It is commonly said in companion planting circles that you can plant marigolds as companion plants to vegetables in order to prevent or to control nematodes. If only things were that simple!
What are Nematodes?
First, though, a definition: nematodes are microscopic worms most of which are actually harmless or even beneficial (we use certain species to control insect pests, including white grubs in lawns, for example), but some species are harmful to plants. This is especially the case with root-knot nematodes (mosly Meloidogyne spp.). They cause galls to form on the roots of many plants, including vegetables, galls that reduce the circulation of sap 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 nematode pests is that very few home gardeners actually have nematode problems, at least in northern North America. Root-knot nematodes can indeed be a serious problem in the Deep South, but in northern climes, they aren’t nearly as frequent. Few harmful nematode species can survive cold winters and even those that do rarely seem to be common in small 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.
So before you plant marigolds to control nematodes, why not check and see if you even need to bother dealing with nematodes?
Even If You Do Have Nematodes
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. Here’s why:
Marigolds, especially the French marigold, Tagetes patula (other species are less effective against North American species of nematodes) do not repel nematodes as one sometimes hears, at least not in a good way. True enough, root-knot nematodes do not like marigolds and will avoid them if possible… but that only drives them to invade the roots of any nearby plants, including your vegetables. So they 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
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 an entire season. The roots of marigolds are toxic to root-knot nematodes, at least those of the genus Meloidogyne, the most common kind. In the absence of any other food source (since marigolds are being used exclusively, they have nothing else to eat), the hungry nematodes will have no choice but to migrate to and move into marigold roots where toxins inside will kill them before they can reproduce. The following year, therefore, you can grow plants susceptible to nematodes without any risk of damage, since the marigold cover crop will have eliminated the nematodes entirely.
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?!
Marigolds Can Still Be Useful
If we remove nematodes 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.
Also, marigolds make a good trap crop for certain garden pests. They are highly susceptible to spider mites 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 on… to your vegetables.
So, marigolds are indeed the vegetable gardener’s friend… just not in the way you may have thought.