Companion planting strategically places different crops near each other to foster mutual benefits, such as pest control or improved growth. Not all pairings are helpful; some can actively sabotage your tomato harvest by creating an environment for disease, spurring intense competition for resources, or drawing in destructive pests. Avoiding these negative relationships is crucial for preventing crop failures and ensuring your tomato plants reach their full potential.
Plants That Harbor Tomato Diseases
The greatest threat to a healthy tomato crop comes from plants that share pathogens, allowing diseases to build up and easily spread. This is particularly true for other members of the nightshade family (Solanaceae), which includes many popular garden vegetables. Planting tomatoes near these relatives dramatically increases the risk of soil-borne illnesses and fungal infections.
Potatoes are the most notorious companion to avoid, as they are a primary host for Phytophthora infestans, the oomycete responsible for late blight. This aggressive organism spreads rapidly from infected potato foliage to nearby tomato plants, especially in cool, damp conditions. Potatoes can also harbor the pathogen in unharvested tubers over winter, providing a source of infection for the next season.
Eggplants and peppers, also in the Solanaceae family, are susceptible to the same devastating soil-borne diseases as tomatoes. These shared vulnerabilities include Verticillium wilt and Fusarium wilt, caused by fungi that can remain active in the soil for years. Concentrating these crops accelerates the build-up of these pathogens, making it nearly impossible for any of them to thrive.
While not nightshades, members of the cabbage family (Brassicas) can pose a different type of disease risk to tomatoes. Brassicas can carry fungi that affect tomatoes, and some species are susceptible to shared nematode issues that stunt root growth. Planting them in close proximity can promote an unhealthy environment where various soil pathogens flourish.
Aggressive Competitors for Water and Nutrients
Tomatoes are heavy feeders that require consistent moisture and high amounts of nutrients, especially nitrogen, to support fruit production. Pairing them with other plants that have similar, aggressive appetites leads to direct competition that can stunt tomato growth and reduce fruit set. This competition is most pronounced in plants with large, dense root systems.
Corn is a significant competitor because it is a tall, heavy feeder that demands large amounts of nitrogen and water. When planted too closely, the deep, fibrous roots of corn effectively steal nutrients and moisture from the shallower tomato roots. Corn’s height also casts dense shade over the tomato plants, limiting the sunlight needed for robust fruit development.
Large, sprawling varieties of squash, such as pumpkins or winter squash, can also starve nearby tomatoes of resources. These plants develop extensive root systems and massive leaves that cover the soil, competing for both water and light. The physical presence of the squash vines can also make it difficult to access the tomato plants for necessary pruning and harvesting.
Fast-growing, shallow-rooted weeds are a major source of competition. While not cultivated plants, they are the most immediate threat to resource depletion in the upper soil layers. Allowing weeds to flourish near tomatoes will quickly deplete the available nitrogen and moisture before the tomato roots can fully establish.
Plants That Attract Shared Pests or Stunt Growth
Some plants create a negative environment for tomatoes by luring in shared pests or by chemically inhibiting the tomato’s ability to grow. These subtle forms of interference can be as damaging as disease or nutrient competition. Proper placement is essential to avoid these undesirable interactions.
Fennel is the most widely cited example of a plant that chemically inhibits the growth of almost all its neighbors, including tomatoes, through allelopathy. It releases allelochemicals from its roots that suppress the growth of other plants, leading to stunted tomato plants and smaller fruit production. Fennel is also known to attract pests like aphids, which can easily migrate to the vulnerable tomato foliage.
Dill presents a complex situation because it is often recommended as a beneficial companion for attracting helpful insects like ladybugs. However, mature dill plants can also host species like spider mites and tomato hornworms, which are major pests for tomatoes. Placing dill too close to the tomato plants can act as a bridge for these destructive insects once the dill plant begins to decline or bolt.
Certain flowers, when used as trap crops, can become problematic if positioned too near the tomatoes they are meant to protect. A trap crop works by attracting a pest away from the main crop, but if the pest population is high, the trap crop can become a breeding ground. An overabundance of pests on a nearby trap crop can result in a spillover effect, moving pests onto the tomato plants once their preferred host is overwhelmed.