Companion planting involves placing specific plant species near one another to provide mutual benefits. This natural method creates a diverse ecosystem where plants assist their neighbors by enhancing growth, improving soil health, or protecting against pests. Pairing flowering plants with tomatoes is an effective approach, offering a chemical-free way to manage common garden challenges and improve the overall health and yield of the tomato crop. Floral companions work through chemical repulsion, attracting predatory insects, or providing physical support to the plants and soil.
Flowers That Repel Common Tomato Pests
Certain flowers actively deter harmful insects through the release of potent aromatic compounds or root exudates. The French Marigold (Tagetes patula) is the most well-known example for its ability to combat microscopic soil pests. Its roots release alpha-terthienyl, a compound toxic to plant-parasitic root-knot nematodes (Meloidogyne spp.). These nematodes damage tomato roots, hindering the plant’s ability to absorb water and nutrients.
For the marigold’s nematicidal effect to be fully realized, it requires dense planting and often a season of growth. The strong, pungent scent from marigold foliage and flowers also repels above-ground pests. This aroma helps mask the scent of the tomato plant, deterring pests like whiteflies and the five-spotted hawk moth, which lays the eggs of the tomato hornworm.
Calendula, often called pot marigold, also has a strong repellent fragrance. Its musky scent discourages many pests and specifically helps ward off the tomato hornworm. By confusing the adult moths attempting to locate the tomato plants for egg-laying, these aromatic flowers create a natural perimeter of defense.
Flowers That Attract Beneficial Insects
Other flowers provide food and habitat for beneficial predatory insects and pollinators rather than repelling pests. These flowers act as a continuous source of nectar and pollen, sustaining the adult stages of insects that prey on tomato pests. This strategy is an indirect, yet highly effective, form of biological pest control.
Borage (Borago officinalis) is effective in attracting parasitic wasps, which lay their eggs inside pests like the tomato hornworm. The nectar-rich, star-shaped blue flowers provide the necessary energy for these wasps to patrol the tomato plants. Borage is also an excellent source of pollen for native bees, ensuring proper pollination and fruit set.
Zinnias and Cosmos, with their open flower heads, serve as landing platforms for generalist predators. Zinnias draw in hoverflies, parasitic wasps, and ladybugs, which consume soft-bodied pests like aphids. Cosmos attracts green lacewings and hoverflies, whose larvae feed on thrips, aphids, and scale insects. Choosing orange varieties of Cosmos is often recommended, as this color strongly attracts green lacewings.
Flowers That Provide Physical and Soil Support
Some floral companions offer structural or environmental benefits rather than purely chemical or insect-based ones. Using flowers as a “trap crop” diverts pests away from the primary tomato plants. Nasturtiums are a classic example, as their stems contain a sweet sap that aphids find appealing.
Aphids congregate on Nasturtiums, leaving the tomato plants relatively untouched. Once the trap crop is heavily infested, the gardener can remove and dispose of the plants to eliminate the pests. Other low-growing flowers, such as Sweet Alyssum (Lobularia maritima), act as a living mulch.
This dense, low canopy of Sweet Alyssum foliage helps shade the soil, retaining moisture and keeping the root zone cooler during summer. Crimson Clover is another flowering plant that offers substantial soil benefits, as it fixes atmospheric nitrogen into the soil. As a dense cover crop, it also suppresses weed growth, reducing competition for water and nutrients.
Practical Planting Considerations and Flowers to Avoid
Successful companion planting depends on correct placement and maintenance to ensure companions do not become competitors. Taller flowers like Zinnias and Cosmos must be planted far enough away to prevent excessive shading, which reduces sunlight needed for optimal fruit production. A distance of at least 18 inches between root zones is often recommended.
Maintaining consistent soil moisture is important, so low-growing companions like Sweet Alyssum should be planted densely for effective ground cover. Deadheading spent flowers encourages continuous blooming, ensuring a steady supply of nectar and pollen to sustain beneficial insects.
Conversely, some plants should be entirely avoided near tomatoes because they can actively harm the crop. Fennel is problematic due to its allelopathic properties, as its roots release compounds that inhibit the growth of many nearby plants. Mature Dill can also stunt tomato growth and compete for nutrients. Heavy-feeding plants like those in the Brassica family aggressively compete for soil resources, leading to stunted tomato plants and reduced yields.