What Grows Well Together in a Vegetable Garden

Most companion planting charts are copied from each other and trace back to a handful of mid-century books with no trials behind them. A small part of it is real, and the way to tell the difference is to ask what the mechanism is supposed to be.

The four mechanisms that do something

Support and shade is the least contested. Lettuce under taller crops bolts later because the soil stays cooler; beans climb corn instead of needing a frame.

Nitrogen fixation is real but slower than most charts imply. Beans and peas fix nitrogen into root nodules, and the benefit reaches the next crop when those roots break down — so the pairing that works is in sequence, legume then heavy feeder, rather than side by side in the same season.

Trap cropping is well documented commercially. Nasturtiums draw black bean aphid away from beans; the trap plant is expected to be ruined, which is the point.

Host confusion has evidence in specific cases, most clearly alliums and carrot fly. The effect is real but partial, and it fails if the onions are harvested mid-season.

What to plant together, by crop

• Tomatoes — basil and parsley at the base, lettuce beneath for the shade. Keep brassicas and fennel away.
• Carrots — onions, leeks or chives through the row, for carrot fly. Keep spacing generous; carrots compete badly.
• Beans and peas — corn or sunflowers for support, nasturtium as a trap. Keep onions and garlic away, which suppress them.
• Brassicas — follow a legume crop. Dill and coriander nearby draw parasitic wasps. Keep away from strawberries.
• Squash and pumpkin — under corn, on the ground. They want the space nothing else uses.
• Lettuce and leafy greens — in the shade of anything taller, which buys weeks in summer.

What genuinely conflicts

These four have mechanisms behind them, unlike most of the “enemies” lists:

• Fennel inhibits the growth of a wide range of vegetables around it. Grow it in a pot or at the far end.
• Onion family with beans and peas — alliums appear to suppress legume growth, and the pairing consistently underperforms.
• Brassicas and strawberries compete directly for the same shallow nutrients and both suffer.
• Black walnut releases juglone from roots, leaves and husks. Tomatoes, peppers, potatoes and aubergines are highly sensitive and will not grow in the root zone.

The Three Sisters, and why it is the exception

Corn, beans and squash is the system worth copying, because every plant in it has a job:

• Corn is the pole the beans climb, so no frame is needed.
• Beans fix nitrogen, feeding the corn, which is a heavy nitrogen user.
• Squash sprawls across the ground, shading out weeds and keeping soil moisture in, and its prickly stems deter some animals.

It is the clearest demonstration of the principle: three mechanisms, all physical, all verifiable. If a pairing you read about cannot be described that plainly, it probably does nothing.