When we talk about companion planting we’re talking about a set of techniques. I’ve written a blog on effective companion planting techniques so I won’t repeat myself here.


How plants communicate with microbes – the basics!

All plants communicate with microbes using secondary metabolites some of which are excreted into the soil. These are called root exudates. They are compounds a plant excretes to attract and/or repel particular microbes. I’ve written a blog covering the basics of this science, so I won’t go into much detail here.

Understanding these basic functions is necessary when looking at companion planting. If for example we choose a companion plant for its scent shielding  qualities, that plant may also excrete root exudates that repel say: other plants… or feed other pathogen microbes.

Therefore, we may us a plant’s companionship in one way and also get a result in another perhaps unexpected way. Nature is complex.

The key reason for companion planting with marigolds is usually based on root exudates and their effect on plant parasitic nematodes. I’ve written a blog on nematodes so have a read of that to understand nematode types. Most nematodes are great for gardens and plants.

Having a knowledge of plant susceptibility/resistance and types of nematodes we start to understand the importance for species specific plants for species specific problems. All plants evolve with particular antagonists. Marigolds are no exception.

Some Marigolds have effective defences against some plant feeding nematodes, others just don’t. Likewise, some plants in our gardens may attract the nematodes we want to kill with our Marigolds. So one plant may feed the nematodes and one might kill them.

However, nature is complex and the result of that scenario is likely to be more of those nematodes rather than less. I’ll tell you why in a bit.


Why Marigolds?

Many of our ancestors throughout the world grew and/or used Marigolds. Marigolds tend to have internal and external medicinal uses. They are beautiful and in some cultures they are used in religious and spiritual situations. They are also said to be good for gardens and soil.

Some horticulturalists and gardeners suggest they can be used as a sacrificial plant to attract some herbivores such as aphids. Others suggest companion planting with  Marigolds to kill of nematodes in the soil. I sought to know what the science suggests.

Before we go on, I’ll link to this scientific article that covers the literature on the issue. As I have found, it suggests conflicting findings on the use of Marigolds on nematode populations. Those researchers have stated what we all know: nature is complex.

Research also shows a difference between the ways we use Marigolds and the effects on key nematodes. Do we use them as a cover crop or as a companion plant? Do we need to plant near their root zones or is digging in a dead Marigold plant enough? Is it more effective to use a spray based on steeped flowers or should we use the leaves?

Overall, it appears that marigolds hold the greatest effect when still growing. That tends to suggest companion planting with marigolds is key. However, not all marigolds are the same…


Which Marigolds do we use for which nematode?

Our environment and our soil is – or should be – a breeding ground for all types of nematodes. I’ve written a blog on the types of nematodes so check that out so we understand that most nematodes are our friends.

When we use Marigolds to reduce plant parasitic nematode numbers we’re looking for a Marigold that has the thiophene a-terthienyl secondary metabolite as a root exudate. Apparently that’s a very toxic compound. That compound can and often does detriment other microbes in the soil.

We also need to be mindful that the Marigold suppression of nematodes tends to only be effective when the soil temperature is between 10º C and 30ºC. So what do we do in winter and summer?

Well, we use organic matter in the soil and mulch on the soil to moderate soil temperatures.  We also do that with water in the soil and a canopy of plants so there is no bare soil.

Also, in the colder seasons microbes tend to go to sleep. Most enter a dormancy period in the colder months. That’s why it’s important to store any Bt (Bacillus thuringiensis) we have under 4º C until use.

I’ve looked into a number of different Marigolds and found three with very positive effects on some microbes. However before we go on, I just need to add that the science in this area is in its infancy. And, most of the research is being done on a handful of microbes that harm the agricultural industry. That’s probably less than 0.0001% of the microbe species as a whole.

1. Tagetes minuta – Stinking Roger/ Mexican Marigold

Tagetes minuta is considered to be a weed in most countries including several jurisdictions of Australia. Despite this,  research conducted at the University of Florida shows that Tagetes minuta are resistant to two root knot nematodes (Meloidogne incognita and Meloidogne javanica) but susceptible to another root knot nematode (Meloidogne arenaria).

These nematodes I’ve just named are considered to be global problems and are also an issue throughout Australia. Therefore, for Australian gardens, we can see that if we plant Tagetes minuta and we have M.arenaria in our soil, we are likely to perpetuate a nematode issue. Why?

Because that Florida study showed that Tagetes minuta (Stinking Roger) is susceptible to M.arenaria root knot nematode. Being susceptible it can become a host for that nematode.

That’s why we need to know what nematode we have. Nature is complex…

Companion planting with marigolds of this type (Tagetes minuta) with tomatoes and eggplant is likely to be beneficial because both tomato and eggplant are susceptible to both M.incognita and M.javanica: and research suggests Tagetes minuta is resistant to those two root knot nematodes.

Therefore companion planting with marigolds in that scenario is likely to work.

Companion planting with Tagetes minuta have also shown suppressed number in Rotylenchulus reniformis and Tylenchorhynchus brassicae when inter-planted with tomato, eggplant, cabbage and cauliflower.

