Plants That Tend to Thrive Together—and Combinations Worth Avoiding

This is one of the most popular companion pairings in home gardens, and it has both culinary and practical appeal. Basil is thought to repel thrips and aphids, two insects that routinely damage tomato plants. Some gardeners also report that basil planted nearby improves tomato flavor, though this specific claim is difficult to test reliably.

What's more certain: basil and tomatoes have similar needs — warm temperatures, full sun, and consistent moisture — which makes them easy to manage together. Plant basil 12 to 18 inches from tomato stems to give both plants adequate airflow.

This trio has been cultivated together by Indigenous farmers in North America for centuries, and it works through three complementary roles. Corn provides a vertical structure for bean vines to climb. Beans fix nitrogen from the air into the soil, feeding the other two plants. Squash spreads wide at ground level, shading out weeds and retaining soil moisture with its large leaves.

This is one of the most well-documented companion planting systems. It's especially worth trying if you have a larger bed or patch of ground, since each plant needs room to play its role effectively.

Carrots and onions are often paired because their respective pests find each other's scent off-putting. Carrot flies dislike the smell of onions; onion flies tend to avoid the smell of carrots. While the repellent effect isn't absolute, interplanting these two crops can reduce pest pressure compared to planting either in a solid block.

Both crops are also relatively compact, making this pairing well-suited to raised beds or small-space gardens. Plant them in alternating rows or mixed patches for the best effect.

Marigolds (Tagetes species) are one of the most versatile companion plants for vegetables. Their roots release a compound called alpha-terthienyl that is toxic to certain soil nematodes — microscopic worms that damage root systems. French marigolds (Tagetes patula) in particular have been shown in research settings to reduce nematode populations when planted in affected soil for a full season.

Marigolds also attract pollinators and some beneficial predatory insects. Plant them as a border around vegetable beds or scatter them between crops. Note that the nematode-suppression effect works best when marigolds are grown densely over a full growing season — a few plants tucked in at the last minute won't produce the same result.

Fennel is a useful herb in the kitchen, but it's one of the most problematic plants to include in a mixed vegetable garden. It releases allelopathic chemicals — natural compounds that inhibit the germination and growth of nearby plants. Tomatoes, peppers, beans, and many herbs tend to perform poorly when grown close to fennel.

The practical solution is simple: grow fennel in its own container or a dedicated bed well away from your main vegetable garden. It will thrive on its own and attract beneficial insects like swallowtail butterflies without disrupting your other crops.

Brassicas (cabbage, broccoli, kale, Brussels sprouts) and strawberries compete aggressively for similar nutrients, and the two plant families have a documented history of suppressing each other when grown in close proximity. Strawberries placed near brassicas often show reduced vigor and smaller fruit yields.

Additionally, some of the same soil-borne pathogens that affect brassica roots can persist and affect strawberry plants. Keeping these groups well separated — ideally in different beds — avoids both competition and shared disease risk.