Flats of marigolds can be found in nurseries and lawn and garden centers this time of year. (Photo by Rick Bogren)

Marigolds thrive in a planting bed well into fall. (Photo by Allen Owings)

The results of root-knot nematode can be seen on the roots of this eggplant. (Photo by Charles Overstreet)

For Release On Or After 05/16/14

By Dan Gill
LSU AgCenter Horticulturist

You may sometimes read or hear information about planting certain plants around other types of plants to prevent insect problems. This is commonly called companion planting. Generally, research does not substantiate the claims of companion planting.

I have frequently seen marigolds recommended as a companion plant for vegetables to prevent insect and nematode problems. Gardeners generally use marigolds by planting them among the vegetables in the garden. As far as repelling insects, no real benefit is apparent. Indeed, marigolds have their own pest problems. Spider mites are particularly attracted to marigolds, and spider mites are also a leading pest of tomatoes and other vegetables. Populations of spider mites can build up on marigolds and then move onto vegetable plants.

Nematode control is another reason marigolds are recommended for planting around vegetables. And here’s the kernel of truth: Marigolds can help control some of the most damaging nematodes that attack vegetables in our gardens.

Parasitic nematodes are microscopic worms that attack the roots of many vegetables, reducing both yield and quality. Tomatoes are a favorite host, and most tomato varieties we grow these days have been bred to be resistant to nematodes. Okra is well known for its susceptibility to the root-knot nematode.

The marigold is one of the few plants that produces substances detrimental to nematodes, says LSU AgCenter nematologist Charles Overstreet. I asked him about this topic, and he says asparagus, pangola grass, neem and castor beans also produce substances toxic to at least one or more kinds of nematodes.

Most of the effect of marigolds is not from these natural nematicides, however, but because the plants act as a “trap crop.” The nematode enters the marigold’s roots and is killed because it is not able to set up a successful feeding site.

Although marigolds control several types of nematodes, the greatest effect seems to be on root-knot and lesion nematodes, Overstreet says. The southern root-knot nematode is Louisiana’s biggest nematode pest in the home garden.

Most marigolds, with the exception of the Signet types, appear to be effective against the southern root-knot nematode. The French marigolds (Tagetes patula) are more effective than the African types (Tagetes erecta). You will occasionally find catalogs offering seed for marigolds selected especially for nematode control.

Unfortunately, planting marigolds as companion plants next to crops that are susceptible to root-knot nematode doesn’t seem to work. Although some slight advantages of putting marigolds and vegetable plants together may exist, the root-knot nematode is still able to find and infest the roots of susceptible vegetable plants.

The most effective way to use marigolds is as a cover crop in the rows or areas where the gardener wants to reduce a nematode problem. Early-summer crops such as tomatoes, snap beans, cucumbers, squash, onions and garlic are often pulled up in June or July when they finish production. Marigolds can then be planted in their place.

Overstreet cautions gardeners not to expect the marigold’s influence to be effective for more than one crop or one season’s reduction of the nematode before this pest builds its populations back to damaging levels.

All soils have nematodes. The question is, are they parasitic nematodes and what are the populations? Parasitic nematodes may be present but not cause enough damage to affect the plants’ harvest. Populations have to be high enough to really cause adverse effects.

Nematodes are microscopically small, so if you suspect a nematode problem in your garden, you cannot evaluate population levels with the naked eye. Gardeners who want to verify nematodes are present should contact their parish LSU AgCenter Extension office about getting their soil analyzed for nematodes. This involves submitting soil samples to the LSU AgCenter nematology lab where the types and population levels of the nematodes in the sample are determined.

If the assay determines that nematode levels are high enough for concern, a cover crop of marigolds is one option for control. Other techniques that will help reduce problems with nematodes include working generous amounts of organic matter into the soil (this encourages natural predators that attack nematodes), planting nematode-resistant varieties when available, solarizing (covering the soil of an empty bed with clear plastic to heat the soil to a temperature that kills the nematodes) and using nematode-control products that contain chitin (the primary component of crawfish and shrimp shells).

“The marigold is a common flower grown in Louisiana and could be readily used in the garden both for control of nematodes and to add beauty to the landscape,” Overstreet says. “So if root-knot nematodes have been giving your desired plants problems in the home garden or ornamental bed, try planting marigolds as a colorful cover crop.”

Rick Bogren