Deck Gardening

Marigolds and tomatoes can be good companion plants because they have similar growing conditions and marigolds can help protect tomato plants:
Repel pests
Marigolds’ essential oils can repel insects that are harmful to tomatoes, such as the moth that lays tomato hornworms. Marigolds can also attract beneficial insects like bees, parasitic wasps, ladybugs, lacewings, and hoverflies that eat aphids and other pests.
Protect from nematodes
Research suggests that planting marigolds between tomato plants can protect them from root-knot nematodes in the soil.