Companion planting means putting plants next to each other that help each other grow — repelling pests, attracting beneficial insects, or sharing nutrients.
The combinations with real evidence behind them:
Tomatoes + Basil — basil may repel aphids and whiteflies through volatile oils. At minimum, basil fills space between tomato plants that weeds would otherwise take. Plant basil 12 inches from tomato stems.
The Three Sisters (corn + beans + squash) — a Native American system that works beautifully. Corn provides a trellis for beans. Beans fix nitrogen from air into soil, feeding corn and squash. Squash leaves shade the ground, keeping it moist and suppressing weeds. Plant corn first, add beans 2 weeks later, squash a week after that.
Carrots + Onions — carrot fly and onion fly reportedly avoid each other's scent. Interplant them in alternating rows.
Nasturtiums as trap crops — aphids love nasturtiums more than almost anything. Plant nasturtiums near vegetable beds and aphids cluster there instead of on your food plants. Pull and discard infested nasturtiums every few weeks.
Marigolds — French marigolds (Tagetes patula) produce a root chemical that deters nematodes in the soil. Plant densely for 1–2 seasons in nematode-infected beds before planting vegetables.
What doesn't work: Most "companion planting charts" online are folklore with no scientific backing. Garlic near roses, for example, has no proven pest-repelling effect. Focus on the combinations above — they have at least some evidence or a clear logical mechanism.
Keep it simple: Pick one or two combinations that match what you're growing. Interplanting also increases yield per square foot, which matters in small gardens.