Useful companion planting is about pest management and sharing light, water and space — not folklore pairings. Trap crops and shading are the best-supported tactics.
Trap crop example
Nasturtium draws aphids away
Scent masking
Basil near tomatoes may deter thrips
Shading
Sunflowers shelter heat-sensitive lettuce
Competition
Zucchini and potatoes compete heavily
Use sacrificial trap crops at bed edges to pull pests off the main crop.
Pair tall crops with heat-sensitive greens for afternoon shade.
Keep heavy feeders apart so they do not compete for the same nutrients.
Avoid planting large brassicas beside tomatoes, peppers and potatoes.
Treat any claimed pairing with scepticism unless a trial backs it.
Every figure above comes from these pages.