Companion Planting Guide 2026: Science-Backed Combinations That Work

Separating Science From Folklore

Companion planting sits at the intersection of agricultural science and garden folklore. "Carrots love tomatoes" has been repeated for generations, but when Cornell University tested the pairing in a 2019 controlled trial, they found zero statistically significant yield difference compared to monoculture planting.

Other claims, however, are solidly proven:

• Marigolds (Tagetes patula) suppress root-knot nematodes: Demonstrated in dozens of peer-reviewed studies since the 1970s. The roots exude alpha-terthienyl, a compound toxic to nematodes. Effect is species-specific — French marigolds work, African marigolds (T. erecta) do not.
• Basil interplanted with tomatoes reduces thrips and whitefly: A 2020 University of Florida study found 30-40% fewer thrips on tomato plants intercropped with basil compared to tomatoes alone.
• Nasturtiums act as trap crops for aphids: Aphids preferentially feed on nasturtiums over brassicas, beans, and squash. A 2018 study in Biological Control confirmed that nasturtium trap crops reduced aphid populations on adjacent broccoli by 58%.

The rest of this guide focuses on combinations with research support, not just tradition.

The Three Sisters: 5,000 Years of Proof

The Indigenous American "Three Sisters" system — corn, beans, and squash planted together — is the oldest and most thoroughly validated companion planting system on Earth. The corn provides a trellis for the beans. The beans (as legumes) fix nitrogen in the soil, feeding the nitrogen-hungry corn. The squash sprawls across the ground, shading out weeds and conserving soil moisture with its large leaves. The spiny squash stems also deter raccoons and other pests.

How to plant Three Sisters:

1. Build mounds 18 inches diameter × 6 inches high, spaced 4 feet apart center-to-center.
2. Late May-June (soil 65°F+): Plant 4 corn seeds in the center of each mound, 6 inches apart in a square pattern.
3. When corn reaches 6 inches (about 2 weeks): Plant 4 pole bean seeds around the corn, 3 inches from each corn stalk.
4. One week after beans emerge: Plant 3 squash seeds around the edge of each mound.
5. Thin to the strongest plants: 2-3 corn stalks, 2-3 bean vines, 1-2 squash plants per mound.

Yield per 4-foot × 8-foot bed (4 mounds): 30-40 ears of corn, 5-8 lbs of dried beans (or 15-20 lbs fresh), 15-25 lbs winter squash. This is roughly 3,000-5,000 calories per square foot over a season — competitive with modern intensive agriculture.

Research-Backed Pairings by Crop

Tomatoes

• Basil: Reduces thrips and whiteflies (University of Florida, 2020). Also improves tomato flavor in blind taste tests — the mechanism is unknown but confirmed in multiple trials. Plant basil 12 inches from tomato stems.
• Marigolds (French): Suppress root-knot nematodes when planted as a cover crop and tilled in before tomato planting. Border planting (marigolds around the bed perimeter) reduces above-ground pests by 15-25%.
• Borage: Attracts predatory wasps that parasitize tomato hornworms. One borage plant per 4 tomato plants.
• AVOID: Fennel (secretes allelopathic compounds that inhibit tomato growth). Dill (attracts tomato hornworm — plant far from tomatoes, let dill act as a trap/treat crop). Brassicas (cabbage, broccoli, kale) stunt tomato growth through root exudates.

Cucumbers

• Nasturtiums: Trap crop for aphids. Plant nasturtiums 12-18 inches from cucumber hills. When aphids colonize the nasturtiums, remove and dispose of the infested plants (do not compost).
• Dill: Attracts parasitic wasps and hoverflies that control cucumber beetles and aphids. Allow dill to flower — the tiny blooms are what attract beneficial insects.
• Radishes: Plant radishes among cucumbers as a trap crop for flea beetles and cucumber beetles. The radishes take the damage; the cucumbers stay clean. Harvest the radishes early before they bolt.
• AVOID: Potatoes (compete for nutrients and water). Aromatic herbs like sage (inhibit cucumber growth).

Brassicas (Cabbage, Broccoli, Kale, Brussels Sprouts)

• Dill: Attracts braconid wasps that parasitize cabbage worms. Plant dill at the corners of brassica beds. Allow flowering.
• Nasturtiums: Trap crop for aphids and cabbage white butterflies, which lay eggs on nasturtiums instead of brassicas.
• Thyme: Reduces cabbage worm infestation when planted as a living mulch between brassica plants (Washington State University, 2017).
• AVOID: Strawberries (compete for calcium, share verticillium wilt). Tomatoes (root exudates stunt brassicas).

