Companion planting is a practice that uses the natural properties of certain plants to create a mutually protective environment for tomatoes. This strategy provides a natural, chemical-free method for managing common garden pests that target tomato plants. By strategically placing specific herbs, flowers, and vegetables near your tomatoes, you can leverage plant chemistry and insect ecology to foster a healthier garden ecosystem. The goal is to either drive away destructive insects or invite helpful predators that feed on those pests.
Understanding Companion Planting Mechanisms
The protective effect of companion plants operates through several distinct biological mechanisms. One primary function is scent masking, where strongly aromatic plants emit volatile organic compounds (VOCs) that confuse pests with highly specific odor recognition systems. This strong non-host scent essentially hides the tomato plant’s natural aroma, making it difficult for the pest to locate its target.
Another mechanism relies on chemical repellency, where the companion plant releases specific VOCs or root exudates that pests find distasteful or toxic. These compounds can irritate insects or interfere with their feeding and egg-laying behaviors, causing them to avoid the area. Finally, some plants function as trap crops, which are purposefully planted to be more attractive to a pest than the tomato itself, luring the pests away to a sacrificial plant.
Plants That Directly Repel Tomato Pests
Certain companion plants are highly effective because they actively drive specific pests away through chemical or scent-based deterrence.
French Marigolds
French Marigolds (Tagetes patula) are particularly valuable because their roots release thiophenes, compounds that suppress root-knot nematodes, which are microscopic worms that damage tomato roots. The strong, pungent scent from the foliage and flowers also helps deter whiteflies and may confuse the moths that lay the destructive tomato hornworm eggs.
Basil and Alliums
The herb Basil (Ocimum basilicum) is one of the most effective aromatic companions for tomatoes due to its potent, confusing scent. This strong odor helps mask the tomato’s smell, deterring pests like whiteflies, thrips, and the challenging tomato hornworm. Similarly, members of the Allium family, such as garlic and onions, emit sulfurous compounds with a powerful odor. Planting garlic near tomatoes can help create a pest barrier that deters aphids and spider mites.
Borage and Mint
The flowering herb Borage (Borago officinalis) provides a dual defense, working as both a repellent and a beneficial insect attractant. Its presence is noted for repelling the destructive tomato hornworm and the larvae of cabbage worms. The herb Mint, when grown in nearby containers, is another strongly scented option that helps confuse and deter many flying insect pests.
Plants That Attract Beneficial Predators
Instead of repelling pests directly, other companion plants work indirectly by attracting predatory insects that feed on common tomato pests. These plants typically offer easily accessible nectar and pollen, which serve as food sources for adult beneficial insects.
Herbs for Predators
Dill (Anethum graveolens) is highly attractive to parasitic wasps, which lay their eggs inside the larvae of the tomato hornworm, and to ladybugs, which are voracious consumers of aphids. Allowing herbs like Cilantro (Coriandrum sativum) to flower, or “bolt,” makes them excellent food sources for ladybugs and syrphid flies (hoverflies). Syrphid fly larvae are particularly effective predators of aphid colonies.
Flowering Attractants
Sweet Alyssum (Lobularia maritima) is a low-growing plant that produces tiny, nectar-rich flowers that are favored by minute parasitic wasps and lacewings. The larvae of lacewings and the parasitic wasps are effective biological controls for aphids and small caterpillars like fruitworms. Other composite flowers, such as Yarrow and Cosmos, also provide a broad source of pollen and nectar that supports diverse populations of beneficial insects. Maintaining a consistent source of these flowers throughout the season ensures that predators like ladybirds and lacewings remain in the garden to control pest outbreaks.
Practical Placement and Plants to Avoid
Successful companion planting depends on the proper placement of the protective plants. For scent masking and direct repellency, companion plants should be interplanted closely with the tomatoes, rather than grouped in a separate area. Low-growing varieties, such as French Marigolds or Sweet Alyssum, should be planted along the edges or between tomato plants to avoid competing for sunlight.
Plants to Avoid
Some plants can actively harm tomato growth or increase pest pressure.
- Fennel should be strictly avoided as it releases allelopathic chemicals that inhibit the growth of many neighboring plants, including tomatoes.
- Plants in the Brassica family, such as cabbage and broccoli, are heavy feeders that compete intensely with tomatoes for essential soil nutrients.
- Avoid planting Potatoes near tomatoes because both belong to the nightshade family and share many of the same devastating pests and diseases.
- Growing potatoes and tomatoes together increases the risk of spreading early or late blight, as well as attracting shared pests like the tomato hornworm.
Strategic spacing and avoiding these antagonists are just as important as choosing the right allies for a healthy tomato harvest.