Do marigolds repel nematodes and garden pests?
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Only when grown as a dense cover crop. Living marigold roots are significantly nematicidal through alpha-terthienyl, but University of Hawai'i Extension advises sowing about 20 plants per square meter and growing them for two to four months before termination. A few interplanted marigolds are far less potent, and some cultivars, such as 'Cracker Jack' and 'Polynema', host reniform nematode and can increase it.
What do people believe about marigolds?
Planting marigolds among vegetables suppresses root-knot nematodes.
What does the evidence show?
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 meter, 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 neighboring 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 works 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.
Key takeaways
- Only living marigold root systems are significantly nematicidal.
- The active compound, alpha-terthienyl, has limited activity once residues are incorporated.
- Effective use means about 20 plants per square meter grown for two to four months.
- Tagetes erecta 'Cracker Jack' and T. hybrid 'Polynema' are good hosts for reniform nematode.
- University of Florida IFAS treats marigold as an allelopathic cover crop, not ornamental interplanting.
References
Frequently asked questions
Which marigold species affect nematodes?
Marigolds are Tagetes species, and cultivar choice matters. University of Hawai'i Extension warns that Tagetes erecta 'Cracker Jack' and the T. hybrid 'Polynema' are good hosts for reniform nematode and can increase it. University of Florida IFAS publishes cultivar susceptibility tables, so match the cultivar to your nematode species.
How long do marigolds need to grow?
Two to four months, sown densely at around 20 plants per square meter, as a cover crop or rotation before termination. Only living marigold roots are significantly nematicidal, so the plants need that full growing period in the ground.
Do marigolds still work after they are tilled in?
Much less. Only living root systems are significantly nematicidal, and alpha-terthienyl has limited activity once residues are incorporated into the soil.
Does interplanting a few marigolds help?
Little. University of Hawai'i Extension says interplanting may not be as potent and works only by permeation into neighboring soil, so treat a few marigolds among vegetables as flowers rather than nematode control.
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