French marigold Companion Plants

Claim. A dense marigold cover crop in the season before carrots can reduce root-knot nematodes that fork and gall carrot roots; a few marigolds beside carrots will not.

Mechanism. Nematicidal root compounds from a full stand of Tagetes.

Studied in. Review of Tagetes cover-crop trials. (field)

Conditions. Season before carrots. Whole-bed dense stand. Not required; rotation practice.

Management. Full stand for 2–3 months; Incorporate before sowing carrots.

Applies when. Where nematodes are confirmed.

Limitations. Species-specific; not a cure for carrot fly or other problems.

Evidence. Tier B: One or more relevant controlled field studies or an authoritative synthesis tied to the crop, effect and arrangement. (extension synthesis).

Sources. Hooks 2010

Reviewed August 15, 2026 by Garden Planner editorial team (pending independent horticultural review). Record marigold--carrot--pest_suppression v1. Request a correction.

Claim. French marigolds interplanted with tomatoes reduced glasshouse whitefly numbers in a UK glasshouse study; the effect was attributed to airborne limonene from the marigolds.

Mechanism. Volatile limonene released by marigold foliage repels or deters whitefly on nearby tomatoes.

Studied in. Glasshouse compartments with tomato and French marigold (Tagetes patula) interplanted; limonene dispensers tested separately. (protected)

Conditions. Marigolds established before whitefly arrive; present throughout. Marigolds interplanted among tomatoes at close spacing. Marigolds must be growing (not just flowering) while tomatoes are at risk.

Management. Enough marigold foliage per tomato for volatiles to accumulate; Works best in enclosed spaces.

Applies when. Most applicable in a greenhouse, polytunnel or sheltered corner; outdoors, volatiles disperse and the effect is uncertain.

Limitations. Glasshouse evidence only; no outdoor home-garden trial; no effect shown on other tomato pests.

Evidence. Tier C: Indirect evidence: greenhouse or pot work, a related crop or species, or credible extension guidance with important uncertainty. (greenhouse).

Sources. Conboy 2019

Reviewed August 15, 2026 by Garden Planner editorial team (pending independent horticultural review). Record marigold--tomato--pest_suppression v1. Request a correction.

Claim. Marigolds (Tagetes) suppress some root-knot nematodes, but mainly when grown as a dense cover crop for a season before tomatoes, not as a few plants beside them.

Mechanism. Alpha-terthienyl and related compounds in Tagetes roots are toxic to certain nematode species; suppression requires the roots to occupy the soil volume.

Studied in. Review of marigold cover-crop and intercrop trials, mainly USA. (field)

Conditions. Marigolds grown for 2–3 months in the season before the tomato crop. Dense stand (about 15–20 cm spacing) across the whole bed. Does not require the two plants to grow together; this is a rotation practice.

Management. Choose tested cultivars (T. patula, T. erecta); Keep the stand weed-free; Incorporate residues before planting tomatoes.

Applies when. Only where root-knot nematodes are an identified problem (mainly warm climates and sandy soils).

Limitations. Interplanted marigolds have a much smaller effect; some nematode species are unaffected or even increase.

Evidence. Tier B: One or more relevant controlled field studies or an authoritative synthesis tied to the crop, effect and arrangement. (extension synthesis).

Sources. Hooks 2010

Reviewed August 15, 2026 by Garden Planner editorial team (pending independent horticultural review). Record marigold--tomato--pest_suppression-2 v1. Request a correction.

Claim. Marigolds are said to deter Mexican bean beetle. No controlled evidence found.

Mechanism. Proposed by tradition; not demonstrated in a controlled comparison.

Conditions. Planted together Both plants must be present at the same time.

Applies when. Optional experiment, not a guarantee.

Limitations. Tradition only.

Evidence. Tier D: Traditional knowledge or structured community observations without a controlled comparison. (tradition).

Sources. University of Minnesota Extension 2023

Reviewed August 15, 2026 by Garden Planner editorial team (pending independent horticultural review). Record marigold--bush-beans--pest_suppression v1. Request a correction.

Claim. Marigolds are widely claimed to repel cabbage pests. The marigold evidence concerns glasshouse whitefly on tomatoes (limonene) and soil nematodes (cover crop); neither transfers to cabbage caterpillars or root fly.

Mechanism. Proposed by tradition; not demonstrated in a controlled comparison.

Conditions. Planted together Both plants must be present at the same time.

Applies when. Optional experiment, not a guarantee.

Limitations. No brassica-specific evidence found.

Evidence. Tier D: Traditional knowledge or structured community observations without a controlled comparison. (tradition).

Sources. University of Minnesota Extension 2023; Conboy 2019

Reviewed August 15, 2026 by Garden Planner editorial team (pending independent horticultural review). Record marigold--cabbage--pest_suppression v1. Request a correction.

Claim. We did not find a source that establishes a benefit or harm between pepper and marigold.

Mechanism. None documented.

Conditions. Planted together Both plants must be present at the same time.

Applies when. The planner evaluates spacing, light, water and timing normally.

Limitations. The glasshouse whitefly study was on tomato; no pepper trial found.

Evidence. Unknown: No source meeting the inclusion rules. (none).

Sources. University of Minnesota Extension 2023

Reviewed August 15, 2026 by Garden Planner editorial team (pending independent horticultural review). Record pepper--marigold--other v1. Request a correction.