Beans and Onions
Onions and beans should never be grown together due to the allelopathic properties of onions. "Onions emit allicin, an antibacterial gas, which kills the beneficial urease bacteria," says Linda Langelo, horticulture specialist at Colorado State University. The urease is what beans need to fix nitrogen. Planting these two vegetables together can inhibit the growth of beans in their early stages, and produce smaller, weaker crops.
Tomatoes and Potatoes
Tomatoes and potatoes are susceptible to similar pests and diseases, says Carrie Spoonemore, co-creator of Park Seed's From Seed to Spoon app. "Growing these plants in close proximity increases the risk of these issues spreading," she says.
Corn and Tomatoes
Corn and tomatoes should not be planted together due to their different growth habits. "Both of these plants are heavy feeders and require nutrient-rich soil," says Spoonemore. "When planted together, this increases the competition for vital nutrients and may inhibit the growth and development of the plants." Additionally, tomatoes require full sun to grow and tall corn stalks can shade tomato plants.
Tomatoes and Brassicas
Never plant tomatoes and brassicas together as they are both heavy feeders and require nutrient-rich soil to thrive. "Planting these together can lead to greater competition for nutrients and leave tomatoes undernourished, hindering their growth," says Spoonemore.
Cucumber and Squash
Cucumbers and squash come from the same family and both need nutrient-rich soil in order to thrive. "Planting cucumbers and squash together increases the competition for vital nutrients and will quickly exhaust the nutrients in their soil," says Spoonemore.
Lettuce and Celery
Lettuce and celery should never be planted together because celery can attract pests, including aphids and whiteflies, which increases the risk of infestation of the lettuce and damages both crops.
Fennel and Tomatoes
Fennel is a poor companion plant for many garden vegetables, but it can especially inhibit the growth of tomatoes. "It’s best to keep fennel away from many of your vegetables," says Spoonemore. "It should be grown in a container so its roots don’t touch surrounding plants."
Peppers and Cabbage
Avoid planting peppers and cabbage next to one another, as peppers that are in the nightshade family will have their growth inhibited by the cabbage in the brassica family, says Langelo.
Potatoes and Zucchini
If planted together, potatoes will utilize available nutrients and starve out the zucchini. "Two fast growers and heavy feeders do not give good results come harvesting time," says Langelo. "Potato tubers will be small if they are not given regular applications of phosphorus and potassium. They will be malformed and will not last as long in storage. Zucchini will have stunted growth."
Carrots and Parsnips
Carrots and parsnips are both in the Apiaceae family, meaning they share pests and diseases. "Placing vegetables in the same family will always bring about a higher risk of a disease or pests," says Langelo. "In this case, carrot root fly can attack both carrots and parsnips." Additionally, growing several root crops in a garden increases the risk of attracting the same pests to one and spreading that pest to others.
Asparagus and Onions
Asparagus and onions do not make good companion plants because they are plagued by some of the same pests and diseases, such as cutworms, which destroy asparagus stalks and eat new onion transplants. Additionally, the allelopathic properties of onions can inhibit the growth of asparagus, says Langelo.
Pumpkins and Summer Squash
Pumpkins and summer squash will cross-pollinate if grown together. "This produces very odd-looking fruits that do not taste very palatable," says Langelo. Both of these crops also share insects and diseases, including powdery mildew, squash vine borers, hairy back legs, and squash bugs.
Fennel and Eggplant
Fennel releases a substance from its roots that can inhibit the growth of other vegetables, especially eggplant. For best results, plant fennel far away from other vegetables or in a container where its roots are contained.
Peas and Garlic
Garlic produces sulfur compounds that can stunt the growth of peas, says Langelo. Peas and garlic also compete for the same nutrients in the soil. In addition to peas, garlic also stunts the growth of asparagus, beans, sage, parsley, and strawberries.