Companion Planting Guide: What to Grow Together

My grandmother planted marigolds around everything. Tomatoes, beans, squash, peppers — every bed had a border of marigolds. When I asked why, she said, 'They keep the bad things away.' I was skeptical. Then I studied horticulture and learned that she was, in fact, mostly right — marigold roots release alpha-terthienyl, which suppresses root-knot nematodes, and their strong scent confuses and repels several common pest insects. My grandmother couldn't cite the research, but she had decades of observational data that told her the same story.

Companion planting — the practice of growing certain plants together for mutual benefit — occupies an interesting space between gardening tradition and scientific validation. Some companion planting claims are well-supported by research. Others are garden folklore with little evidence. And some are genuinely useful even if the mechanism isn't fully understood. Here's what we actually know.

The Three Sisters: Ancient Wisdom, Proven Science

The most famous companion planting system is the Three Sisters, developed by Indigenous peoples of the Americas centuries before European contact: corn, beans, and squash grown together. It's not just folklore — it's an elegant, scientifically valid polyculture.

How it works: Corn grows tall, providing a natural trellis for the beans to climb. Beans are legumes that fix atmospheric nitrogen through symbiotic bacteria in their root nodules, providing free fertilizer for the nitrogen-hungry corn and squash. Squash sprawls across the ground, its large leaves shading the soil to suppress weeds and retain moisture, and its prickly stems deter raccoons and other animals from walking through the planting.

To plant the Three Sisters: Create mounded hills about 4 feet apart. Plant 4-6 corn seeds in the center. When corn is 6 inches tall, plant 4 bean seeds around it. A week later, plant 2-3 squash seeds at the base. The timing matters — corn needs a head start or beans will outgrow and smother it.

Science-Supported Companions

Basil + Tomatoes: Research shows basil repels thrips (a common tomato pest) and may confuse the moths that lay tomato hornworm eggs by masking the tomato scent. The combination also makes culinary sense — you're growing the ingredients for caprese salad in the same bed.

Marigolds: Extensive research confirms that French marigolds (Tagetes patula) suppress root-knot nematodes in the soil for up to a year after the plants are removed. This isn't just repellency — the roots produce compounds that kill nematode eggs. Studies also show some effectiveness in repelling whiteflies and certain beetle species.

Nasturtiums as trap crops: Studies confirm that aphids preferentially colonize nasturtiums over many vegetable crops. Planted at the edges of beds, nasturtiums draw aphids away from your food plants. When they become heavily infested, pull them up (aphids and all) and replace with fresh plants.

Dill, fennel, and cilantro: The tiny umbrella-shaped flowers of these herbs attract parasitic wasps — beneficial insects that lay eggs inside aphids, caterpillars, and other pest insects. This is well-documented biological control. Let some of your herb plants flower specifically to attract these beneficial insects.

Clover as living mulch: Growing white clover between vegetable rows suppresses weeds, fixes nitrogen, and provides habitat for ground beetles and other beneficial predators. Research shows that clover intercropping can reduce pest damage by 30-60% in some systems compared to bare soil between rows.

The 'Bad Neighbors' List

Some plants genuinely do inhibit each other through allelopathy (the release of growth-suppressing chemicals) or competition:

Black walnut: Produces juglone, a compound toxic to many plants. The 'death zone' extends roughly to the tree's drip line and persists in soil for years after the tree is removed. Tomatoes, peppers, eggplant, blueberries, and many other plants cannot grow near black walnut. Notably tolerant plants: beans, beets, carrots, corn, and squash.

Fennel: Inhibits the growth of most garden vegetables, particularly beans, tomatoes, and brassicas. Grow fennel in a separate bed or in a container, well away from the vegetable garden. (Regular dill is fine — it's fennel that's the problem.)

Alliums (onions, garlic) near beans and peas: Allium family plants release compounds that can inhibit the growth of legumes. The effect isn't dramatic, but spacing them apart is a good practice.

The Honest Truth About Companion Planting

Many companion planting claims are untested or weakly supported. 'Carrots love tomatoes' (the title of a famous companion planting book) isn't backed by controlled studies. Many traditional companion planting charts are passed from book to book without verification.

That said, the absence of evidence isn't evidence of absence. Gardens are complex systems, and many companion effects may be real but difficult to isolate in experiments. The Three Sisters system worked for centuries before anyone understood the nitrogen-fixing mechanism.

My approach: Use the well-documented companions (marigolds, basil + tomatoes, trap cropping, beneficial insect plants) with confidence. Treat other traditional recommendations as 'probably can't hurt, might help.' Interplant flowers with vegetables regardless — it attracts pollinators (improving fruit set), supports beneficial insects, and makes your garden more beautiful. Even if the specific companion effects are marginal, the biodiversity benefits are real and well-documented.

And plant marigolds around everything. My grandmother knew what she was talking about.