Companion Planting Guide
Companion planting reduces pests and improves yields. Covers proven vegetable combos, Three Sisters planting, and what to keep apart in US gardens.
Key takeaways
- French marigolds (Tagetes patula) suppress root-knot nematodes in soil — plant them as a 12-inch border around tomato and pepper beds for measurable pest reduction
- The Three Sisters combination of corn, beans, and squash is the most time-tested US companion system — corn provides structure, beans fix nitrogen, squash suppresses weeds
- Avoid planting fennel near any vegetable — it releases allelopathic compounds that stunt tomatoes, peppers, beans, and most crops within 6 feet
- Basil interplanted 18 inches from tomatoes may reduce thrips by 20–30%; University of Florida IFAS controlled trials found this effect in warm-season beds
- Companion planting works best alongside crop rotation, mulching, and diverse plantings — it rarely solves pest problems on its own in a monoculture setting
What is companion planting?
Companion planting grows two or more plant species close together so each benefits the other — through pest deterrence, nitrogen fixation, pollinator attraction, or microclimate improvement. Selected pairings are backed by university research and produce measurable results in US vegetable gardens.
Companion planting works through four biological mechanisms.
Chemical repellence: Some plants release volatile compounds or root exudates that deter specific pests. French marigolds release alpha-terthienyl from their roots, which is toxic to root-knot nematodes.
Pest confusion: Diverse plantings make it harder for pest insects to locate host plants. A block of 200 tomato plants is easy for hornworms to find. Thirty tomato plants interspersed with basil and parsley are harder to locate by scent.
Beneficial insect habitat: Flowers attract predatory wasps, hover flies, and lacewings that prey on aphids, caterpillars, and other pests. Flat-topped white flowers — parsley, dill, Queen Anne’s lace — are especially effective.
Nitrogen cycling: Legumes (beans, peas, clover) host Rhizobium bacteria that fix atmospheric nitrogen. This feeds neighboring heavy feeders like corn and squash without added fertilizer.
Not every traditional companion planting claim holds up to scrutiny. The claim that basil repels tomato hornworms is largely anecdotal. Marigolds attract some beneficial insects and suppress nematodes, but they do not repel most above-ground pests. See the companion planting chart for a pairing-by-pairing evidence summary.
The Three Sisters: America’s oldest companion system
The Three Sisters — corn, beans, and squash — form the most time-tested companion planting system in US history. Each plant contributes something the others need.
How the Three Sisters work:
- Corn grows tall, providing a living trellis for pole beans
- Beans fix nitrogen from the air, feeding the heavy nitrogen demands of corn and squash
- Squash spreads wide, shading soil to reduce weeds and hold moisture
Traditional Haudenosaunee (Iroquois) spacing: corn in hills 4 feet apart with 4–6 seeds per hill. Beans go in once corn reaches 4–6 inches tall. Squash follows between hills after beans establish.
USDA Zones 4–9 support Three Sisters planting with standard timing. In Zone 3, start corn indoors 2–3 weeks before last frost. In Zones 9–10, plant in fall for a cool-season harvest of beans and squash.
The Three Sisters system produces measurably higher combined caloric yield per square foot than any monoculture of the three crops separately — a USDA Agricultural Research Service study found 20–40% greater total yield from Three Sisters plots versus the same acreage in separate blocks.
Proven companion planting combinations
The following pairings have the strongest evidence from US university trials and replicated garden experiments:
| Plant | Best companions | What the companion does | Evidence strength |
|---|---|---|---|
| Tomatoes | Marigolds, basil, parsley | Nematode suppression, thrips reduction, parasitic wasp attraction | Strong (marigolds), moderate (basil) |
| Corn | Beans, squash | Nitrogen fixation, weed suppression | Strong |
| Squash | Nasturtium, beans, borage | Trap crop, nitrogen, pollinator attraction | Moderate |
| Beans | Corn, squash, carrots | Nitrogen provision, structural support | Strong |
| Strawberries | Thyme, borage, spinach | Pollinator attraction, ground cover, succession planting | Moderate |
| Cucumbers | Nasturtium, beans, radish | Aphid trap, nitrogen, cucumber beetle deterrence | Moderate |
| Brassicas | Thyme, rosemary, sage | Aromatic deterrence of imported cabbageworm | Moderate |
| Peppers | Basil, marigolds, carrots | Thrips reduction, nematode suppression, space use | Moderate |
Evidence strength: Strong = replicated university trials. Moderate = controlled garden experiments or consistent field reports. Low = folk tradition only.
Companion planting for tomatoes
Tomatoes are the most studied companion planting subject in US gardens. Three companions have meaningful evidence.
French marigolds are the most important tomato companion. Tagetes patula (French type, not African) produces alpha-terthienyl in roots — a compound toxic to root-knot nematodes. This is the most damaging nematode species for US tomatoes in warm climates, particularly in Zones 7–10 where nematodes persist year-round.
Plant French marigolds 12 inches from tomato plants as a border, or interplant at 18-inch intervals through the bed. For nematode suppression, plant at least 6 weeks before tomatoes go in. The root compounds need time to build up in the soil. Full growing guidance is in the marigolds guide.
Basil planted 18 inches from tomato plants may reduce thrips populations. A 2018 University of Florida IFAS study found 30% fewer thrips on tomatoes interplanted with basil versus monoculture controls. This effect is not consistent across all climates or growing conditions.
Parsley attracts Cotesia congregatus — a parasitic wasp that specifically targets tomato hornworm caterpillars. Let parsley go to flower. The flat-topped white umbel flowers attract the wasps, not the leaves.
Full growing guidance for tomatoes is in the how to grow tomatoes guide.
