The practice of companion planting involves strategically placing different plant species near each other to create a mutually beneficial environment in the garden. This method works by harnessing natural plant interactions to improve growth, manage pests, and maximize the efficiency of garden space. For the home gardener focused on producing a successful tomato harvest, understanding these beneficial relationships is key to maximizing yield without relying on chemical interventions. The following pairings explore how specific plants can act as nature’s defenders, soil enhancers, and flavor boosters for your tomatoes.
Plants That Protect Tomatoes
Defending tomato plants from garden threats relies on utilizing the chemical defenses and attractive qualities of companion plants. Certain flowers and herbs are effective because they either release compounds that repel pests or attract beneficial insects that prey on tomato enemies. This defensive strategy provides natural protection for the vulnerable tomato plant.
The French marigold, a popular annual flower, offers a powerful defense both above and below the soil line. Its roots exude a group of allelopathic compounds, known as thiophenes, which are toxic to root-knot nematodes. These microscopic, parasitic worms attack tomato roots, but planting marigolds densely nearby can significantly reduce their population in the soil. Above ground, the marigold’s strong, pungent scent helps to deter whiteflies and other small flying insects that might otherwise target the tomato foliage.
Basil provides a similar, yet distinct, chemical defense against pests like the tomato hornworm and various flies. The herb’s strong aroma, derived from volatile compounds such as eugenol and citronellol, creates a natural barrier that confuses and deters these insects from landing on the tomato plant. Beyond simple repulsion, basil has also been shown to “prime” the tomato’s internal defenses, enhancing the plant’s wound response system through airborne signaling.
The flowering herb borage, easily identified by its bright blue, star-shaped flowers, attracts natural predators. Borage attracts beneficial insects, including honeybees and tiny parasitic wasps, to the area. The parasitic wasps are important because they lay their eggs inside destructive pests like the tomato hornworm, controlling the population. Borage also provides a useful ground cover; as the leaves age, they can be dropped to create a natural mulch that helps cool the soil.
Plants That Enhance Growth and Flavor
Other plant companions are chosen for their ability to improve the physical and chemical environment of the tomato’s root zone. These plants enhance soil structure, manage moisture, and contribute nutrients that support the tomato’s vigorous growth and heavy fruiting cycle. The placement of these companions is important to ensure their root systems complement, rather than compete with, the tomato’s own extensive network.
Carrots and onions are effective companions because their root structures operate at different depths than the tomato’s, preventing competition for resources. Carrots grow deep taproots that naturally loosen and aerate the soil, which improves water penetration and allows the tomato’s deep feeder roots easier access to oxygen and moisture. Onions, like garlic, emit sulfurous compounds that are thought to act as a general pest deterrent and are also known to discourage nematodes in the soil.
Certain herbs like parsley and chives offer benefits by attracting beneficial insects and enhancing soil health. Parsley’s delicate flowers, for instance, attract hoverflies, whose larvae are voracious predators of aphids and other soft-bodied tomato pests. Chives, a member of the allium family, are often credited by gardeners with improving the overall flavor profile of the tomato fruit when planted nearby.
Legumes, such as beans and peas, are unique in their ability to enrich the soil through nitrogen fixation. These plants form a symbiotic relationship with rhizobia bacteria, which colonize nodules on the plant roots and convert atmospheric nitrogen into a usable form for plant growth. As heavy feeders, tomatoes particularly benefit from this natural process, especially if the legume roots are left in the soil after the plant is finished producing.
Plants to Keep Away From Tomatoes
While many plants offer mutual benefits, some pairings can actively harm a tomato crop through competition, shared vulnerabilities, or chemical antagonism. Gardeners must avoid these incompatible neighbors to prevent stunted growth, pest outbreaks, and the spread of destructive diseases.
The Brassica family, which includes cabbage, broccoli, and cauliflower, should be planted far away from tomatoes due to their aggressive feeding habits. These plants are heavy feeders, demanding large amounts of nitrogen and other micronutrients from the soil, directly competing with the tomato plant. This competition often results in stunted growth and reduced fruit yield for the tomato.
Both corn and members of the Solanaceae (nightshade) family present unique risks to tomatoes. Corn’s tall, dense growth competes heavily for sunlight, shading out the sun-loving tomato plants, and it also attracts the corn earworm, a pest that readily crosses over to attack tomato fruit. Planting nightshades like potatoes and eggplant nearby is problematic because they are susceptible to the same soil-borne diseases, such as blight, allowing pathogens to multiply and spread easily between crops.
Fennel is one of the most consistently poor companions for tomatoes because of its allelopathic nature. This herb releases specific biochemicals from its roots that are known to inhibit the growth of many other plants, including tomatoes. These compounds can suppress seed germination and stunt the development of nearby plants, making it advisable to grow fennel in a separate container or in an isolated area of the garden.