Companion Planting: Which Plants Support Each Other (and Which Compete)

This combination, developed by Indigenous peoples of North America over thousands of years, is one of the most complete companion systems in existence. Corn provides a vertical structure for pole beans to climb. Beans are legumes that fix atmospheric nitrogen into the soil, feeding the corn and squash. Squash spreads wide along the ground, its large leaves shading out weeds and retaining soil moisture while its prickly texture discourages some pests and foraging animals.

Each plant fills a different niche — vertical, mid-level nutrient cycling, and ground cover — making the system remarkably self-supporting. Plant corn first, allow it to establish several inches of height, then sow beans around the base and squash at the outer edges of the mound.

Basil is widely planted near tomatoes, and while the evidence is more anecdotal than conclusive, the pairing remains a staple of kitchen gardens for practical reasons. Basil's strong volatile oils are thought to deter thrips and aphids, and some gardeners report it may help confuse whitefly. More concretely, basil attracts pollinators and beneficial insects that also serve the tomato plants nearby.

From a purely practical standpoint, grouping herbs you'll harvest together makes garden management easier. Keep basil in full sun adjacent to tomato plants rather than in their shadow — both crops need direct light to thrive.

The traditional logic here is that carrot fly and onion fly are each repelled by the scent of the other's companion crop. Carrot fly is attracted to the smell of bruised carrot foliage, and onions mask that scent effectively. Conversely, onion fly is confused by the presence of carrots. It's a mutual disguise strategy.

Chives work similarly to onions and are easier to integrate into mixed beds because of their compact form. They also deter aphids on a range of plants when planted at intervals throughout a bed — a useful all-around border plant. Avoid planting carrots near dill, which can inhibit carrot development at certain growth stages.

Nasturtiums are one of the most versatile companion plants in the garden, primarily because aphids love them. This makes them effective trap crops — plants positioned to lure pests away from more valuable crops. Plant nasturtiums near brassicas (broccoli, kale, cabbage) or beans to draw aphid colonies away from those plants and concentrate them where they're easier to manage or where natural predators can find them.

Nasturtiums also attract hoverflies, whose larvae are voracious aphid predators. Rather than viewing an aphid-covered nasturtium as a failure, treat it as the system working as intended. Remove and dispose of heavily infested stems periodically to prevent populations from spreading back into main crops.

Not all companion relationships are beneficial — some plants actively harm their neighbors. Fennel is one of the most aggressive allelopathic plants in common kitchen gardens, meaning it releases compounds from its roots that inhibit the growth of many nearby plants. Keep fennel well away from tomatoes, peppers, and most brassicas.

Dill and fennel should not be planted close to each other either, because they cross-pollinate readily, producing seeds with muddled flavor. Dill itself is a good companion for brassicas and cucumber when it's young, but as it matures and flowers it can inhibit those same plants. Plant dill in a dedicated spot where it can be harvested young, or allow it to go to seed in a controlled area where it will self-sow and attract beneficial wasps.

French marigolds (Tagetes patula) produce a chemical from their roots called alpha-terthienyl, which has been shown in research to suppress certain nematode populations in the soil — specifically root-knot nematodes that damage tomatoes, peppers, and other crops. For this effect to work, marigolds need to be planted densely and grown as a full-season cover or interplanted over multiple consecutive seasons.

Above ground, marigolds' strong scent deters some flying pests and their open flowers attract hoverflies and parasitic wasps. They're one of the few companion plants where both above- and below-ground mechanisms are reasonably well supported. They're also a practical addition to any mixed border — the annuals vs. perennials guide covers how to mix seasonal bloomers like marigolds into a longer-term planting strategy.

Legumes — including peas, beans, clover, and vetch — host nitrogen-fixing bacteria (Rhizobium) in nodules on their roots. These bacteria convert atmospheric nitrogen into a plant-available form that enriches the surrounding soil. This makes legumes invaluable companions for heavy nitrogen feeders like corn, leafy greens, and brassicas.

The benefit is most pronounced after the legume crop ends: when you cut the plant at the base rather than pulling it, the nitrogen-rich root system decomposes in place, feeding subsequent plantings. Rotating legumes through your beds each season is one of the most practical, low-cost ways to build long-term soil fertility without relying solely on added fertilizers. For gardens incorporating native plants, some native legumes like wild lupine also serve this function — worth exploring if you're interested in native plants for ecological benefit.