Borage is thought to repel the tomato hornworm moth. 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 sometimes dill. These also attract predatory flies that attack various tomato pests.
Plants with strong scents, like alliums (onions, chives, garlic), mints (basil, oregano, spearmint) and French marigold, (Tagetes patula) are thought to mask the scent of the tomato plant, making it harder for pests to locate it and provide an alternative landing point, less chance of the pest on the tomatos. These plants may also subtly affect the flavor of tomato fruit.
Ground cover plants, including mints, stabilize moisture loss around tomato plants and other Solanaceae, which come from very humid climates, these can help prevent moisture-related problems like blossom end rot.
Tap-root plants like dandelions break up dense soil and bring nutrients from below a tomato plant's reach, possibly benefiting their companion.
Tomato plants can protect asparagus from asparagus beetles, because they contain solanine that kills the beetle, while asparagus plants contain Asparagusic acid that repels nematodes known to attack tomato plants. Marigolds also repel nematodes.