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Partial Garden Myth: Marigolds Can Eliminate Nematodes

French marigold with orange flowers

By Larry Hodgson

The idea that French marigolds (Tagetes patula) or other species of marigolds (Tagetes spp.) can control nematodes is not entirely a myth, but its usefulness is so limited that it’s not far from being one. Maybe we could call it a half-myth? Or a myth under most circumstances? At any rate, the nematocidal capacities of marigolds are not going to be useful to most temperate climate gardeners. Read on to understand why. 

What’s Being Claimed

The common belief in companion planting circles is that you can plant French marigolds as companion plants next to vegetables in order to prevent or to control nematodes. The usual theory is that marigolds exude some chemical from their roots that chases the pest away. If only things were that simple!

What Are Nematodes?

A root-knot nematode penetrating a tomato root
A root-knot nematode (Meloidogyne incognita) penetrating a tomato root. Photo: William Wergin and Richard Sayre, Wikimedia Commons.

So, what are nematodes? They’re microscopic roundworms, sometimes called eelworms, and most of the many thousands of species are actually harmless to plants. Some, indeed, are predators on plant pests and thus beneficial: gardeners use certain species to control white grubs, weevils, thrips, ants, slugs and even termites, for example. But there are also many nematodes that are harmful to plants and, when it comes to vegetables, the ones to be most concerned about would be root-knot nematodes, especially those in the genus Meloidogyne

Roots showing galls caused by root-knot nematodes.
Roots showing galls caused by root-knot nematodes. Photo: NMSU-PDC

These nematodes provoke the formation of galls on the roots of many plants, including such common vegetables as beans, beets, carrots, corn, eggplants, melons, okra, onions, peas, potatoes, squash, sweet potatoes and tomatoes. The galls reduce the flow of water and nutrients to the plant above and can therefore seriously reduce the plant’s capacity to produce a normal crop.

Why Treat for a Problem You Don’t Have?

The main “problem” with the use of marigolds to control root-knot nematode is that very few home gardeners actually have root-knot nematode problems, at least in more northerly climates. 

Most root-knot nematode species are tropical or subtropical and even those that are hardier don’t proliferate as well in cooler climates. They need warm soil to start their season and are put off by springs that take time to heat up and summers that are short. They can indeed be a serious problem in the tropics and the Deep South, but in northern climes, they aren’t nearly as frequent. The average home gardener may never see a single case of root-knot nematodes in a lifetime of gardening!

And it’s not just a question of poor adaptation to cool soils. Root-knot nematodes just don’t seem to be common in small gardens, like home gardens or community gardens. They’re more a problem for farmers who grow crops—say tomatoes or onions—on a large scale and repeatedly grow the same crop in the same field year after year. Yes, they’re very much linked to vast monocultures and lack of crop rotation. Again, that’s especially true in the North. In hot climates, root-knot nematodes are much more widespread.

So, before you plant marigolds to control nematodes, why not check—pull up a weak plant or two and check their roots for galls—to see if nematodes are even a problem in your garden?

Even If You Do Have Nematodes

French marigolds used as companion plants in a raised bed surrounding pole beans.
French marigolds don’t repel nematodes, at least not from nearby plants. Photo: seedparade.co.uk

Even if you belong to the minority of home gardeners with a root-knot nematode problem, planting marigolds as companion plants, that is, around or among vegetables likely to be infested, will actually do absolutely nothing to correct the situation. In fact, it will probably make it slightly worse. Here’s why:

Marigolds, especially certain cultivars of the French marigold, Tagetes patula, like ‘Nema-Gone’ (other species are less effective against most species of root-knot nematodes), do not repel nematodes as one sometimes hears, but rather poison them once they penetrate their roots. Root-knot nematodes seem to sense the toxicity of marigold roots and will avoid them if possible … but that only drives them to invade the roots of any nearby plants, including your vegetables. So, marigolds will actually tend to concentrate the problem on your veggies rather than help them.

