Companion Planting Calculator — What to Grow Together

Evidence is mixed, but several combinations are well-tested: basil with tomatoes, marigolds to repel whitefly and aphids, and onions interplanted with carrots to mask the scent from carrot fly. At worst it does no harm and increases biodiversity. The RHS notes the underlying principle is mixing plants to confuse pest insects.

An ancient Native American method: sweetcorn provides support for climbing beans, the beans fix nitrogen, and squash shades the soil to retain moisture. Per the RHS, set out a block of sweetcorn 45 cm apart, then sow climbing beans at the base once the corn reaches 30 cm. Plant in blocks, not rows.

Tomatoes and potatoes (both nightshades, sharing blight). Fennel is allelopathic and inhibits most neighbours. Alliums (onions, garlic) and beans can stunt each other. Keep brassicas and strawberries apart as they compete for the same nutrients.

Close enough to benefit each other but not so close they compete for light, water and nutrients. As a rule of thumb interplant companions 15-30 cm from the main crop, then follow each species own spacing needs. Use the Spacing Adjustment Factor here to scale that gap for vigorous or compact varieties.