Dr. Ploeg advises planting taller, French marigold varieties where you plan to plant melons. Tall marigolds have large root systems that attract nematodes from deep in the soil. The variety 'Single Gold', which can grow 3 to 4 feet tall, worked well in his study. In Dr. Ploeg's field trials, the marigolds were planted in early spring. At flowering, the marigolds were mowed and left to dry, then tilled into the soil. Melons were then direct-seeded into the plots.
Gardeners with shorter seasons can attain similar results by planting marigolds where nematodes are a problem, then planting melons in the marigold plot the following year. (The recommended mowing and tilling methods are the same regardless of climate.) As with other pest management strategies, it is a bad idea to rely exclusively on marigolds for nematode control each year, says Dr. Ploeg. Alternating marigolds with other control methods (like solarization or rotation) is essential.