Companion Planting: Which Plant Pairings Help and Which Ones Hurt

Gardeners have paired tomatoes and basil for generations, and there is reasonable evidence to support the habit. Basil is thought to repel thrips and aphids through volatile compounds it releases, and some studies suggest it may improve the flavor of nearby tomatoes — though the flavor claim remains more anecdotal than proven. What's better established is that basil attracts pollinators when allowed to flower, which benefits the entire bed. Keep basil trimmed to prevent it from bolting too quickly and shading lower tomato branches.

One of the most well-documented examples of companion planting comes from Indigenous agricultural traditions of North America. Corn provides a vertical structure for pole beans to climb. Beans fix atmospheric nitrogen into the soil, feeding the heavy-feeding corn. Squash spreads low and wide, shading the ground to retain moisture and suppress weeds. Each plant fills a different niche — vertical, nutritive, and horizontal — making the system more productive per square foot than monoculture rows. This is a foundational example when mixing plant types for a layered garden.

French marigolds (Tagetes patula) produce a root exudate that is toxic to root-knot nematodes — microscopic soil pests that damage tomatoes, peppers, and other crops. Research published in horticultural literature supports this effect, particularly when marigolds are grown as a dense border or interplanted for a full season rather than scattered sparsely. Marigolds also attract hoverflies, whose larvae prey on aphids. Plant them around the perimeter of vegetable beds or between rows for the most consistent impact.

Fennel is one of the most allelopathic plants commonly found in home gardens. Allelopathy refers to a plant's ability to release chemicals that inhibit the germination or growth of other plants nearby. Fennel has been shown to suppress tomatoes, peppers, beans, and many herbs when planted in close proximity. It's not that fennel is a bad garden plant — it attracts beneficial insects and pollinators — but it should be grown in its own container or in an isolated bed well away from your vegetable patch. Most companion planting guides agree: fennel is a loner by nature.

The logic here is straightforward: carrot fly is deterred by the scent of onions, while onion fly dislikes the smell of carrots. Interplanting the two creates an olfactory confusion that may reduce pest pressure on both crops. The evidence is observational rather than robust in controlled trials, but the pairing is low-risk and widely practiced. It also makes efficient use of space since onions grow upright while carrots occupy the soil below. Avoid planting onions near beans or peas, as alliums are known to stunt legume growth.

Strawberries and brassicas (cabbage, broccoli, kale, Brussels sprouts) are frequently cited as incompatible neighbors. Strawberries can harbor verticillium wilt, a soil-borne fungal disease that also affects many brassicas, effectively acting as a disease bridge in your garden. Additionally, brassicas may inhibit strawberry growth through root competition and light blocking when plants mature. If you're growing both, give them separate, clearly defined sections of the garden — ideally with a season of rotation between them to reduce soil pathogen buildup.

Nasturtiums are a useful companion not because they protect neighbors directly, but because they attract aphids away from more valuable plants. This strategy is called trap cropping: you deliberately plant something that pests prefer, drawing them away from crops you want to protect. Aphids cluster heavily on nasturtiums, where they can then be manually removed or targeted with a water spray. Plant nasturtiums on the upwind side of your garden or at the edge of susceptible crops. Just monitor them regularly — an unchecked aphid colony on your nasturtiums can eventually spread if left alone.