The research-backed companion planting chart
— 12 crops, every pairing sourced.

Why companion planting works (and where it doesn't)

Companion planting works through five distinct mechanisms, and most of the popular advice you'll see online mixes them together. Here is what the research actually supports.

1. Volatile-priming defence (the strongest evidence)

Some plant pairings work because volatile compounds from one species switch on defence genes in the other before any pest arrives. The classic case is the 2024 Kagoshima University study showing that basil's linalool, chavicol, and alpha-terpineol prime tomato wound-response signalling (jasmonic acid plus reactive oxygen species). Caterpillars feeding on basil-primed tomato leaves gained roughly half the weight of caterpillars on unprimed leaves. This is the most rigorous companion-planting mechanism on the books today.

2. Pest disruption by scent confusion

The carrot-onion pairing reduces carrot fly egg-laying by up to 70% in University of Bristol trials. The mechanism: visual and chemical complexity makes host-plant location harder for specialist pests. The same principle explains why marigold-bordered beds receive fewer cucumber beetle visits and why garlic interplanted with brassicas reduces aphid colonisation.

3. Trap cropping

Radishes for cucumber beetles, nasturtiums for aphids, and bolted radish for spinach leafminer all work by drawing pests preferentially onto the trap plant, where they can be removed before the main crop is damaged. Cornell Cooperative Extension specifically recommends 2-3 radish seeds in each cucumber hill for this reason.

4. Nitrogen fixing

Legumes (beans and peas) host Rhizobium bacteria in their root nodules that convert atmospheric nitrogen into plant-available forms — potentially 30 to 50 lb of nitrogen per acre for following crops. Corn, leafy greens, and brassicas all benefit when planted after beans or peas. This is also why alliums are bad neighbours for legumes: allium sulfur compounds suppress the Rhizobium bacteria.

5. Microclimate and ground cover

Living mulches (squash leaves shading soil for the Three Sisters), shade provision (tomato canopies cooling lettuce), and root-layer separation (carrot taproots + lettuce surface roots) all qualify as companion effects without involving any chemistry. These are observational rather than experimental, but the logic is sound and the practice is universally recommended.

Where the science is weakest

Many pairings popularised by Louise Riotte's 1975 Carrots Love Tomatoeshave never been replicated under controlled conditions. The carrot-tomato pairing itself is mostly folklore; basil-pepper has only anecdotal support; the "marigolds repel everything" claim is true for root-knot nematodes (and only when marigolds are grown as a cover crop the season before) but overstated for general garden pests. Treat anything labelled "traditional" on our pages as gardener consensus rather than confirmed effect.

How to use this chart in a real garden

Start with the 5-step vegetable garden plan, pick 4-6 of these 12 crops to grow this year, then arrange the bed so that each crop has at least one good neighbour from this chart and no antagonists within 3 feet. If you're planning multiple beds, see our vegetable garden layout guide for spacing and crop-rotation rules.

Timing matters as much as pairing — companion planting can't fix wrong-zone planting dates. Cross-reference your zone in the USDA hardiness zone map and your monthly planting calendar in the Growli planting calendar before committing the bed plan.

Frequently asked questions

What is companion planting and does it actually work?

Companion planting is the practice of growing two or more crops near each other so each benefits the other — usually through pest deterrence, nitrogen fixing, pollinator attraction, or microclimate effects. Some pairings have strong peer-reviewed evidence (tomato + basil, carrot + onion, cucumber + radish). Others are folklore that survives because gardeners enjoy them. We flag every pairing on this page with its evidence level.

What is the best companion plant for tomatoes?

Basil is the most evidence-backed tomato companion. A 2024 study in Plant Cell Reports identified three basil volatile compounds (linalool, chavicol, alpha-terpineol) that prime tomato wound-defence genes — caterpillars on primed plants gained about half the weight of caterpillars on unprimed plants. West Virginia University intercropping trials also recorded roughly 20% yield gains for tomatoes grown with basil at adequate density.

What should you never plant next to tomatoes?

Brassicas (cabbage, broccoli, kale) and tomatoes are both heavy feeders and compete for nitrogen — most extension services recommend keeping them at least 3 feet apart. Potatoes share the late-blight pathogen Phytophthora infestans with tomatoes, so keep them 10+ feet apart or in completely separate beds. Fennel releases anethole, an allelopathic compound that suppresses tomato growth within 3-4 feet.

Do marigolds really repel pests?

Partially. French marigolds (Tagetes patula) release alpha-terthienyl from their living roots, which kills root-knot nematodes when used as a cover crop the season before planting tomatoes — this effect is well documented by UF/IFAS and Hawaii CTAHR. A simple marigold border planted alongside an established tomato bed has much weaker evidence, and marigolds can actually increase populations of some other nematode species (stubby-root, sting). Use them as a pre-crop, not a border.

Why can't you plant beans with onions or garlic?

Alliums (onions, garlic, chives, leeks) release sulfur compounds — primarily allicin — that suppress the Rhizobium leguminosarum bacteria living in legume root nodules. Those bacteria are what let beans and peas fix atmospheric nitrogen. Multiple extension trials show measurably reduced nodulation when beans grow within 2-3 rows of onions or garlic. Keep alliums and legumes on opposite sides of the garden.

Does the Three Sisters method (corn, beans, squash) work?

Yes — it is one of the best-documented polycultures in horticulture, originating with the Haudenosaunee (Iroquois) people. Corn provides a living trellis for beans; beans fix nitrogen for both corn and squash; squash leaves shade the soil and suppress weeds. A modern study found the Three Sisters delivered more total energy and protein per acre than any local monoculture, though individual crop yields are slightly lower than each grown alone.

How far apart do incompatible plants need to be?

For allelopathic pairs (fennel, walnut, mature dill), 4+ feet usually clears the chemical effect. For heavy-feeder competition (tomato + cabbage), 3 feet of root separation or a different bed is enough. For shared-disease pairs (tomato + potato), the spores travel — 10+ feet or separate beds entirely. For allium-legume interference, 2-3 row separation prevents the worst nodulation loss.

Sources and methodology

Every claim in this chart is sourced from one or more of: peer-reviewed horticultural literature (e.g. Tian et al. 2024, Plant Cell Reports on basil-tomato volatile priming), US Cooperative Extension publications (UMN, WVU, Cornell, UF/IFAS, UVM), the Royal Horticultural Society's vegetable companion guidance, or the evidence reviews maintained by garden-myths.com. Pairings with only anecdotal or historical support are labelled traditional rather than presented as established science. Curated by the Growli editorial team, last reviewed May 2026.

Keep going

• Complete companion planting guide (article)
• How to start a vegetable garden — 5-step plan
• Vegetable garden layout — bed plans + spacing
• Monthly planting calendar
• USDA hardiness zone map