edible gardening
Companion planting guide — what works, what is folklore
Research-backed companion planting guide: 4 strongest-evidence pairings, 4 to always avoid, and the science behind every claim.
Companion planting guide — what works, what is folklore
Most companion planting charts mix together peer-reviewed science, garden folklore, and outright marketing — and don't tell you which is which. This guide is the opposite. Every claim below is sourced from horticultural literature, US Cooperative Extension publications, or the Royal Horticultural Society, and we explicitly flag anything that is traditional consensus rather than confirmed effect.
Plan your beds in Growli: Tell Growli what you want to grow and the app builds a bed plan that puts good companions together and keeps antagonists 3+ feet apart — calibrated to your USDA zone or UK growing zone.
The five mechanisms behind companion planting
Companion planting is not one thing. It's five separate biological mechanisms that the gardening community has lumped together. If you treat them as one, half the advice you'll read online is contradictory. If you separate them, the picture is clear.
Mechanism 1 — Volatile-priming defence (the strongest evidence)
This is the mechanism that turned tomato + basil from folklore into established science. Researchers at Kagoshima University showed in 2024 that three basil volatile compounds — linalool, chavicol, and alpha-terpineol — prime tomato wound-response signalling (the jasmonic acid + reactive oxygen species pathway). When caterpillars then arrived, they gained roughly half the weight of caterpillars on unprimed plants over three days. West Virginia University intercropping trials separately recorded ~20% yield gains for tomatoes grown with basil at adequate density.
This is the strongest companion-planting evidence on the books. Other crops in the same family probably benefit from similar priming, but only the tomato-basil pair has been replicated in controlled conditions.
Mechanism 2 — Pest disruption by scent confusion
The carrot-onion pairing is the standout here. Mixed-row plantings reduce carrot fly egg-laying by up to 70% in University of Bristol Botanic Garden trials. The mechanism is not "onions repel carrot fly" — it's that visual and chemical complexity makes host-plant location harder for specialist pests like Chamaepsila rosae. The same principle applies in reverse: carrot scent disrupts onion fly egg-laying on the alliums.
This is also how garlic interplanted with brassicas reduces aphid colonisation (recent research in Journal of Agricultural and Food Sciences), and probably how marigold-bordered beds receive fewer cucumber beetle visits. Where the host-finding behaviour is olfactory, scent diversity disrupts it — the same scent-confusion principle (with the same evidence caveats) is what underpins the popular mosquito-repellent plants claims.
Mechanism 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. Cornell Cooperative Extension specifically recommends 2-3 radish seeds in each cucumber hill for this reason — beetles concentrate on the radish foliage during the cucumber's vulnerable young-vine phase.
Trap cropping is the most actionable companion mechanism for new gardeners. It costs almost nothing (radishes are cheap and mature in 25-30 days), the effect is visible within weeks, and the worst-case outcome is that you get free radishes.
Mechanism 4 — Nitrogen fixing
Legumes (beans, peas, lentils) host Rhizobium leguminosarum bacteria in their root nodules. The bacteria convert atmospheric nitrogen into plant-available forms — potentially 30 to 50 lb of nitrogen per acre for following crops. Corn, leafy greens, brassicas, and most heavy-feeding fruiting vegetables all benefit from being planted after (or near) beans and peas.
This mechanism is also why alliums are bad neighbours for legumes: garlic and onion sulfur compounds (allicin) suppress the Rhizobium bacteria, dropping nitrogen fixation measurably. Keep alliums and legumes on opposite sides of the garden.
Mechanism 5 — Microclimate and ground cover
The Three Sisters polyculture (corn + beans + squash) is the showpiece. 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.
Living mulches, shade provision (tomato canopies cooling lettuce in zones 7+), and root-layer separation (carrots dig deep + lettuce stays shallow) all qualify as microclimate companion effects. These are observational rather than controlled-trial, but the logic is sound and the practice is recommended by virtually every extension service.
The 4 strongest companion pairings
| Pairing | Mechanism | Evidence level |
|---|---|---|
| Tomato + basil | Volatile-priming defence | Strong (peer-reviewed 2024) |
| Carrot + onion | Scent confusion (carrot fly) | Strong (University of Bristol) |
| Cucumber + radish | Trap cropping (cucumber beetle) | Strong (Cornell Extension) |
| Three Sisters (corn + beans + squash) | Microclimate + nitrogen fixing | Strong (Haudenosaunee documented) |
These are the pairings worth restructuring a bed plan around. Detailed pairing pages: tomatoes + basil, carrots + onions, cucumbers + radishes.
