Companion planting: what the evidence actually supports
Companion planting mixes two different things: mechanisms with field evidence (nitrogen fixation, trap cropping, habitat for beneficial insects) and folklore about plants "liking" each other. The first group is worth planning around. The second is mostly unproven, and a few negative pairings are real.
Claims, sorted by evidence
| Claim | Evidence status | What to do instead |
|---|---|---|
| Legumes add nitrogen for the next crop | Well established (rhizobia fixation) | Rotate beans/peas before heavy feeders; leave roots in the ground. |
| A trap crop pulls pests away from the main crop | Established for specific pests (e.g. nasturtium for aphids, trap rows for flea beetles) | Plant the trap crop early and accept that you will sacrifice it. |
| Flowers feed predatory insects and pollinators | Established in field studies (flower strips) | Sow a strip of umbellifers and daisies at the bed edge. |
| Marigold roots suppress nematodes | Real but specific: Tagetes species and variety matter, and the effect is inconsistent | Treat it as a maybe, not a control measure; rotate crops anyway. |
| A herb's smell repels a pest | Weak or untested in the field | Use row cover or timing instead of relying on scent. |
| "These plants like each other" | Folklore | Check that they want the same light, water and spacing — that is the real compatibility test. |
Classic pairings, honestly
| Pairing | What is true | Verdict |
|---|---|---|
| Tomato + basil | Same sun and water needs; convenient to harvest together. No reliable yield or disease effect. | Fine, not magic. |
| Three sisters (corn, beans, squash) | Beans fix nitrogen and use corn as a trellis; squash shades the soil. Needs fertile ground, wide spacing and a long season. | Works with planning, fails when crowded. |
| Marigold + anything | Nematode suppression depends on species and density; most garden plantings are too sparse. | Good companion plant, weak pesticide. |
| Nasturtium + brassicas | Aphids often colonise nasturtium first, which can protect the crop if you remove the infested growth. | Useful trap crop. |
| Alliums + beans/peas | Allium root exudates can inhibit legume growth. | Avoid interplanting. |
| Fennel + vegetables | Fennel is allelopathic and suppresses many neighbours. | Give it its own corner. |
What actually decides whether a bed works
- Light: two plants with the same sun need can share a bed; a sun lover and a shade plant cannot.
- Water: putting thirsty crops next to drought-tolerant ones means one of them is always wrong.
- Spacing: competition for root space is stronger than any "companion" effect. Use mature spacing.
- Rotation: moving a crop family to a new bed next year does more against soil-borne disease than any pairing.
- Flowers and habitat: the most reliable "companion" is a strip of flowers that feeds predators and pollinators.
If you want to test a claim in your own garden, change one bed at a time and keep the same spacing, water and variety; otherwise you are measuring the season, not the pairing.
FAQ
Does companion planting work?
Some of it: nitrogen fixation, trap crops and flower strips have field evidence. Scent-based pest repellence is mostly unproven.
What is the best companion for tomatoes?
Legumes before them for nitrogen, flowers nearby for pollinators. Basil is harmless but not a proven yield booster.
Which plants should not be grown together?
Fennel suppresses many vegetables; alliums can inhibit legumes. These negative effects are better documented than most positive ones.
Related: plant spacing chart · raised bed layout · yard survey checklist