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. The greatest disadvantage to using marigolds is they attract large populations of spider mites to many gardens and landscapes.
If you enjoy growing marigolds, French dwarf varieties have shown the most consistent control (of nematodes, not insects!). Plant these marigolds in an infested garden and maintain a solid stand for three to four months to reduce nematodes. After the appropriate amount of time, plow the plants under as green manure. The nematode population should be decreased and the garden ready for planting. Be sure to keep the garden weed-free until planting time.