The 4 pairings to always avoid
| Pairing | Reason | Source |
|---|---|---|
| Tomato + potato | Shared late blight (Phytophthora infestans) | UMN Extension, UVM |
| Brassicas next to tomato | Heavy-feeder nutrient competition | RHS, Old Farmer's Almanac |
| Alliums next to peas or beans | Allicin suppresses Rhizobium | Multiple extension trials |
| Fennel near anything | Anethole allelopathy within 3-4 ft | Cultivated horticultural literature |
Detailed antagonist pages: peas + onions, beans + garlic.
What about marigolds?
Marigolds are the most over-claimed companion plant in popular gardening writing. Here is what the research actually supports:
True: French marigolds (Tagetes patula) release alpha-terthienyl from their living roots, which kills root-knot nematodes (Meloidogyne spp.) when used as a full-season cover crop the year before planting tomatoes or melons. UF/IFAS and Hawaii CTAHR have documented 40-50% nematode population reduction in field trials.
Partially true: Marigold borders may modestly reduce beetle activity through scent confusion, but the effect is much smaller than the nematode-suppression effect.
Overstated: "Marigolds repel everything." They don't. Marigolds actually increase populations of some other nematode species (stubby-root, sting, awl). They are not a general-purpose pest deterrent.
The actionable rule: Plant marigolds as a cover crop the season before you grow tomatoes in a nematode-prone bed. As a border around an active tomato bed, treat them as decorative — not as pest control.
Common companion planting myths
-
"Carrots love tomatoes." This pairing was popularised by Louise Riotte's 1975 book of the same name, but there is no peer-reviewed evidence for a yield benefit either way. Tomato canopies do provide useful afternoon shade for carrots in warm zones, and carrots tolerate partial shade — so they share a bed without competing. But treat the "love" framing as folklore.
-
"Basil improves tomato flavour." The 2024 priming study is real, but no peer-reviewed taste test has found a flavour difference between tomatoes grown with vs. without basil companions. Triple-blind tasting at WVU specifically found no consistent preference.
-
"Beans help tomatoes." Most extension services recommend keeping legumes and tomatoes 3+ feet apart. Tomatoes are heavy feeders that outcompete beans for water; the bean nitrogen is more useful in the next rotation in that bed than in the same season.
-
"Mint repels everything." Mint's strong scent does disrupt some pest host-finding, but mint is so aggressively rhizomatous that it will take over any shared bed within two seasons. If you want mint's deterrent effect, grow it in a separate sunken pot adjacent to the main bed.
How to use companion planting in your bed plan
Start with the 5-step vegetable garden plan to nail the fundamentals (sun, soil, water, spacing), then layer companion planting on top.
Step 1 — Pick 4-6 crops for the bed
Pick from the 12 crops in our companion planting chart. For a first vegetable garden, two strong starting combinations:
- Salsa garden: tomatoes + basil + peppers + onions + garlic (warm-season, all evidence-backed pairings).
- Spring salad bed: lettuce + radishes + carrots + spinach + peas (cool-season, all compatible).
Step 2 — Place companions within 12-18 inches
That's the spacing that lets volatile compounds and scent overlap — close enough for the beneficial effect, far enough that the plants don't crowd each other. For trellised crops (peas, cucumbers, pole beans), allow extra clearance for vine spread.
Step 3 — Keep antagonists 3+ feet apart
For allelopathic pairs (fennel, walnut, mature dill), 4+ feet usually clears the chemical effect. For heavy-feeder competition (tomato + cabbage), 3 feet or a different bed is enough. For shared-disease pairs (tomato + potato), 10+ feet or separate beds entirely.
Step 4 — Plan the rotation
Companion planting helps within a season; family rotation matters across seasons. Don't grow nightshades (tomato, pepper, eggplant, potato) in the same bed two years running, regardless of companions. Rotate to alliums, then to legumes, then to brassicas — a four-year rotation eliminates most soil-borne disease cycles.
For the bed-design fundamentals (sizing, spacing, path widths, sun exposure), see our vegetable garden layout guide.
Companion planting in containers and raised beds
The volatile-based mechanisms (scent confusion, defence priming) actually work better in dense raised-bed plantings because the volatile cloud stays concentrated. Three practical rules for containers:
- Pair within the same container only when both crops want the same water schedule. Tomato + basil ✅. Tomato + lavender ✗ (lavender wants drier soil).
- Use a container at least 12 inches deep for two-crop plantings — most companion benefits depend on root systems coexisting, not competing.
