Companion Planting is not only the placement of mutually beneficial crops in close physical proximity but also naturally involves separating plants whose development is antagonistic to each other.
A plant that attracts a certain class of insect pest, fungal or microbial pathogen should not be placed near another plant that is adversely effected by the same pests and pathogens.
Corn Attracts many of the same pests that feed on Tomatoes. Such as the Corn Earworm, ironically also known as the Tomato Fruitworm. Tomato hornworm is another pest that feeds on both corn and tomato plants. Potato aphid, Beet Armyworm, cutworms, fall armyworm are a few others and there are more. Planting Corn and Tomatoes together is a bad idea.
Potatoes - Also attract many of the same pests and are susceptible to the same disease. Tomatoes and Potatoes are actually closely related - of the same genus. The same can be said of Peppers, eggplants and Okra. Potatoes planted with Tomatoes is a no no.
Fennell should be kept away from most vegetable garden plants , Tomatoes are no exception.
Kohlrabi supposedly stunts tomato growth, although this is not a scientifically established fact, it is prone to some of the same blights which is a good reason to keep the two separated.
Nuts - Many Nut trees secrete a phytotoxin which is toxic to other plants encroaching on the nut tree - Tomatoes can not survive this. Don't plant tomatoes near nut trees. Some fruit trees , such as peaches exhibit this behavior to a lesser degree.
Basil - contrary to popular belief is not truly as good a choice as a companion plant for tomatoes, they draw basically the same nutrients from the soil at the same times, there is no proof that basil enhances the tomato flavor at all.
Dill is touted as a trap crop for Tomato Hornworms. The hornworm prefers the dill over the Tomato and it is used to lure them in. Theoretically, you are supposed to handpick the hornworms off the dill and save the Tomato plants. This works fine so long as you keep up with it and don't miss any. You should also plant dill a safe distance from any tomato plants as it is believed to stunt their growth.
is another plant that is supposed to repel tomato hornworms. It may be simply folk lore it may not, but many gardeners swear by it.
Carrots can be inter-planted with Tomatoes if you time it correctly. Carrots mature more rapidly than tomatoes so your first harvest of Carrots should be when the Tomatoes are still seedlings. Mature carrots and mature tomatoes should not share the same soil as they compete for the same nutrients at the same time. A second planting of carrots is also possible in some zones, calculating their maturity to be after the bulk of the Tomatoes have been harvested. Parsley, a close relative of Carrots also works fine in the same fashion.
Spinach and Lettuce are good companions for Tomatoes They are relatively small, and will fare better in the summer heat when shaded by the mature tomato plants. They also act as living mulch for the Tomato, shading the soil and helping to retain moisture and reducing the odds of soil borne pathogens being splashed on the tomato plants.
Asparagus and tomatoes are good neighbors. Tomatoes are helpful to Asparagus in that they repel asparagus beetles. Asparagus grows early in the season and does not deplete the soil of the same nutrients as tomato plants. The tomato plants mature after the asparagus has been harvested. Sort of like time sharing a section of your garden
Chives, Onion, garlic emit a pungent aroma which repels many insects. Garlic also releases sulfur into the soil that is beneficial to Tomatoes and detrimental to soil borne pathogens.
Marigold, Nasturtium, Limas and Sage are Good Companion Plants for Tomatoes. They act as insect repellents of various pests.
Sage can be planted with cabbage, carrots, strawberries and tomatoes to enhance their growth.