Marigolds repel most garden pests and protect the whole garden.
Marigolds don't protect the whole garden. Their best-known trick is underground. A dense planting of the right kind, grown at least two months before your crop, suppresses root-knot nematodes. Dotted among the vegetables, it doesn't work. Against insects the results are mixed. Some pests, like thrips and whiteflies, drop in trials; others, like Colorado potato beetles, don't budge. Grow them for color and treat any pest help as a bonus.
What is actually known
Marigolds are not a garden-wide shield. Their best-established effect is on root-knot and some other nematodes in the soil. A dense stand of the right Tagetes, planted at least two months before the crop, suppresses them. Whether that is through the root compound alpha-terthienyl or because marigold is not a host is not settled. Dotting marigolds among the vegetables does not appear to work, and a susceptible variety can make nematodes worse. Against insects the evidence is mixed. Studies have found fewer thrips, cabbage worms (in mixed plantings) and squash bugs, and a glasshouse study found marigold scent slowed whiteflies, but marigolds do not deter Colorado potato beetles and there is little research behind the flea-beetle claim.
The evidence
Evidence: checked against 3 sources in the research-based literature.
The 3 sources
The presence of alpha-terthienyl inhibits the hatching of nematode eggs
Many gardening articles mention marigolds as a deterrent for Colorado potato beetles, but multiple studies have shown this to be untrue.
Here we show that association with marigolds significantly slows whitefly population development.
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Milpa Gardens. “Marigolds repel most garden pests and protect the whole garden.” Evidence grade: Contested. https://milpa.garden/beliefs/marigolds-marigold-pest-repellent/
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