- Inoculate legume seeds with Rhizobium if you're growing them in containers. Container soil rarely has the bacteria naturally, so the nitrogen-fixing benefit is lost without inoculation.
How companion planting interacts with your hardiness zone
Companion planting doesn't change when you plant — only what you plant next to what. Use your zone to set the timing first, then layer companions on top:
- Zones 3-5 (cold winters): Short season — focus on warm-season pairings that work fast (tomato + basil, pepper + onion). Cool-season pairings (lettuce + radish + carrot) get a spring and a fall window.
- Zones 6-8 (moderate): All 12 of our crops thrive. Run a spring salad bed and a summer salsa bed back-to-back in the same space.
- Zones 9-11 (hot/mild): Cool-season pairings move to winter; warm-season crops run nearly year-round with a brief midsummer heat pause for tomatoes and peppers.
For zone-specific planting dates, see our USDA hardiness zone map and the monthly planting calendar.
CTA — Build the bed in Growli
Plan it in Growli: Tell Growli your bed size, your zone, and the crops you want — and the app lays out spacing, companions, antagonists, and rotation for you. iOS + Android, free.
Related articles
- The full companion planting chart — pairwise compatibility matrix for 12 crops
- How to start a vegetable garden — 5-step beginner plan
- Vegetable garden layout — bed sizing and spacing
- Easiest vegetables to grow — what to start with
- USDA hardiness zone map — find your zone
- Monthly planting calendar — what to plant this month
Reviewed and curated by the Growli editorial team. For questions about anything here, open Growli and ask — or email hello@getgrowli.app. Sources include the Royal Horticultural Society, Cornell Cooperative Extension, University of Minnesota Extension, University of Florida IFAS, the 2024 Plant Cell Reports basil-tomato priming study, and the evidence reviews maintained at garden-myths.com.
Frequently asked questions
What is the single best companion plant for tomatoes?
Basil — by a wide margin. A 2024 study in Plant Cell Reports identified three basil volatile compounds (linalool, chavicol, alpha-terpineol) that prime tomato wound-defence genes. Caterpillars feeding on primed plants gained roughly half the weight of caterpillars on unprimed plants. West Virginia University intercropping trials separately recorded ~20% yield gains for tomatoes grown with basil at adequate density.
Do marigolds actually repel pests?
Partially. French marigolds (Tagetes patula) release alpha-terthienyl from their living roots, which kills root-knot nematodes when grown as a cover crop the season before tomatoes. UF/IFAS and Hawaii CTAHR document 40-50% nematode reduction. As a decorative border around an active vegetable bed, the effect is much smaller — and marigolds can actually increase populations of some other nematode species.
Why can't you plant beans with onions or garlic?
Alliums 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.
What plants should never be planted together?
Tomato + potato (both nightshades, share late blight Phytophthora infestans — 10+ feet apart or separate beds entirely). Brassicas next to tomato or pepper (heavy-feeder nutrient competition — 3+ feet apart). Alliums next to peas or beans (allicin suppresses nitrogen fixation). Fennel near almost anything (anethole allelopathy stunts growth within 3-4 feet).
Does the Three Sisters method actually work?
Yes — it is the best-documented companion polyculture in horticulture. 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. Individual crop yields are slightly lower than each grown alone, but the overall productivity of the bed is higher.
Can companion planting replace pesticides and fertilizer?
Not entirely. Companion planting reduces pest pressure (sometimes substantially) and nitrogen-fixing legumes provide free nitrogen for following crops. But heavy infestations still need physical removal, neem, or row covers, and most crops need supplemental phosphorus and potassium that legumes can't supply. Think of companion planting as a multiplier on good basics, not a replacement for them.
Does companion planting work in raised beds and containers?
Yes — and the volatile-based mechanisms work even better in dense raised-bed plantings because the volatile cloud stays concentrated. For containers, pair only crops with the same water schedule (tomato + basil works; tomato + lavender doesn't), use a container at least 12 inches deep, and inoculate legume seeds with Rhizobium because container soil rarely has the bacteria naturally.
How far apart should incompatible plants be?
For allelopathic pairs (fennel, walnut, mature dill), 4+ feet usually clears the chemical effect. For heavy-feeder nutrient competition (tomato + cabbage), 3 feet of root separation or a different bed is enough. For shared-disease pairs (tomato + potato, with their shared late blight), 10+ feet or completely separate beds. For allium-legume interference (onion + bean), 2-3 rows of separation prevents the worst nodulation loss.