Does companion planting really work?

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.

What is the Three Sisters planting?

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.

What plants should never be grown together?

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.

How close should companion plants be?

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.

GardenCalcs

Companion Planting Calculator — What to Grow Together

Find companion planting combinations and spacing for common vegetables.

Main CropThe vegetable you are planning the bed around — companions and spacing are matched to it

How We Calculate This

This calculator provides companion planting guidance for common UK vegetable garden crops, drawn from RHS grow-your-own advice (the Three Sisters, companion-planting and per-crop spacing pages).

Each crop shows recommended companions (plants that benefit each other), plants to avoid (allelopathic or competing species, or shared pests and diseases), spacing guidance, and an explanation of why the pairing works.

The Spacing Adjustment Factor scales the RHS-typical companion gap for the chosen crop: a factor below 1.0 tightens spacing for compact varieties, above 1.0 widens it for vigorous ones. The adjusted distance appears in the breakdown.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.