Lettuces make good companion plants for tomatoes because they fit nicely beneath the trailing plant. Also, ground crops like lettuce help keep the soil cool and moist, which tomatoes like.
Tomatoes are a member of the nightshade family, as are peppers, potatoes and eggplant. They’re all vulnerable to the same pests and diseases, so keeping them near each other in a small space is a recipe for disaster.
Planting carrots in the same soil as your tomato plants helps to keep it loose, which the tomato roots prefer.
They can stunt the growth of tomato plants, but only when kept in the same soil. It’s therefore a fairly easy problem to avoid if you’re growing crops in pots.
Alliums are the onion family, including garlic, leeks and onions. Gardening lore states they make good companion plants for tomatoes because their strong smell deters plenty of pests.