Rotylenchulus reniformis is a plant parasitic nematode that having infected a plant, the plant shows signs of water or nutrient deficiency. Apparently it’s a global problem affecting a lot of edible plants.

Tylenchorhynchus brassicae is known as the ‘stunt nematode’. She feeds on the roots of key brassicas and many other plants such as rice, tomato, eggplant etc and stunts their growth.

Another study found companion planting with Marigolds of this type effective when growing sugarbeets and perhaps also beetroot and chards. The antagonist there was Tetanops myopaeformis the Sugarbeet maggot.

Therefore, inter-planting Tagetes minuta with plants from the Solanacea and Brassica families may be effective with some root feeding nematodes.


2. Tagetes patula – French Marigold

Tagetes putula has a sap known to irritate and burn skin. She flowers in an array of colours ranging from yellows, to oranges and maroons. A spray made of her flowers may be useful in reducing cankers, early blight, wilt, blossom end rot and even perhaps sun scald!

Like Tagetes minuta, companion planting with marigolds of this type (T. patula) can be effective against Rotylenchulus reniformis and also M. incognita and other nematodes

An Indonesian study looked at a number of host and non-host plants and found, among other things, that the populations of plant parasitic nematode (Rotylenchulus reniformis) couldn’t develop with Tagetes patula and Tagetes erecta.

Another study found Tagetes patula effective in suppressing a lesion nematode (Pratylenchus penetrans [Cobb]). That study found an ongoing desirable effect using Tagetes patula as a cover crop and then planting strawberries. In fact, that study found the use of this plant as a cover crop more effective than soil fumigation. It is possible companion planting with marigolds of this type would also be beneficial to strawberries against that lesion nematode.

Therefore, it appears that where specific plant parasitic nematodes are problematic, cover cropping or companion planting with marigolds of this type might benefit other plants including members of the Solanacea family and also may have a positive effect with Cucurbits, Brassicas and Rosaceaes (strawberries etc).

3. Tagetes erecta – Aztec Marigold

Tagetes erecta is often regarded as a poisonous plant just like T. patula. She can cause skin rashes but what a lovely flower! She has known medicinal qualities and is also popular in gardens.

There does not appear to be as many scientific papers on this type of marigold. Most of the studies are approaching 30 and 40 years old. Some are more recent.

One study found companion planting with marigolds of this type successful in suppressing a number of different nematodes with cowpea. In fact they found the growth rates enhanced when Tagetes erecta were interplanted with cowpea.

Another study found a positive effect of a number of marigolds, including T. erecta with tomatoes. That study looked at four types of root galling nematodes. However, that study also showed a difference between cultivars of marigolds and also how one type of marigold acted as a host.


The key to reducing plant parasitic nematodes is creating a healthy soil

We’re not going to get rid of all the root feeding nematodes in the soil. Why would we want to? As Zen Master Thich Naht Hanh says: No mud, no lotus.

What we want is a healthy and diverse microbial population to keep the root feeding nematodes in check. Most of the nematodes in our soil are beneficial. Like everything, we need balance, homeostasis. This is a key message from soil microbiologist Dr Elaine Ingham.

As the research shows, companion planting with marigolds can be effective in reducing plant parasitic nematode populations. It depends on the type of marigold and also the cultivar. It also depends on the type of nematode and other factors such as soil temperature.

Unless we send a soil sample to someone like Dr Elaine Ingham or the Soil Foodweb Institute (Australia) how would we know if we have a nematode problem? How would we identify the type of nematode? There are many different types of root knot nematodes for example. Therefore by looking at the effects of a root knot nematode we still don’t know what type of root knot nematode we’re dealing with.

Are we dealing with Meloidogne incognita, Meloidogne javanica) or Meloidogne arenaria or some other species of Meliodogne…???

The reality is, if we have a diverse microbe population in our soil, we’re less likely to have a plant parasitic microbe problem. A diverse microbe population would typically house plenty of omnivore nematodes and these would feast and feast if there were too many plant parasitic nematodes.

The presence of all the other microbes help to sustain a healthy plant. As my nitrogen fixing blog shows us, plants select for particular microbes.  Healthy plants release exudates that attract and feed microbes beneficial to its survival. We get these results from having a healthy soil and gardening organically.

Therefore, I think companion planting with marigolds should be done with due caution. If we don’t have a plant parasitic problem, then why use a plant that could in fact cause a nematode problem?

Companion planting with marigolds can produce excellent results. This is particularly the case with susceptible plants such as tomatoes, eggplants etc as shown above. However, relying on marigolds to do the work rather than feeding the soil and the billions of microbes as nature intended, might be a bit short sighted.

Did you find this blog interesting? What are your thoughts on companion planting with Marigolds?

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Sherrill · September 27, 2019 at 8:25 pm

Very interesting article but I was hoping you would say they are good for keeping aphids away, sadly this is not so, so I am wanting to ask what plant might keep aphids away from my dill please.

    Gabi · December 18, 2019 at 2:49 pm

    Maybe tansy will do that, you need to research it

Beverley H. · July 10, 2018 at 2:47 pm

Excellent article Amber. Full of good information. Thank you.

    Amber Hall · July 12, 2018 at 12:34 pm

    Glad you found it interesting Beverley 🙂

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