Although the sunny annuals have been used in this capacity for many generations, little documentation exists showing marigolds actually repel insects. In fact, they may attract harmful pests damaging not only the marigolds but also your vegetables. While marigolds are rumored to deter certain beetles, they are more widely known for their effects on nematode populations in the soil.
Nematodes are microscopic worm-like organisms that destroy the root systems of plants.
Certain species of marigolds release compounds from their roots that are toxic to some species of nematodes. Unfortunately, the great variety of nematode species in Alabama soils significantly reduces the potential for adequate control. When the right combination of marigold and nematodes species does exist, visible results can take up to four months to appear. During this time, if the marigolds are planted around or intermingled with your vegetable or ornamental plants, they can act as weeds, competing for water and essential nutrients and causing additional stress for the plants. Also, the marigolds don't draw nematodes away from the other plants, so the plants still run the risk of nematode infestation and damage.