Carrots

• Onions, leeks, chives: The allium family repels carrot rust fly through odor masking. Plant 1 row of onions for every 2 rows of carrots. Chives are the most effective — the strong scent confuses the fly, which locates carrots by smell.
• Rosemary and sage: The strong volatile oils repel carrot fly at low levels. Plant at carrot bed margins.
• AVOID: Dill (cross-pollinates with carrots, producing bitter-tasting hybrid seeds if both flower). Parsnips (attract the same pests, doubling the target).

Squash, Zucchini, Pumpkins

• Nasturtiums: Trap crop for squash bugs and aphids. Plant a ring of nasturtiums around each squash hill.
• Marigolds (French): Reduce cucumber beetle and squash bug populations when planted at the bed perimeter.
• Radishes: Deter squash vine borer when planted around the base of squash stems (the mechanism is unclear but replicated in multiple field trials).
• AVOID: Potatoes (compete heavily; both are heavy feeders).

Beans (Pole and Bush)

• Corn: Beans climb corn stalks (Three Sisters method). The nitrogen beans fix becomes available to corn after bean plants are tilled in at season''s end.
• Savory (Satureja hortensis): "The bean herb" — traditional German pairing. Savory improves bean flavor and reduces Mexican bean beetle infestation. Plant at 6-inch spacing among beans.
• Marigolds: Suppress root-knot nematodes that affect beans. Plant as a preceding cover crop (grow marigolds for one season, till in, plant beans the next season).
• AVOID: Alliums (onions, garlic, leeks — exude compounds that inhibit bean growth). Fennel (inhibits most plants).

Lettuce and Greens

• Carrots: Carrots and lettuce share space efficiently — lettuce is shallow-rooted (6-12 inches), carrots are deep-rooted (12-18 inches). Interplant rows 6 inches apart.
• Radishes: Radiolucent (radishes, lettuce, and spinach together form a classic intercropping triad — radishes mature in 25 days and are harvested before they compete with slower lettuce).
• Strawberries: Lettuce grows well in the partial shade of strawberry leaves. The combination uses vertical space efficiently.
• AVOID: Parsley (both attract the same aphid species, concentrating pest pressure).

Plants That Belong in Every Garden (Not Crop-Specific)

| Plant | Function | Where to Plant | Research Source | |-------|----------|---------------|-----------------| | French Marigold (Tagetes patula) | Nematode suppression, general pest repellent | Bed borders, between tomato/cucumber rows | 50+ peer-reviewed studies since 1970s | | Sweet Alyssum (Lobularia maritima) | Attracts hoverflies and parasitic wasps — the #1 beneficial insect attractor | Bed edges, under-planting between crops | Oregon State University IPM studies | | Borage (Borago officinalis) | Attracts predatory wasps and bees. Accumulates potassium, calcium, and trace minerals in leaves — chop and drop as green mulch | 1 plant per 10 sq ft | Multiple university trials | | Dill (Anethum graveolens) | Attracts parasitic wasps, hoverflies, and lacewings. A single flowering dill plant hosts 50+ beneficial insects daily | At bed corners, allow to flower | Cornell IPM program | | Nasturtium (Tropaeolum majus) | The universal trap crop for aphids. Also repels squash bugs and whiteflies | Among squash, cucumbers, beans, and brassicas | Biological Control journal, 2018 |

Crop Rotation vs. Companion Planting

Companion planting and crop rotation address different problems and work best together. Companion planting is spatial (what grows next to what this year). Crop rotation is temporal (what grows in this spot next year).

The basic 4-year rotation for a 4-bed garden:

• Year 1: Bed 1 — Legumes (beans, peas). Bed 2 — Brassicas (cabbage, broccoli). Bed 3 — Solanaceae (tomatoes, peppers). Bed 4 — Roots (carrots, beets).
• Year 2: Shift each family forward one bed. Bed 1 gets brassicas, Bed 2 gets solanaceae, Bed 3 gets roots, Bed 4 gets legumes.
• Years 3-4: Continue shifting.

This prevents: (1) soil-borne disease buildup (tomato blight spores survive 3 years in soil), (2) nutrient depletion (each family draws different nutrients), (3) pest population establishment (flea beetles overwinter where brassicas grew last year — they emerge to find legumes instead and starve).