Open any old gardening book and you'll find a chart of who "loves" and "hates" whom in the vegetable bed, as if your tomatoes are keeping a grudge against your cabbage. Some of that is real, tested, and genuinely useful. Some of it is folklore that got copied from one chart to the next for a hundred years. Since we'd rather you spend your season on what works, here's the honest version.
What companion planting actually does
Strip away the mysticism and companion planting comes down to a handful of down-to-earth mechanisms — the same ones a farm uses, just at backyard scale.
Some plants feed the soil. Beans, peas, and other legumes pull nitrogen out of the air and leave it in the ground for hungry neighbors. That's the engine behind the classic Three Sisters — corn for the beans to climb, beans to feed the soil, and squash sprawling below to shade out weeds and hold moisture.
Some plants confuse or repel pests. Strongly scented plants — onions, garlic, chives, basil — make it harder for pests to sniff out the crop they're after. Onions planted near carrots throw off the carrot fly; basil tucked among tomatoes discourages thrips.
Some plants take the bullet for you. This is trap cropping: nasturtiums draw aphids and flea beetles to themselves and away from your brassicas, so the pests feast on the sacrifice plant instead of your kale.
Some plants call in the pollinators. Borage, thyme, rosemary, and cosmos bring in the bees and hoverflies that both pollinate your crops and eat the aphids. More flowers in the bed usually means more fruit set and more beneficial insects on patrol.
Five pairings worth starting with
You don't need the whole chart. Pick a couple of these and watch what happens: tomatoes with basil, carrots with onions, peppers after marigolds (marigolds suppress root-knot nematodes only as a dense, solid planting or cover crop grown the season before — a few dotted among the peppers won't do it), strawberries with borage or thyme, and the Three Sisters if you've got the room for corn. These are the best-supported pairings on the old charts — some for reasons that have been tested, some mainly because they do no harm and bring in pollinators.
The part where we stay honest
Those elaborate "enemies" charts? Treat them with a raised eyebrow. A few real ones exist — keep heavy feeders from crowding each other, and give sprawling plants their space — but most "X hates Y" claims never had much evidence behind them and won't make or break your harvest. The strongest, best-tested move in companion planting isn't avoiding enemies at all — it's adding flowers and herbs to attract pollinators and predatory insects. If you do one thing this year, do that.
Where the moon comes in
Here's how companion planting and lunar timing fit together: each plant still gets sown in its own moon window. Leafy greens and above-ground growers like basil go in during the waxing moon; root crops like carrots and onions go in on the waning moon — the same rhythm we walked through in Planting by the Moon: Your First Month. Companion planting decides who sits next to whom; the moon decides when each one goes in the ground. Plan the bed once, and the timing takes care of itself.
Timing them together: see when to plant each crop where you live, so companions actually go in the ground at the same time.