By Terry Twigg
(August 4, 2023) — There is something so cozy about the term “companion planting.” It brings to mind a long-married couple with comfy armchairs pulled close to a fire, he with pipe and slippers, she with her knitting. It’s been practiced successfully for generations, yet is often dismissed as just another old wives’ tale. As it happens, though, there’s a fair bit of science behind the concept.
How does it work? As with nearly everything horticultural, the key is diversity. When thousands of the same plant are grown together, they are all competing for the same nutrients at the same time—and the soil becomes nonproductive. Masses of a single plant attract matching swarms of their typical pests, while the absence of other plants means there’s nothing to attract other insects, which otherwise might keep the bad guys in check—and so pesticides become essential. As agricultural practices go, the typical commercial monoculture model, in which acres upon acres of the same crop are planted, is about the worst choice possible.
But you have a home garden, and can make smarter choices. When companion planting is mentioned, most people think first of adding flowers. Marigolds among the tomatoes are a common beginner’s move, but the more you diversify, the better plants will work for you. To attract a maximum number of beneficial insects, you’ll want host plants available for the longest period possible, so include early spring bulbs, long-blooming summer annuals, and late-season bloomers.
Many herbs are great for pollinators, but they have secondary strategies: their odors attract beneficial insects while repelling unwanted ones, and help mask the scent of target crops. Anything in the allium family, which includes onions, garlic and leeks, will help to deter pests. Your efforts will be aided by legions of predatory and parasitic insects with satisfyingly blood-thirsty names, like assassin bugs, ambush bugs, and minute pirate bugs. Keep in mind, though, that you’ll never get rid of the bad bugs altogether. Your goal is to reduce them to a manageable number.
The layout of your companionable garden will affect your harvest. Plant early-season crops to the south of later-growing varieties, so the first will be gone by the time the second need the sunshine. Later, use taller plants to provide shade to cool-preferring plants, like lettuce, thereby extending their season longer into the hotter months. Think architecturally: Tall plants need vertical space; unsupported climbers or spreading varieties take up square footage; root crops shouldn’t need to compete for underground space.
You can plant a sacrificial “trap crop” to sidetrack insects from more desirable varieties. One of the best-known traps is blue hubbard squash, which is a favorite of squash borers, squash bugs, and cucumber beetles. A handful of these squash, planted around the perimeter of the garden, can protect a hundred of your preferred plants, from the fanciest pumpkins to humble zucchini. Just make sure the trap crop plants are a few weeks older than the real crop, so the pests find them first, and remove or destroy the bugs as soon as they appear.
Even the mere presence of other plants, acting as a living mulch, can reduce infestation by confusing pests or blocking easy access. At the same time, a living mulch can improve soil structure, reduce runoff and evaporation, and lower the soil’s temperature. Make sure your main crops have a good head start before seeding the mulch crop, and choose one that won’t compete with your chosen crops for the same nutrients.
Rule of thumb: Most leafy vegetables and fruits are considered heavy nitrogen feeders, herbs and root vegetables are light feeders, and members of the legume family are considered “givers,’ since they fix nitrogen in the soil. One of each type will generally make a successful grouping. Some good choices for living mulch are clover, herbs, lettuce, and short annuals like nasturtiums and calendula. All will grow quickly to crowd out weeds. Yes, you may need to do some culling to prevent them from taking over, but unlike the crabgrass and other weeds that would otherwise take over, they’re improving your garden.
One of the oldest and probably best known examples of companion planting is the famous “Three Sisters” combination of corn, beans and squash. While European settlers planted crops separately in neat rows or blocks, Native Americans planted these staples together. Their approach was efficient: the corn grew tall, providing readymade support for the beans, while the broad squash leaves crowded out weeds. The trio was nutritionally balanced: corn and beans combine to make a complete protein, while squash provides carbohydrates and vitamins.
In fact, one study found that the plants provided more calories and nutrients when grown together than as separate crops. Horticulturally, it was hard to beat: beans fix nitrogen in the soil, where it can be taken up by notoriously nitrogen-hungry corn. A conventional block of corn quickly exhausts the soil, requiring added fertilizer after only a few years, but a Three Sisters patch will remain productive for decades. Companion planting, nearly perfected, centuries before scientists figured out the “why.”
My own vegetable garden is an example of particularly unsuccessful companions: I ordered a decorative dwarf peach to plant in the center, thinking its spring bloom would attract early pollinators, but the mislabeled specimen I received is now twenty-five feet tall. It crowds out half the space and shades the rest. There’s enough sunshine for a few tomatoes and some determined chard, but that’s about it. Still, despite the late spring frost, the tree is laden with peaches, so I’m letting it stay for the summer. Autumn will be soon enough to decide between the sad prospect of cutting it down and the daunting job of trying to transplant it. My newly cleared vegetable garden, redesigned with companion plants, will be fragrant with herbs, bright with annuals, and bursting with produce…next year.
