Companion planting advice fills gardening books with confident specificity. Plant basil with tomatoes! Never plant fennel near anything! The Three Sisters! Most of it is repeated folklore with no mechanism — and some of it actually works. Here's how to tell the difference.
The Proven Combinations
Tomatoes + Basil: Basil deters aphids and thrips. The evidence is mixed on the mechanism (volatile compounds vs. physical disruption), but the pest reduction is real enough to be worth doing.
Three Sisters (corn + beans + squash): Corn provides trellis, beans fix nitrogen, squash shade suppresses weeds. This one works — it's been validated by both indigenous practice and modern research.
Brassicas + Nasturtiums: Nasturtiums are aphid magnets. Plant them as trap crops adjacent to cabbage and kale to pull aphids away from your harvest.
The Myths
Marigolds repel everything: French marigolds (Tagetes patula) repel nematodes in soil over multiple seasons — but the common claim that they deter all garden pests above ground is overblown.
Fennel suppresses all neighbors: Fennel does produce allelopathic compounds but the effect is much smaller than commonly stated. Keep fennel out of raised beds for other reasons (it's invasive), not because it "kills" neighbors.