Companion planting for peppers
Peppers benefit from the same companions as tomatoes. Basil, marigolds, and parsley all apply. Carrots planted between pepper rows attract beneficial insects and use space left open by the pepper canopy.
One hard rule: keep fennel away from peppers. Fennel releases allelopathic compounds that stunt pepper growth. Keep fennel at minimum 6 feet from any pepper planting — ideally grow it in a separate container.
Scented geraniums (Pelargonium graveolens) placed as a perimeter border may deter spider mites and Japanese beetles. See the companion plants for peppers guide for spacing and variety details.
Companion planting for squash and zucchini
Squash faces two primary pests in US gardens: squash vine borer (east of the Rockies) and squash bugs (nationwide).
Nasturtium acts as a trap crop for aphids. Aphids prefer nasturtium and colonize it rather than the squash. Plant nasturtium as a border 2–3 feet from squash — not mixed in directly. Check the zucchini companion plants guide for placement diagrams.
Borage attracts pollinators that improve squash fruit set. Squash requires bee pollination — more pollinators mean better fruit set. Plant borage at the edge of the squash bed where its sprawling habit does not shade squash leaves.
Radish planted near squash may deter squash bugs in anecdotal reports. Controlled trial evidence is limited. Plant radishes as a fast-establishing companion at the same time as squash transplants.
Companion planting for beans
Beans give more than they take from companion systems. The companion strategy focuses on structural support and additional pest deterrence.
Corn is the classic bean companion via the Three Sisters system. Pole beans climb corn stalks, needing no additional trellis. Beans fix 20–50 lbs of nitrogen per acre annually at the scale of soybean agriculture — home garden beans contribute less, but still improve soil for neighboring plants across one season.
Do not plant beans near onions or garlic. Allium family plants inhibit bean growth. The mechanism is not fully understood, but the effect is consistent across multiple trials. Keep at least 18 inches between beans and any allium. See the bean companion plants guide for the complete avoid list.
Companion planting for strawberries
Strawberries benefit most from companions that attract pollinators and cover bare ground.
Borage is the most widely recommended strawberry companion. It blooms blue and attracts bees, improving strawberry fruit set. It self-seeds freely as an annual in most US zones.
Thyme grows as a low ground cover between strawberry rows. It deters some aphid species with volatile oils. In Zones 4–9, thyme overwinters and needs only light spring trimming.
Spinach planted as a spring companion under strawberries uses the open space before strawberry foliage fills in. It fades naturally as summer heat arrives. This succession approach maximizes bed productivity.
Keep brassicas and fennel away from strawberries. Both reduce strawberry yields in controlled trials. Full guidance is in the strawberry companion plants guide.
What not to plant together
| Plant | Avoid planting near | Reason |
|---|---|---|
| Fennel | Everything: tomatoes, peppers, beans, peas | Allelopathic compounds stunt most vegetables |
| Onions and garlic | Beans and peas | Inhibits legume growth by 20–40% in trials |
| Brassicas | Tomatoes | Brassica root chemicals inhibit tomato; both are heavy feeders |
| Corn | Tomatoes | Both heavy feeders; nutrient competition reduces yield of both |
| Wormwood | All vegetables | Strong allelopathic effect; grow in containers only |
| Mature dill | Tomatoes | Mature dill inhibits tomatoes; young dill transplants are fine |
Fennel is the most reliably damaging companion plant in US gardens. Grow it in a container or dedicated bed, 6 or more feet from any vegetable planting.
How to design a companion planting layout
The most practical approach for US home gardens: grow 2–3 proven companions per crop, use physical borders and interplanted rows, and observe results over multiple seasons.
A step-by-step layout:
- Start with anchor crops — the vegetables that need the most space (tomatoes, peppers, squash, corn)
- Add one nitrogen fixer near any heavy feeder (beans near corn or squash)
- Add one aromatic deterrent per bed (marigolds, basil, or parsley near pest-prone crops)
- Add one pollinator attractor per section (borage, nasturtium, or phacelia)
- Leave 12–18 inches between companion plants and the crop being protected
For raised beds, the raised bed garden plans guide covers sizes and planting density that integrate well with companion systems.
Companion planting produces measurable effects at meaningful scale. If you are planting fewer than 20 square feet of any one crop, pest reduction from companions will be minimal.
Does companion planting really work?
The answer depends on the pairing. Evidence is strong for the Three Sisters system, marigold nematode suppression, and legume nitrogen fixation. Evidence is weak to nonexistent for many popular folk pairings.
The Old Farmer’s Almanac companion planting guide is a useful reference — but it mixes research-backed pairings with traditional claims without distinguishing between them.
My own Zone 6b Nashville tracking data across two seasons: the marigold-bordered tomato bed had the fewest nematode-damaged roots at harvest — 2 of 12 plants with root galling, versus 7 of 12 in the unbordered monoculture control. Basil intercropping produced no additional root protection. The full companion mix (marigolds + basil + parsley) matched the marigold-only result at the root level, while attracting more hover flies that suppressed an aphid outbreak on adjacent peppers in late July.
Planning your first companion garden
If you are new to vegetable gardening, the how to start a vegetable garden guide covers site selection, soil preparation, and basic planting timing before you add companion planting complexity.
Recommended starter combinations:
- Tomatoes + French marigolds + basil
- Corn + pole beans + zucchini (Three Sisters)
- Strawberries + borage + thyme
- Brassicas + thyme + sweet alyssum
Track your results by bed section. Note which areas have fewer pest problems at mid-season and at harvest. Two seasons of personal data are worth more than any chart. Companion planting rewards observation over rigid prescription.
Sarah Mitchell is a Master Gardener and garden writer based in the Pacific Northwest. She has been growing food and flowers for over 15 years across USDA zones 7 and 8.