For Release On Or After 05/15/09

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, there is no real benefit. 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 vegetable plants, 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 also is well-known for its susceptibility to the root-knot nematode.

The marigold is one of the few plants that produce substances detrimental to nematodes, Dr. Charles Overstreet, an LSU AgCenter nematologist, said when I asked him about this topic. He added that 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 but because the plants act as a trap crop. The nematode enters the plants’ 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 said. 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 seed catalogs offering 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 there may be some slight advantages of putting marigolds and vegetable plants together, 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 you want to reduce the nematode problem. Early-summer crops such as tomatoes, snap beans, cucumbers, squash, onions, garlic and strawberries are often pulled up in June or July when they finish production. Marigolds can then be planted in their place.

Overstreet cautions 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. 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 and request a nematode assay kit. This will allow you to submit soil samples to the LSU AgCenter nematology lab, which will determine the types and population levels of the nematodes in the sample.

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 into the soil generous amounts of organic matter (this encourages natural predators that attack nematodes), planting nematode-resistant varieties when they’re 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 said. “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