Several years ago we planted a marigold that was designated as a suppressant of nematodes, and we have not had to replant since! The five foot stalks bear tiny yellow marigold flowers which produce copious seeds, which then scatter throughout the garden. They sprout later in summer and we leave them throughout the fall. This time of year these marigolds are hard at work!
Studies show that the marigolds need to be in high concentrations and companion planted for at least two to three months to be effective and that working them into the ground as green manure releases other nematicidal compounds. If you do not want them reseeding, it is best to dig them in before they set seed.