Brassicas: These include cabbage, broccoli, cauliflower, and kale. They share many of the same diseases as tomatoes, like early and late blight, and can easily spread them to your precious fruits. Additionally, they are heavy feeders, competing with tomatoes for valuable nutrients in the soil.
Potatoes: Like tomatoes, potatoes belong to the nightshade family and are susceptible to similar diseases, including blight and potato scab. Planting them close together increases the risk of both plants succumbing to these problems.
Eggplant: While they look beautiful together, eggplant and tomatoes are both nightshades with overlapping disease vulnerabilities. Additionally, eggplant attracts the Colorado potato beetle, a harmful pest for both plants.
Fennel: This feathery herb emits a chemical that can stunt the growth of tomato plants. It's best to keep fennel at least several feet away from your tomato patch.
Dill: Although some gardeners swear by dill as a companion plant for tomatoes due to its supposed pest-repelling properties, research suggests the opposite.
Corn: While corn and tomatoes may seem like a classic summer duo, they have different watering needs. Corn requires frequent, deep watering, which can lead to overwatering for tomatoes, potentially causing root rot.
Walnuts: These majestic trees release juglone, a chemical compound that can inhibit the growth of many plants, including tomatoes. If you have a walnut tree, plant your tomatoes far away from its root zone.