Most 'bad neighbour' rules come down to three real problems: plants in the same family share the same pests and diseases (so trouble spreads fast), some plants are simply too greedy to share a bed, and a handful release substances that genuinely check what grows beside them. The folklore around this is thick, so below are the ones that actually hold up.
Keep these apart
Same family — both get blight, and grown together it sweeps through both. The most important pair to separate.
Allium root secretions can stunt the growth of legumes. Keep them in different beds.
Long-standing poor partners that seem to check each other and share pests.
Fennel releases compounds that inhibit many neighbours — give it a bed of its own.
Close relatives that share carrot fly and other umbellifer pests.
Two greedy crops competing for the same food and water, and a nightmare to harvest together.
Not a bad companion so much as a bully — it runs everywhere. Always grow it in a sunken pot.
Common questions
Why can't tomatoes and potatoes grow together?
They're in the same family (the nightshades) and share the same diseases — most importantly blight. Grown side by side, an outbreak can wipe out both at once, so keep them in separate parts of the plot.
Is companion planting 'don't' folklore actually true?
Some of it is, some isn't. The reliable rules have real reasons behind them — shared family diseases, heavy competition, or genuine growth inhibition (as with fennel). Vaguer claims with no mechanism are best taken with a pinch of salt. This list sticks to the ones that hold up.
What's the single most important thing to keep apart?
Tomatoes and potatoes, because of blight. If you remember only one rule, make it that one.
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