Marigolds for nematode control

The claim

Planting marigolds among vegetables suppresses root-knot nematodes.

What the evidence actually shows

This is the most useful entry in the registry, because the popular version and the evidenced version differ so sharply.

University of Hawai'i Extension states that only living marigold root systems are significantly nematicidal - alpha-terthienyl, the active compound, has limited activity once residues are incorporated. To work, marigold must be sown at high density, around 20 plants per square metre, and grown for two to four months as a cover crop or rotation before termination.

Interplanting a few plants, in their words, may not be as potent, and works only by permeation into neighbouring soil. So the technique is real; the domestic version of it mostly is not.

Cultivar choice is not a detail. The same publication warns that Tagetes erecta 'Cracker Jack' and the T. hybrid 'Polynema' are good hosts for reniform nematode and can increase it. Plant the wrong marigold against the wrong nematode and you have built a nursery. University of Florida IFAS frames marigold explicitly as an allelopathic cover crop with cultivar susceptibility tables - not as ornamental interplanting.

What to do instead

If you have the ground and the season, do it properly: dense sowing, the right cultivar for your nematode species, two to four months of growth. If you do not, use a targeted control and manage soil organic matter, and treat any marigolds you plant as flowers.