in various studies,” said Ms. Walliser, who plants it liberally in her home garden near Pittsburgh.
Studies show that it can help against thrips that stunt tomato plant growth and cause early fruit drop, and may limit egg laying by adult moths whose larvae are the all-too-familiar (and voracious) tobacco and tomato hornworms. It also works against yellow-striped armyworm moths.
Pair your potatoes with catmint (Nepeta) to deter Colorado potato beetles; partner calendula with collards to keep aphids at bay. To limit cabbage worms, plant brassicas with sage, hyssop or chamomile. (The bonus with chamomile: plenty to harvest and dry for tea.)
Other useful herbs include many umbellifers, members of the carrot family (Apiaceae). Flowers of dill, fennel, cilantro, parsley and chervil may attract predators of various species of aphids and caterpillars, luring helpers like parasitic wasps, tachinid flies, ladybugs and lacewings.
Umbellifers can help with aphid control among lettuce and leafy greens; with cabbages, try leaving the cilantro to flower. Dill’s ferny foliage has also been shown to protect broccoli from imported cabbage worms, reducing their egg-laying behavior. And if you have Colorado potato beetle problems with your eggplants, dill and cilantro may help there, too.
More Flower Power
Besides the nasturtiums and the flowering herbs, there are two familiar annual flowers that Ms. Walliser has come to rely on after consulting the research: sweet alyssum (Lobularia maritima) and cosmos.
In organic lettuce farms in California, she said, strips of sweet alyssum are alternated with lettuce to enhance biological aphid control.
Cosmos help against aphids, too, particularly around brassicas like broccoli and cauliflower. And so do other aster family members, including sunflowers, Rudbeckia, Coreopsis and zinnias, by attracting beneficial predatory insects.
Tolerance and Vigilance (Just Don’t Overdo It)
One thing is certain: There is seldom a season in the vegetable garden without a pest infestation of some kind. As organic gardeners, Ms. Walliser recommends that we develop a realistic tolerance for some pest pressure.
She also urges that we do our part by practicing good garden hygiene, removing faded or diseased foliage as it appears.
“Partnering companion planting with smart mechanical controls, for me, is key,” said Ms. Walliser, who puts row covers on vulnerable crops to protect against flying pests during sowing and transplanting.
“I know squash bugs will be problematic,” she said. “And even if I plant nasturtiums with my squash, I still keep an eye out.”
She wraps a piece of duct tape, sticky side out, around the flattened fingers of one hand, she said, “and wherever I see cluster of eggs on the leaf undersides I tap them, and they end up on the tape.”
The same technique works on the undersides of brassica leaves for cabbage white butterfly eggs, and at the base of squash stems, if female vine borers have deposited their progeny.
And please, she said, give everybody enough room. In the enthusiasm to cram in every possible plant partner, she warned, we can go overboard. Make a garden plan, and be strategic.
Plants, including the companions, compete for resources like water and nutrients, and there can be too much of a good thing.
Margaret Roach is creator of the website and podcast A Way to Garden, and a book of the same name.
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