Marigolds interplanted among vegetables control root-knot nematodes.
Confidence in this refutation: Refuted
This is one the planner refuses to enforce. The claim is recorded here with what is actually known, rather than dropped — a rule that was checked and failed is worth more to you than one that was never checked.
What is actually known
Tagetes patula suppresses Meloidogyne spp. via alpha-terthienyl and related thiophenes. Suppression requires a dense sole-crop stand held for a full season, i.e. a biofumigant cover crop, not an interplant. Scattered marigolds among vegetables do approximately nothing.
Marigolds do suppress root-knot nematodes, but only as a solid stand grown for a full season as a cover crop. Dotting them between your tomatoes will not do it.
The evidence
Evidence: confirmed against 1 source in the research-based literature.
the source
UF/IFAS EDIS ENY-056/NG045, 'Marigolds (Tagetes spp.) for Nematode Management', edis.ifas.ufl.edu/publication/NG045 (read 2026-07-11): intercropping — 'Can marigolds have an effect on plant-parasitic nematodes when grown in an intercropping setting? Probably not.'; 'marigold should be planted at least two months before the desired vegetable crop' at 'the same site in which the vegetable crop will be planted'; 'Planting should be dense to ensure the best nematode control'; 'Marigold produces a substance called alpha-terthienyl, which can aid in the reduction of root-knot nematodes'; 'Marigolds cannot eradicate nematodes ... it must be grown every season before the actual crop is planted.'
Where this rule stops
THIS RULE HAS NO SURFACE. Its trigger reads plan.role_assignment, which nothing in the app populates, and nematode_suppression is not a role any guild defines — so it fires in conformance case C-015 and nowhere else. That is deliberate, not rot: firing on adjacency instead would scold a gardener whose plan is fine, which is the distinction the trigger_note above draws. B-011 carries the same correction, reachable, to anyone who asks about marigolds. See ISSUES #23.
Plants this fires on
Change history
Every change to this rule is one commit. This is what the project's version history records — the date and the reason — not a summary of it.
- corpus: another rule here says that it has no surface (ISSUES #23)
- promote to verified — interplanted marigolds do not control root-knot nematodes
- cite UF/IFAS EDIS for marigold nematode suppression (rule confirmed)
Cite this page
Milpa Gardens. "Marigolds interplanted among vegetables control root-knot nematodes." Evidence grade: Refuted. https://milpa.garden/rules/marigolds-interplanted-among-vegetables-control-root-knot/
A garden-planning corpus: every claim states its evidence grade and links its sources on the page. Cite it with the grade, and add the date you read it.