Tagetes patula · daisy family
French Marigold (Tagetes patula) is an annual in the daisy family.
Every number above carries a confidence tier. These are the ones the corpus does not stand behind — estimated from general knowledge, or contested between sources. They are shown because hiding them would be the dishonest half of showing the rest.
Mechanism: Ice nucleation in leaf tissue.
Evidence: settled without a trial - climatology.
Sow or set out after the last frost date, or start indoors and transplant once it has passed.
Mechanism: Bottom rot (Rhizoctonia solani) and lettuce drop (Sclerotinia sclerotiorum and S. minor) survive in soil as mycelium and sclerotia for years and build where lettuce repeats; a three-year break holds bottom rot down. Drop's fungi infect alliums, brassicas, nightshades, cucurbits and legumes as well, so a lettuce rotation slows them without starving them, and only cereals are safe ground.
Evidence: confirmed against 4 sources in the research-based literature.
Cornell Vegetables, 'Do Rotations Matter within Disease Management Programs?', vegetables.cornell.edu/pest-management/disease-factsheets/do-rotations-matter-within-disease-management-programs (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): its rotation table gives lettuce bottom rot (Rhizoctonia) 3 years and drop (Sclerotinia) 3 years.
NC State Extension, 'Lettuce Drop', content.ces.ncsu.edu/lettuce-drop (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'Sclerotinia sclerotiorum and S. minor have a wide host range infecting allium (onion, garlic), brassica (cabbage, broccoli), solanaceae (pepper, potato), cucurbits (cucumber, squash), lettuce, and sweetpotato crops'; 'Cereals (maize, wheat, sorghum) are less susceptible to these pathogens'; 'the broad host range of both pathogens significantly limits the efficacy of crop rotation.'
Utah State University Extension, 'Drop (Sclerotinia Rot)', extension.usu.edu/planthealth/ipm/notes_ag/veg-drop-sclerotinia (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'The sclerotia can survive dormant in the soil for extended periods'; 'S. sclerotiorum affects a wide range of hosts, including root crops, legumes, and solanaceous crops however, drop is specific to lettuce'; rotate 'with nonsusceptible crops such as grains or corn to reduce disease build-up.'
UMass Vegetable Notes 2024 vol. 36:2, umass.edu/agriculture-food-environment/vegetable/newsletters/vegetable-notes/vegetable-notes-2024-vol-362 (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): bottom rot - 'Rhizoctonia survives in the soil, as mycelium or sclerotia'; lettuce drop - 'It produces sclerotia that can survive in the soil for many years'; 'Its host range includes many vegetables such as tomato and beans.'
Plant lettuce and its family in different ground, or wait until 3 years have passed since this ground last held them. Where drop has shown, the break that counts is one of grains or corn, since most vegetables carry it, and dead plants go in the bin, not the compost: the sclerotia survive it.
This is a claim the planner refuses to enforce — recorded here with what is actually known, rather than dropped.
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.
Evidence: confirmed against 1 source in the research-based literature.
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.'
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.
Mechanism: Cold hardiness. Hardy crops tolerate hard frost and half-hardy crops tolerate light frost, so they can occupy the ground weeks before frost-tender crops are allowed in. The offset is measured from the last spring frost.
Evidence: confirmed against 4 sources in the research-based literature.
University of Illinois Extension, 'Time Your Vegetable Plantings by Cold Hardiness' (2018-03-26), extension.illinois.edu/blogs/ilriverhort/2018-03-26-time-your-vegetable-plantings-cold-hardiness: very hardy vegetables 'withstand freezing temperatures and hard frosts without injury' (planted earliest in spring); frost-tolerant vegetables 'can withstand light frosts, but not freezing temperatures' (planted a couple of weeks later). LLM-summarised from a read of the page; it establishes the CATEGORIES but states no week offsets.
Colorado State Univ. Extension, GardenNotes #720, 'Vegetable Planting Guide', extension.colostate.edu/resource/vegetable-planting-guide (read 2026-07-25): hardy vegetables go in 'As early as 2-4 weeks before the date of the average last spring frost'; semi-hardy 'As early as 0-2 weeks before the date of the average last spring frost'; tender from seed 'around the date of the average last spring frost'; very tender 'Typically 2+ weeks after'. This is the source that states the offsets in weeks.
Where this rule sits in that range: hardy at 3 weeks is mid-range of CSU's 2-4. Half-hardy at 2 weeks is the EARLIEST EDGE of CSU's 0-2 - the most aggressive planting CSU sanctions, not the middle of it. The rule is inside published guidance at both ends, but it leans early on the half-hardy side and a gardener following it in a cold spring is taking that boundary.
Do not import an offset from another publication's hardiness scheme without checking the category names: CSU has no 'very hardy' tier, so a '4-6 weeks before frost' figure written against a four-tier scheme does not describe CSU's 'hardy'. Categories are not comparable across sources.
Mechanism: A frost-tender crop with a long days-to-maturity cannot mature if it is only direct-sown after the last frost — the frost-free window is too short. A head start under cover buys the weeks the season lacks; the seedling is set out only once frost has passed — the transplant date is still frost-gated.
Evidence: confirmed against 2 sources in the research-based literature.
University of Minnesota Extension, 'Starting seeds indoors', extension.umn.edu/planting-and-growing-guides/starting-seeds-indoors: seed-starting chart gives tomato '5-6 weeks' and pepper and eggplant '9 weeks' before the outdoor planting date (warm crops set out after the last frost). Read 2026-07-19 (LLM-summarised); read by the maintainer 2026-07-25, which is the read that promoted this rule. Note what the corpus's per-crop values do with the sources: tomato 6 sits at the top of Minnesota's '5-6 weeks' and inside Illinois's '6-8'; pepper and eggplant 8 sit BELOW Minnesota's '9 weeks' and at the top of Illinois's '6-8'. They are a reconciliation of two published ranges, not a figure either source prints.
University of Illinois Extension, 'When should I start my seeds?' (2023-02-10), extension.illinois.edu/blogs/good-growing/2023-02-10-when-should-i-start-my-seeds: peppers, tomatoes, and eggplant started '6-8 weeks before the last frost'. Read 2026-07-19.
Another 108 rules in the corpus apply by situation rather than by species — rotation intervals, frost windows, sun, spacing and soil. Read the full evidence page.
Milpa Gardens. "French Marigold (Tagetes patula)." https://milpa.garden/plants/tagetes-patula/
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.