Will Marigolds Stop Root Knot Nematodes?

Companion planting is a standard recommendation for growing vegetables. One of the most commonly recommended plants for this is the marigold, which is supposed to be good for preventing various pests from eating the vegetables. I will limit the discussion in this post to using marigolds to reduce or eliminate nematodes in the garden.

Nematode – What is it?

Nematodes are microscopic worms that live in soil and other places. All soil has them in varying amounts, and there are many different types. Unless you take a soil sample to a lab you will not know which type you have or how many you have. Some nematodes cause problems in the garden, but many are beneficial. For example, nematodes are used to kill June bugs in lawns.

The nematodes being discussed here invade plant roots and cause various deformities in the roots. That can be a real problem in root crops like beets and carrots since they affect the part of the plant we want to eat. But nematodes also affect other plants like tomatoes. When tomatoes are infected, the nematodes make the plant weaker resulting in a poor crop.

Some damage is always present, but in most cases it is so minor that we don’t even see it. In severe cases you simply can’t grow certain crops.

Nematodes and Marigolds

Many web sites and gardening books promote the idea that marigolds will reduce or eliminate the amount of nematodes in the soil. If this happens, then you no longer have a nematode problem. Seems simple enough. Plant some marigolds in your vegetable garden and you eliminate nematodes.

There is some validity to this idea so it is not a complete myth. The problem is that the real truth of the matter has been vastly exaggerated and over simplified.

How do Marigolds Control Nematodes?

Based on my research, scientists are not really sure. Marigolds do produce compounds that are nematicides, a compound that kills nematodes.  It is possible that these are released into the soil and that the nematodes in the vicinity of the roots are killed. Field testing does not support this hypothesis.

A more plausible explanation is that marigolds act as a host for the nematodes. The nematodes actually invade the marigold roots in the same way they invade a tomato root. The difference is that once the nematode is inside the marigold root, the natural nematicides of the plant kills the nematode and prevent it from breeding. Over time the population of nematodes decreases.

None of this has anything to do with the marigold fragrance which does not attract nematodes, nor does it repel them–that is a myth.

Does Companion Planting Work?

Stated another way, will marigolds, planted along side of other vegetables, control nematodes? The key words here are ‘planted along side’. The answer is NO. This has been tested scientifically many times and it just does not work. Your garden may look pretty, it may smell nice, but this practice does not prevent nematodes from invading your crops.

Marigold Extracts

Added 2023: There is some new evidence that aqueous extracts if the African marigold ( Tagetes erecta) can inhibit the growth of nematodes (Meloidogyne incognita) on tomatoes.

Best Cultural Practice

If you decide to use marigolds to control nematodes you should follow these steps. Plant the marigolds in the exact spot where you will be growing the vegetable. Grow them there for at least 2 months and some references suggest 4 months. After 2 months, you can remove them, or dig them into the soil and then plant your vegetable crop. This will control the nematode problem, provided you are planting the right kind of marigold – see below.

The marigolds attract nematodes and as they invade the marigold root, the number of nematodes is reduced. When the vegetable is planted right after the marigold, it grows in an area that does not have many nematodes and therefore you see less damage to the vegetable crop. To make this work the marigold needs to be in the “same” spot as the vegetable. A foot one way or the other makes a big difference and that is why growing the two plants side by side does not work.

If you are located in climates with shorter growing seasons, for example much of the northern hemisphere, you don’t have a long enough growing season to dedicate 2 months to marigolds. You are out of luck.

Which Marigold Works Best?

You might know that there are a number of different types of marigolds, both species and cultivars. It turns out that each type has a different effect on nematodes and the effect depends very much on the type of nematode. That is right, to have any effect you need to match the right type of marigold to the nematode growing in your soil. How do you figure that out? You need to have your soil analyzed at a lab. Unless you are prepared to do this, you have no idea which marigold to plant.

If you are not prepared to have your soil tested, don’t bother trying to use marigolds to control the nematodes. It’s probably not going to work.

Marigold Facts

The following are some facts related to the use of marigolds in fighting nematodes.

• Most modern day tomatoes have been breed to be nematode resistant–one reason for not using heirlooms.
• Protection lasts one season or for one crop. Nematodes multiply quickly and will return.
• Marigolds do not draw nematodes away from crop plants. They are simply one of the possible hosts.
• Digging the marigolds in is good for the soil, but it does little to help the nematode problem. The natural nematicides are only found in living plant tissue. However, the extra organic matter might encourage natural nematode predators to prosper.
• The effectiveness of marigolds is temperature dependent. Some marigold types work better at higher temperatures, and some work better at lower ones.
• Marigolds are a magnet for thrips and spider mites which could be as big a problem on vegetables as the nematodes.