But you can control root-knot nematodes with marigolds … if you use them as a cover crop, that is, as a monoculture.

Marigolds as a Cover Crop

Field of French marigolds.
Marigolds can successfully control nematodes when used as a cover crop. Photo: marigoldmonday.blogspot.com

This technique should be applied on a plot that was seriously infested with root nematodes the previous year.

Plant nothing but marigolds in the plot for two to four months. In colder climates, that would be the entire season; in mild ones, though, you could have a fall/winter marigold crop eliminating nematodes followed by a spring/summer crop of a susceptible vegetable. 

It’s also important to plant the plot densely: nematodes don’t travel far looking for roots to invade and you’ll want the entire site entirely dominated by marigold roots. 

This will give the nematodes little choice. In the absence of any other food source, the hungry nematodes will have to migrate to and move into marigold roots where toxins inside will kill them before they can reproduce. 

The following growing season, therefore, you will be able to grow plants susceptible to nematodes with little risk of damage, since the marigold cover crop will have reduced the numbers of nematodes so thoroughly there won’t be enough left to do any major damage.

But I repeat, do you really want to apply such a drastic treatment (imagine, an entire season without vegetables!) if you’re not even sure you have a nematode problem?!

But Marigolds Do Repel Pests, Right?

Cat sitting among French marigolds
So, French marigolds repel cats? What’s this one doing in the marigold patch? Photo: Prim with Love

It is a common belief among gardeners that plants that smell strange to us will repel pests. And crushed marigold foliage certainly has an odd smell. Thus, people claim marigolds will repel a whole host of vegetable garden pests: cabbage worm, carrot fly, slugs, even cats! But this seems to be a case of wishful thinking. When you dig a bit into scientific studies on the subject, in general the presence of marigolds either has no effect on pests or slightly increases predation.

Marigolds Can Still Be Useful

Even if we remove nematode-toxicity and pest repellency from the equation, though, marigolds still have their use in companion planting.

First of all, their flowers help attract pollinating insects to the garden and that’s always a good thing. Most of us grow vegetables that need insect pollination, like cucumbers and squash, so any plant that brings in more bees, hoverflies, etc. are going to helpful.

Also, marigolds make a good trap crop for certain garden pests. They are highly susceptible to spider mites, aphids* and thrips, for example. Thus, if you interplant them with your vegetables, there is a very good chance that when pests attack, they’ll settle on the marigolds first. If so, it is, of course, important to pull out and destroy the marigolds before the pest population moves onto your vegetables.

*Amazingly, many companion planting sites claim marigolds repel aphids, yet when you grow them, they’re one of the most commonly affected plants!

So, marigolds are indeed the vegetable gardener’s friend… just not in the way you may have thought.

Larry Hodgson is one of Canada’s best-known garden communicators. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He has written for many garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 60 other titles in English and French. He is a past president of the Garden Writers Association (now Garden Communicators International) and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. He resides in Quebec City, Quebec, Canada.

8 comments on “Partial Garden Myth: Marigolds Can Eliminate Nematodes

  1. I do find that if I plant marigolds in a greenhouse that I get no white or green fly as the smell of them in a confined space can be really overwhelming. I also pop them in for some nectar for ladybirds, hoverflies and bees that go in to aid predation and pollination so I don’t know if it smell or predators that keep them off.

  2. I have read that some nematode will kill the root knot nematode & humus rich organic soil will help protect the Fig tree from the root knot nematode. Being a South Carolina Gardener who has 6 kinds of figs this could be a problem for me.

  3. I use marigolds as slug traps! Also love them on their own for their scent and beauty and for natural dye.

  4. Well, at least they are pretty while they do nothing overly useful.

  5. Ugh! I KNOW I am one of those small home gardeners with RKN. They came in with soil that I had purchased to fill my raised garden beds. I have been struggling with them for several years now. I really wish there was something available to the home gardener to deal with them.

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