Marigolds don’t control eelworm

Reading about using marigolds to suppress eelworms piqued my curiosity, so when the opportunity arose, I tried it for myself.

A volunteer tomato among marigolds.
Photo: Bill Kerr

Reading about using marigolds to suppress eelworms piqued my curiosity, so when the opportunity arose, I tried it for myself. Researchers have incorporated chopped marigold leaves into soil in which marigolds have been grown, and found a definite eelworm suppression. Various theories exist on how marigolds control eelworm. Some claim they repel eelworm, others say that toxic exudates are responsible.

But marigolds do seem to act as a trap crop by preventing the hatched eelworm breeding, thereby reducing the population. Many gardening books advocate companion cropping susceptible crops with marigolds, claiming that marigolds repel eelworms to the crop’s benefit. I was able to disprove this theory.

Practical tests

I plant tomatoes in the same tunnels year in and year out, and treat the soil with compost and chicken litter. There would always be some eelworm damage to susceptible varieties. A few tunnels were vacant in January after tomatoes had been harvested, so I decided to plant marigolds to see if it made a difference. In this case luck played a role as, before an in-depth study, I ordered the cheapest marigold seed available. This just happened to be the Tangerine variety, which is a French marigold (Tagetes patula).

Professional studies provide overwhelming evidence that T. patula has the greatest eelworm-suppressing potential, although various other marigold species are also used. The least effective variety of T. patula is equal to the best variety of African marigold (T. erecta). Many farmers consider khakibos (T. minuta) equally effective, but it’s actually the worst option.

I transplanted the marigold plants 15cm apart in the rows where the tomato plants had stood before. They grew until just before seed-set when I mowed them down with a garden lawnmower. I worked the slashed material into the soil with a garden fork, adding a little chicken litter. But after the marigolds were established, I noted some volunteer-seeded tomatoes coming up between them. I left these to see what would happen. When the marigolds were in flower, I removed some of these tomato plants from areas where I knew the last tomato variety planted had been susceptible to eelworm, and found a sufficient number of galls on the roots. This proves that companion cropping is certainly not a viable solution.

Good for a trap crop?

Subsequently, I found abstracts published by researchers who had planted susceptible tomatoes with marigolds and introduced measured quantities of larval-stage eelworm to the pots. Not only did the tomato crop remain vulnerable to attack, but they had to compete with the marigolds for light – the marigolds actualy suppressed the tomato crop they were supposed to protect. Marigolds can also build up populations of spider mites and thrips, other reasons not to consider them for companion plants.

Many studies, on the land and in pots, gave similar results. I concluded that companion cropping with marigolds is not effective in suppressing eelworm.

It seems that the greatest benefit of marigolds is as a trap crop. Immature eelworm hatchlings need extra nourishment for giant cells to develop into females. However, the marigold roots produce exudates that trigger the eelworm to hatch and invade the roots. In the root, the hatchlings can’t form these giant cells, and without developing into females, the eelworms die without reproducing.    |fw