Companion planting — which plants belong side by side

Companion planting means deliberately putting different species next to one another instead of filling a bed with just one. The idea is old and simple: plants that root at different depths do not compete for the same water. Strongly scented plants can confuse pests that find their host by smell. And legumes gather nitrogen from the air through root nodule bacteria, which their neighbours also benefit from.

The best-known pairing is carrot and onion: the onion scent disturbs the carrot fly, the carrot scent the onion fly. Other classics are beans with summer savory, tomato with basil, and cabbage with celery. Close relatives, by contrast, count as poor neighbours because they draw the same nutrients and share the same diseases — potato and tomato, for instance, both belong to the nightshade family.

It pays to stay honest about the evidence. Some of these rules are well studied, such as the effect of marigold roots against root nematodes. Much of the rest comes from experience and tradition and has never been tested under controlled conditions. Companion planting replaces neither good soil nor crop rotation — it is one tool among several, not a substitute for the basics.