Marigolds are popular annuals widely grown in Louisiana. Most people, however, are not aware of the great potential that these plants have in fighting one of the major pests in the garden or landscape.
Root-knot nematodes are common pests of many vegetables, flowers, and shrubs. About 20% of the gardens that have been checked in our state have this nematode present. Every year these pests cause considerable damage and many gardens show the adverse effects of these pests by mid-summer. Plants that are damaged by root-knot nematode are slow growing, drought prone, flower and fruit poorly, and are often short-lived. Marigolds are one of the few plants that can fight back against certain nematodes.
Only a few plants have been found to produce substances known to be detrimental to nematodes. Asparagus, pangola grass, neem, castor bean and marigold produce substances that are toxic to at least one or more kinds of nematodes. Marigolds are known to be particularly effective against root-knot nematode. Most of the effect of marigolds, however, is not from these natural nematicides but the plants acting as a trap crop. The nematode enters roots of the plant but is unable to develop further in its life cycle or may be actively killed by the plant when it attempts to feed.
Most marigold varieties could probably be used against our common root-knot nematode. Varieties of the French marigold appear to be the most effective in suppressing nematode populations. Tangerine appears to be an exceptionally effective variety, since it doesn't appear to support any reproduction by the root-knot nematode. Unfortunately, one type of marigold (Signet) lacks this natural resistance and shouldn’t be used when root-knot is present.
Although planting marigolds as companion plants for susceptible crops sounds like a good idea, it just doesn't work. The nematodes that enter the marigolds are killed but not the ones that enter the susceptible crops. Eventually they will build up on the suitable host and cause problems. The best way to use marigolds is as a cover crop in the rows or area that you want to reduce the nematode problem. After about three to four months of being in continuous marigolds, you can successfully plant a susceptible crop. Don't expect the influence of the marigolds to be good for more than one crop or maybe one season. Nematodes are quite prolific and populations will quickly return.
If root-knot nematode has been giving you problems in the home garden or even in a ornamental bed, then try planting marigolds as a colorful cover crop. Although you can never completely eliminate them with marigolds, it is possible to manage them.
If you want to check for nematode problems, contact your local county agent who can provide you with information on how to sample for nematodes and other available management options.