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Companion plants

Tomatoes serve, or are served by, a large variety of companion plants.

The devastating tomato hornworm has a major predator in various parasitic wasps, whose larvae devour the hornworm, but whose adult form drinks nectar from tiny-flowered plants like umbellifers. Several species of umbellifer are therefore often grown with tomato plants, including parsley, Queen Anne's lace, and occasionally dill. These also attract predatory flies that attack various tomato pests.

Borage is thought to repel the tomato hornworm moth.

Basil is popularly recommended as a companion plant to the tomato. Common claims are that basil may deter pests or improve tomato flavor. However, in double-blind taste tests, basil did not significantly affect the taste of tomatoes when planted adjacent to them.

Tomato plants can protect asparagus from asparagus beetles, because they contain solanine that kills this pest, while asparagus plants contain asparagusic acid that repels nematodes known to attack tomato plants. Marigolds also repel nematodes.