Gardeners have long known of the reported nematode-fighting qualities of some garden plants, and marigolds are often cited as a good choice for reducing soil-borne nematodes that feed on plants. However research supporting these claims has been scarce. Now studies at the University of California are beginning to provide some answers. Trials of French marigold (Tagetes patula) varieties 'Single Gold' and 'Polynema' against the root knot nematode (Meloidogyne incognita) showed that these varieties are effective at reducing nematode damage to susceptible plants. Both varieties dramatically reduced nematode populations in the soil as well as the number of root galls on susceptible crops planted later in the season. The marigolds also increased total yield on those crops. Researchers stress that there is significant variation in the effectiveness of different species and varieties of marigolds, and not all nematode species are equally susceptible. In general, the small-flowered French marigolds (T. patula) are more effective at reducing nematode numbers than the large-flowered African marigolds (T. erecta). Simply planting a marigold next to a susceptible plant is not enough. In order for marigolds to have the optimum effect, a susceptible bed should be given a "season off" from growing crops, and marigolds should be planted thoughout the bed.