Fennel is an allelopathic plant, meaning it releases chemicals into the soil that inhibit the growth of other plants. This makes it a poor companion for nearly all garden crops, including tomatoes. The chemicals fennel releases can stunt tomato growth
Cabbage, along with other members of the Brassica family like broccoli, cauliflower, and Brussels sprouts, competes with tomatoes for essential nutrients in the soil. These plants are heavy feeders, meaning they consume a lot of nutrients
Pole beans can grow aggressively, sometimes overshadowing nearby plants, including tomatoes. While some gardening guides suggest that beans and tomatoes can be good companions, pole beans in particular can compete with tomatoes
Dill is usually a good companion plant for many vegetables, but not for tomatoes. As dill matures, it can inhibit tomato growth by competing for nutrients and releasing substances that negatively affect tomato root development.
Corn and tomatoes should not be planted together because they compete for similar resources, and corn can grow tall enough to shade tomato plants. This shading reduces the amount of sunlight tomatoes receive, which is crucial for their growth
Okra is another tall-growing plant that can overshadow tomatoes. It produces large leaves that can block sunlight from reaching tomato plants, preventing them from getting the 6 to 8 hours of sunlight they need daily.
Potatoes and tomatoes belong to the same plant family, Solanaceae (nightshade), and are susceptible to the same pests and diseases, such as blight and the Colorado potato beetle. Planting them close together increases the risk of disease
Broccoli, like other brassicas, is a heavy feeder that requires nutrient-rich soil. Planting it near tomatoes can lead to competition for nutrients, leaving tomatoes with less than they need to thrive. To ensure both crops get the nutrients they need.