We know that most humans, and even animals, thrive with companionship.

But what about plants?

As interest in organic, local and sustainable gardening and farming has grown in the past decade, so has the practice of “companion planting” — basically the practice of strategically placing mutually beneficial plants together to improve production naturally.

Plants can be used to attract or harbor pollinators and beneficial insects, repel or “trap” harmful ones, and shade or build up the soil, among other uses.

While modern science is just catching up to studying the practice, veteran farmers and gardeners have been using it for years and their proof is in production.

Some locals, such as Carmen Ketron of MUSC Urban Farm, Rita Bachmann of Rita’s Roots Backyard Harvest and the Container Garten near Local Works Charleston, are enthusiastic advocates for companion planting, especially for home gardens, urban gardens and small farms.

And while academic studies are underway, all say that the variables of gardening including sunlight, rainfall, humidity, wind, soil conditions, geographic location, the severity of the past winter and more, make it challenging to “prove” that companion planting works.

But unless pairing plants that don’t get along, such as allium (onions, garlic, leeks) with legumes (peas and beans), thoughtful companion planting usually doesn’t hurt, not to mention bringing beauty and diversity to a garden.

“Science is done in a vacuum,” says Ketron of the isolated conditions often required in study. “We’re not working in a vacuum. Nature is not in a vacuum … Companion planting is about bringing it (gardening) back to nature and restoring the equilibrium.”

Ultimately, Ketron, who often holds a workshop on the practice every spring, says companion planting is simply an effort to use as many tools in nature as possible to benefit your garden.

Companion planting, even as it is known today, is not new.

In 1975, avid gardener and author Louise Riotte wrote “Carrots Love Tomatoes: Secrets of Companion Planting for Successful Gardening,” which has become the bible of companion planting. Eight years later, she followed it up with “Roses Love Garlic: Companion Planting and Other Secrets of Flowers.”

In fact, one of the most common companion planting practices dates back centuries.

Indigenous people of the Americas planted the “three sisters” of corn, climbing beans and squash. The corn served as a natural trellis for the beans, which then helped fix nitrogen in the soil. The squash spread along the ground, both shading and keeping it moist. Small spikes on the squash vine deters some pests.

Some of the top plants to attract beneficial insects, according to latest issue of "Mother Earth News," are borage, cilantro, coreopsis, cosmos, dill, fennel, Queen Anne's lace, sunflower and sweet alyssum. Rodale's "Organic Life" suggests 26 plant combinations, such as cucumbers and nasturtium, cabbage and dill, and radishes and spinach.

One of the most commonly known, companion planting combinations is tomatoes and marigolds.

Bachmann says that while most people think the scent of the marigolds repels pests from the tomatoes, it's actually the marigold roots that have natural chemicals that control nematodes, microscopic worms that harm tomatoes, as well as serving as a magnet for thrips and spider mites.

Caveats (such as soil temperature affecting the level of benefits) are plenty, but Bachmann urges gardeners not to get stuck on getting it right the first time.

“I think when people think about companion planting, they often get so hung up on making their planting perfect that it can prevent them from planting. They get paralyzed. I say, ‘Don’t worry about it too much. Read the book ("Carrots Loves Tomatoes"). Learn some things. But don’t get hung up on the details.’”

Instead, Bachmann says think generally in terms of creating a mini-ecosystem “where a lot of different insects can thrive, where the good bugs can battle the bad bugs.”

"If you’re just planting one thing, it’s not natural, even on a small scale,” says Bachmann, adding that once she started putting flowers alongside vegetables that she has more beneficial insects, such as lady bugs, lacewings and parasitic wasps, which are tiny, non-stinging wasps that lay eggs in bodies of harmful caterpillars.

One tip that both Ketron and Bachman endorse, and most companion planting fans, is counter to most gardeners' conventional practice: Let some vegetables flower and go to seed.

Bachmann says, for example, when people see cilantro bolting, they want to yank it out, not realizing that it makes ideal flowers for pollinators and then, ultimately, the spice coriander.

She uses similar strategies when planting perennial gardens for clients.

On Friday, she and her team prepared to do a “fruit install” featuring blackberries, plums and figs, but those fruiting plants were intermixed with purple cone flower (echinacea), salvia and other flowering perennials with long bloom periods.

While when she was a farmer on Wadmalaw Island, she didn't practice companion planting, but her husband, Harleston Towles, practices a different form of companion planting at Rooting Down Farms, an organic farm on Johns Island.

She says he plants a row of flowers between rows of vegetables and “trap crops” at the end of rows with the idea of luring pests to those sacrificial plants where he can concentrate his organic pest control methods.

Bachmann says that companion planting, intercropping and trap crops are rarely practiced by corporate industry farms focused on monocultural models, but it has become common with smaller organic and biodynamic farmers. 

Contact David Quick at 843-937-5516. Follow him on Twitter @DavidQuick.