Is companion planting actually science-based?

Mixed. Some pairings have strong research support (Three Sisters, brassicas + alliums for pest deterrence, legumes adding nitrogen for adjacent heavy feeders). Others are folk wisdom passed through gardening books with little experimental rigor. Trust the well-tested pairings; treat the rest as “probably won’t hurt” rather than guaranteed benefits.

How close do companions need to be?

Beneficial pairings work best within 1-3 feet (close enough that aromatic compounds, root exudates, and beneficial insects move between them). Avoidance pairings should be separated by 10+ feet, or in different beds. Container gardening intensifies both effects since roots share confined soil.

What about marigolds — do they really repel pests?

Yes, with caveats. French marigolds (Tagetes patula) produce alpha-terthienyl in roots which is genuinely toxic to root-knot nematodes — research-supported, plant them in soil where you’ll later grow tomatoes/peppers. Above-ground pest deterrence (whitefly, aphids) is more anecdotal. Don’t rely on marigolds alone, but they don’t hurt and add color.

Why is tomato + corn a bad pairing?

Both attract the same pest — corn earworm/tomato fruitworm (Helicoverpa zea). Planting them together creates a pest reservoir that boosts populations on both crops. Standard advice: separate by at least 30 feet, or stagger planting so they’re not at peak fruiting simultaneously.

Can I use companion planting with raised beds?

Absolutely — raised beds are ideal for it because you control the soil and layout precisely. Stick to the well-researched pairings (Three Sisters in a wide bed, tomato + basil + lettuce in a narrow one, alliums around brassicas). Note that raised-bed soil heats faster than ground soil, so cool-season crops (lettuce, peas) may bolt earlier than expected.

What about allelopathy — plants that suppress others?

Real and worth knowing. Black walnut produces juglone which kills tomatoes, peppers, and many other plants within 50-80 feet of the trunk. Sunflowers release allelopathic compounds that inhibit nearby plant germination. Garlic and onions can suppress legume growth. The companion checker flags these — when it says “avoid,” often allelopathy or shared-pest pressure is the documented reason.

Home & Life · Free tool

Companion Plant Checker

Check 20 common garden vegetables for good or bad companion pairings instantly. Includes Three Sisters callout — free online tool with no signup needed.

Updated June 2026

Crop ACrop B

Compatibility

excellent

Basil enhances tomato flavor and repels aphids, whiteflies, and hornworms.

“Three Sisters” callout

The classic Indigenous North American planting: corn + beans + squash. Corn provides a trellis for beans, beans fix nitrogen for all three, and squash leaves shade the soil and deter pests. Plant corn first, add beans 2 weeks later when corn is 6 inches tall, then squash around the perimeter.

Note: Companion planting is a combination of scientific findings (allelopathy, trap crops, nitrogen fixation) and tradition. Results vary by climate, soil, and season — treat these as starting points, not guarantees.

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What it does

Companion planting is the practice of placing mutually-beneficial plants next to each other and avoiding pairings that compete or attract shared pests. The tradition goes back to indigenous American Three Sisters planting (corn provides a stalk for beans to climb, beans fix nitrogen for the corn, squash covers ground to suppress weeds and conserve moisture) — a system documented since at least 1000 CE. Modern companion planting combines folklore with research-backed mechanisms: nitrogen fixation by legumes, pest deterrence by aromatics (basil repels tomato hornworm, marigold repels nematodes), and beneficial-insect attraction (dill and fennel attract parasitic wasps that prey on tomato pests).

The tool covers 20 common vegetable, herb, and flower crops with three-tier compatibility scoring: excellent (proven research support — pair these), neutral (no harm, no special benefit), and avoid (documented competition or shared pests). Classic excellent pairings: tomato + basil (basil deters whitefly, both share similar sun/water needs), carrot + onion (onion masks the carrot fly’s scent target), corn + beans + squash (Three Sisters), cabbage + dill (dill attracts wasps that eat cabbage worm). Classic avoid pairings: tomato + corn (share corn-earworm/tomato-fruitworm), beans + onions (onions inhibit bean nitrogen-fixing bacteria), potato + squash/tomato (share blight pathogens).

