Companion Planting Basics
The Short Answer
Companion planting is growing certain plants near each other for mutual benefit — pest control, pollination, nutrient sharing, or physical support. The most important pairings to know: tomatoes love basil, the Three Sisters (corn + beans + squash) support each other, marigolds repel pests from almost everything, and fennel should be planted alone. Use our Companion Planting Checker to look up any two plants.
How It Works
Companion planting benefits fall into several categories. Pest deterrence is the most common — aromatic plants like basil, marigolds, and nasturtiums confuse or repel pest insects through their scent. Marigold roots also release compounds that suppress root-knot nematodes in the soil.
Nutrient sharing is most famously demonstrated by legumes (beans, peas). Their roots host bacteria that convert atmospheric nitrogen into plant-available form in the soil, benefiting nearby plants that need nitrogen.
Physical support is the principle behind the Three Sisters planting. Corn provides a natural trellis for bean vines to climb. The beans fix nitrogen that feeds the corn. Squash sprawls along the ground, shading out weeds and retaining soil moisture.
Trap cropping means planting something pests prefer near something you want to protect. Nasturtiums attract aphids away from your vegetables — the aphids feast on the nasturtiums and leave your tomatoes alone.
The Key Pairings
Tomatoes + Basil: Perhaps the most celebrated companion pair. Basil may repel certain tomato pests including thrips, aphids, and tomato hornworms. Many gardeners also believe the flavor of both improves when grown together — though scientific evidence for this specific claim is limited.
Carrots + Onions: The strong scent of onions confuses the carrot rust fly, which locates carrots by smell. Conversely, carrots may help deter onion flies.
Corn + Beans + Squash: The Three Sisters — an Indigenous planting system used for centuries. Each plant contributes something the others need.
Marigolds + Everything: French marigolds (Tagetes patula) are the universal companion. Plant them throughout your vegetable garden for broad-spectrum pest protection. Their root secretions suppress nematodes and their flowers attract beneficial predatory insects.
What to Keep Apart
Fennel inhibits the growth of most garden vegetables through allelopathic compounds (chemicals released from its roots). Plant it in its own area, away from your vegetable beds.
Onions/Garlic + Beans/Peas: Alliums can stunt the growth of legumes. Keep them in separate beds.
Tomatoes + Brassicas: They compete for the same nutrients and can attract each other's pests. Separate them where space allows.
A Practical Approach
Companion planting isn't magic, and rigid adherence to companion charts isn't necessary for a productive garden. The most impactful steps: plant marigolds and basil among your vegetables, keep fennel isolated, and don't stress about every possible pairing. Most plant combinations are neutral — neither helping nor hurting.