The rumors have been around for some while and now scientific research has proved it: Certain strains of French marigolds dom get rid of root-knot nematodes , the microscopic worms that burrow inside roots and essentially starve a plant in the process.
Indeed, years of tests in Georgia, North Carolina, and Western Europe have proved that when planted solidly, selected varieties of marigolds can be so effective that no other treatment is necessary.
After a decade of research, first in Holland -- and in the US since 1974 -- plant scientists know this for a fact. Just how the marigolds (Tagetes petula) effect the destruction of the nematodes "remains somewhat speculative," to quote Dr. Ralph E. Motsinger, extension nematologist at the University of Georgia. But there are several plausible theories.
Nematodes are widespread in US soils but are infinitely more active in the warmer soils of the South. Chemical nemacides are effective in destroying the pests, but the harsh chemicals also destroy much beneficial soil life, too. In effect, the blast that routs the enemy succeeds in killing the "innocent civilians" as well.
A nonchemical method of attacking the nematodes is to add heavy applications of partly decomposed compost and manures to the soil. The decaying materials contain many life forms that feed on the nematodes. Also, some of the fungal growths in the compost entrap the nematodes and then digest them. All the evidence now suggests that marigolds be included in an anti-nematode program as well.
It was first thought that the marigold secreted a nemacide from its roots, but none has been identified. On the other hand, there is some evidence to suggest that the marigold allows the nematodes into its roots and then traps them there as does the fungal growth in compost. In any event, the nematodes do not appear to mature in the marigold roots and thus die off before they have a chance to reproduce.
In tests at Athens, Ga., scientists grew a wide variety of French marigolds in pots. Some 50 days later Dr. Motsinger could not find a sign of root-knot reproduction in the marigold pots. In contrast, tomatoes grown under the same conditions were heavily infested.
A North Carolina agronomist, Dave Rickard, did considerable follow-up work on Dr. Motsinger's findings. He suggests that the "Tagetes effect," as he likes to term it, works best when gardeners plant a mass of marigolds in a block and grow them there for a whole season (90 to 120 days). The idea is to enjoy the bright beauty of the marigolds for one year and then to plant vegetables in the practically nematode-free soil the next year. This way, the marigold can be used as a rotation crop every few years -- or whenever the buildup of nematodes warrants it.
All French marigolds have some effect on nematodes, but some species do better than others. In addition, one type of marigold may be effective with one species of nematode but not with another. So a mixture of various French marigold varieties gives the overall best results.
Several seed growers, also have been experimenting along these lines but none more so than the Geo. W. Park Seed Company whose trial grounds in Greenwood, S. C., are situated in Southern nematode country. The company has come up with a marigold seed mixture which it refers to as Nemagold.
Instructions that accompany the marigold Nemagold suggest that the seeds be planted on 7-inch centers. That way the root systems will completely fill the bed. One-sixteenth of an ounce of seed should suffice for every 100 square feet.
Keep the beds weed free until the marigolds crowd out all competition.