Important caveats: companion planting is NOT a magic bullet. The actual yield impact is usually 5-20% — meaningful for a backyard garden, marginal for commercial farming. Many traditional pairings (the “tomato + carrot” dyad in Louise Riotte's 1975 book Carrots Love Tomatoes) lack rigorous experimental support and may reflect confirmation bias more than biological truth. Prioritize the well-researched pairings (Three Sisters, tomato + basil, brassicas + alliums for pest deterrence) and ignore the more speculative claims. Soil quality, water, and sun hours matter far more than companion-plant magic.

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How to use it

Pick the two crops you want to plant near each other.
Read the compatibility tier — excellent, neutral, or avoid.
If excellent: plant within 3 feet of each other for best mutual benefit.
If neutral: plant wherever fits your garden plan; no special concern.
If avoid: separate by at least 10 feet OR pick a different companion from the alternatives list.

When to use this tool

Designing a new vegetable garden bed layout.
Choosing a second crop to plant in the same bed alongside an existing one.
Troubleshooting an underperforming garden — sometimes bad pairings cause subtle yield drops.
Three Sisters or other intentional polyculture planning.
Container gardening — pairings matter more in confined containers where roots compete directly.

When not to use it

Commercial farming where mechanized harvest demands monoculture rows.
Strict crop-rotation planning — that follows different rules (rotate plant families across years to break pest cycles).
Selecting plants based ONLY on companion planting — soil quality, sun, water, and hardiness zone matter more.
Indoor or hydroponic growing — companion-planting effects are largely soil and pest-mediated; less relevant in soilless systems.

Common use cases

Quick use during a typical workday
Pre-decision sanity-check on inputs and outputs
Educational use — demonstrating the underlying concept
Onboarding a colleague who needs the same calculation/conversion

Frequently asked questions

Is companion planting actually science-based?: Mixed. Some pairings have strong research support (Three Sisters, brassicas + alliums for pest deterrence, legumes adding nitrogen for adjacent heavy feeders). Others are folk wisdom passed through gardening books with little experimental rigor. Trust the well-tested pairings; treat the rest as “probably won’t hurt” rather than guaranteed benefits.
How close do companions need to be?: Beneficial pairings work best within 1-3 feet (close enough that aromatic compounds, root exudates, and beneficial insects move between them). Avoidance pairings should be separated by 10+ feet, or in different beds. Container gardening intensifies both effects since roots share confined soil.
What about marigolds — do they really repel pests?: Yes, with caveats. French marigolds (Tagetes patula) produce alpha-terthienyl in roots which is genuinely toxic to root-knot nematodes — research-supported, plant them in soil where you’ll later grow tomatoes/peppers. Above-ground pest deterrence (whitefly, aphids) is more anecdotal. Don’t rely on marigolds alone, but they don’t hurt and add color.
Why is tomato + corn a bad pairing?: Both attract the same pest — corn earworm/tomato fruitworm (Helicoverpa zea). Planting them together creates a pest reservoir that boosts populations on both crops. Standard advice: separate by at least 30 feet, or stagger planting so they’re not at peak fruiting simultaneously.
Can I use companion planting with raised beds?: Absolutely — raised beds are ideal for it because you control the soil and layout precisely. Stick to the well-researched pairings (Three Sisters in a wide bed, tomato + basil + lettuce in a narrow one, alliums around brassicas). Note that raised-bed soil heats faster than ground soil, so cool-season crops (lettuce, peas) may bolt earlier than expected.
What about allelopathy — plants that suppress others?: Real and worth knowing. Black walnut produces juglone which kills tomatoes, peppers, and many other plants within 50-80 feet of the trunk. Sunflowers release allelopathic compounds that inhibit nearby plant germination. Garlic and onions can suppress legume growth. The companion checker flags these — when it says “avoid,” often allelopathy or shared-pest pressure is the documented reason.

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