Do Marigolds Really Work as Companion Plants?

Marigolds are perhaps the most frequently recommended companion plant in vegetable growing — planted alongside tomatoes, brassicas, and a host of other crops in the belief that they deter pests and improve growing conditions. The good news is that unlike some companion planting claims, the evidence for certain marigold effects is surprisingly solid. Understanding which effects are well-documented, which species to use, and how to deploy them gives you a genuinely useful tool in your pest management programme.

Which marigolds to use

There are three commonly grown types: French marigold (Tagetes patula), African marigold (Tagetes erecta), and pot marigold (Calendula officinalis). For companion planting purposes, French marigolds are the most useful — their volatile compounds have been shown to deter whitefly, and their root exudates suppress root-knot nematodes in the soil. African marigolds are also useful for nematode suppression and attract pollinators with their large flowers. Pot marigold (Calendula) is excellent for attracting beneficial insects including hoverflies and parasitic wasps.

Marigolds and whitefly on tomatoes

The most well-studied companion planting effect of French marigolds is the reduction of glasshouse whitefly populations on tomatoes. Research at Rothamsted Research showed that intercropping tomatoes with French marigolds reduced whitefly numbers significantly — the mechanism appears to be both chemical (volatile repellent compounds) and physical (the marigolds act as a visual "interference" confusing the host-finding behaviour of the pest). Plant marigolds in the same rows as tomatoes rather than just around the edge of the bed for maximum effect.

Nematode suppression

Growing French or African marigolds as a break crop in soil with a nematode problem — then digging the plants in as a green manure — reduces root-knot nematode populations measurably. This effect requires a full season of dense marigold planting rather than a few companion plants, but it is a genuinely useful soil management technique for beds that have suffered from nematode damage.