Companion evidence

Do marigolds actually keep pests away?

Marigolds are the most repeated piece of companion planting advice there is — and the version that gets repeated is usually the one the research doesn't support. Here is what the actual sources say, what they don't, and how to tell the difference for every pairing you read about.

The short answer

There is real research behind marigolds — but it's about growing them in the bed before you plant, not about the flowers you put beside a crop. Selected Tagetes cultivars, grown in the future vegetable bed for at least two months before vegetables, can suppress certain root-knot nematodes. That effect depends on the marigold cultivar, the nematode species and soil temperature. A few plants bordering the bed are not a proven broad-spectrum pest repellent.

Why the garden-centre version keeps repeating

The cover-crop result is specific, conditional and boring to repeat, so the version that survives is the simple one: "marigolds repel bugs." It's easy to say, it sells plants, and it's hard to disprove in one season — a marigold border doesn't stop much, but it also doesn't stop you noticing. The fix isn't to distrust every old wives' tale; it's to ask which claim each sentence is making. Is it a pest-control claim, a growth claim, a flavor claim, or a timing claim? Those are different questions and they need different evidence.

Three other pairings, labeled honestly

Every pairing in Smart Garden Planner carries the same label: what is claimed, how strong the evidence is, and where it came from. These three show how differently the categories read.

The four labels we use

Nothing in the explorer sits in a grey area. Each pairing is one of these:

• Research-supported — a study or extension research summary backs the specific claim, in the conditions it was tested.
• Extension guidance — a university extension service states it as practice, without a controlled trial behind it.
• Traditional practice — a long-standing growing system, recorded as tradition rather than as measured benefit.
• Insufficient evidence — we looked and couldn't find a source that supports the claim.

The last one matters most. If you search for a pairing and Smart Garden Planner has nothing to say about it, that's not a gap in the database — it's the honest result of not finding a source. We'd rather show you an empty list than fill it with folklore dressed up as science.

So should you plant marigolds?

If you want the nematode result, plant them as a cover crop in the bed you'll later use for tomatoes, for at least two months, and expect the outcome to depend on which cultivar and which nematodes you have. If you want flowers that attract hoverflies to aphid-prone lettuce, sweet alyssum has the better record — and even that is context-specific. If you want a pretty border, plant marigolds for that reason and don't plan your pest control around them.

The useful habit is the same for every pairing you read: find the claim, find the source, check what it was actually tested on.

Reference version 2026-11-08. Information is educational; local conditions vary — check your local extension office.