Are Marigolds Good for Companion Planting?

There is a lot of talk about companion planting, especially for the vegetable garden, and marigolds seem to be at the top of most plant lists. They make other plants grow better and their strong smell keeps pests away. They even stop root knot nematodes.

Much of this information is anecdotal and I suspect some of it is just made up to sell some popular books. What do marigolds actually do in the garden?

Understand Companion Planting

I have read quite a bit about companion planting in gardening circles and I am struck with one glaring fact. Many claims are made, but almost none provide supporting evidence. The famous book “Roses Love Garlic”, which is routinely referred to, does not contain any supporting evidence. It’s a collection of someone’s ideas, which are now highly followed and promoted.

Saying something works in a book does not make it so.

I started looking into companion planting last year and wrote, Companion Planting: Truth or Myth? Perhaps the most important conclusion in that review is that every claim needs to be investigated individually. A claim such as “marigolds make good companion plants” is completely useless because it does not identify the type of benefit being promoted, nor does it identify the species of the companion plant.

For example. Marigolds could attract aphids and keep them off tomatoes by being a trap crop, but their scent may not keep nematodes from attaching carrots. If this were true, they would be both a good companion plant in one situation and fail as a companion plant in the other.

Given this fact, the question posed in this post,  “are marigolds good for companion planting” can’t be answered and such general statements should never be made on social media because they don’t make any sense.

In this post I will look at some common claims and try to find supporting scientific evidence for them.

What are Marigolds?

Gardeners talk about marigolds as if they are a single type of plant, but in fact they can belong to a number of different species.

The term marigold is a common name for plants in the genus Tagetes, but it is also used for a completely different species, Calendula officinalis. The genus Tagetes has at least 50 different species and the common ones include the African marigold (T. erecta), the French marigold (T. patula) and the signet marigold (T. tenuifolia).

When was the last time a discussion about companion plating included the species name of the marigold?

Lists of companion plants often confuse tagetes and calendula, making discussions even more confusing. This post will deal with tagetes unless stated otherwise.

Claims for Marigold Companion Plants

Here are just some of the claims I found online.

• Enhance the growth of basil, broccoli, cabbage, cucumbers, eggplants, gourds, kale, potatoes, squash and tomatoes. But the term “enhanced growth” is not defined.
• Marigolds deter beetles from melons. Which kind of beetles? Why only melons and what kind of melons are they?
• They deter aphids.
• French marigolds produce chemicals that repel whitefly.
• Nematode damage is common in tomatoes, and marigolds are often effective in suppressing nematodes. However, there are thousands of different kinds of nematodes and most are beneficial. Which ones are suppressed?
• Repels the cabbageworm that attacks plants in the cabbage family. I doubt that the hatched cabbageworm is repelled since it has no place to go – they probably meant the cabbage butterfly.
• They also aid in pollination by attracting bees to your garden.

I almost thought I had found a website that was going to be honest about marigolds, when they said,  “Though evidence for the benefits of companion planting is largely anecdotal,” but then they concluded with, “many combinations make good sense and can help to maximize use your space.” Yes it is almost all anecdotal, but how can planting non-vegetables in a garden “maximize” space”? Planting companion plants that don’t work, is a waste of time and garden space and even if they do work – they use up space.

Will Marigolds Stop Root Knot Nematodes?

I have written about this before. The answer is sort of – but only in specific cases.

You have to select the right species of marigold based on the nematodes you have, and you have to grow the marigolds in the same spot as the following crop. That means you need a growing season that is long enough to grow both crops in one season. That will not work for most gardeners.

Planting marigolds beside crops does nothing to stop root knot nematodes.

Many nematodes are beneficial. What effect do marigolds have on them?

Do Marigolds Repel Pests?

The US Department of Agriculture reports that at least 15 pests are attracted to marigolds, including aphids, Asiatic garden beetle, mite, leaf hopper, Japanese beetles, and spider mites.

The most common explanation as to why marigolds repel pests is that they are heavily scented. A study that looked at various scented plants, including marigolds, peppermint, garden sage, and thyme, to see if they repel cabbage butterflies, found they didn’t work.

The marigold, T. tabaci, did not repel the carrot fly when interplanted with carrots.

Similarly, research has demonstrated that marigolds (Tagetes erecta) did not repel the onion fly or the cabbage root fly.

Tagetes erecta may help tomatoes grow, but only if they are plant 30 or 60 days before the tomatoes, which will only work in warm climates – testing was done in Mexico. Interplanting did reduce the nematode Nacobbus aberrans. “The alate aphid and whitefly populations and incidence of tomato plants with virus symptoms were significantly lower in all intercropped treatments”.  When the marigold was planted too close to the tomato, yield was decreased, while at optimum distance it was increased.

When a plant does repel an insect it is usually a specific insect and the reason for the repellency may not be due to the scent of a plant. Just because we humans can smell something does not mean an insect can smell it. The idea that scented plants deter pests and even mosquitoes is a complete myth!

What can we conclude? Most claims made by gardeners and garden writers are not correct. However, it is quite likely that marigolds do repel some insects – we just don’t have enough research to know which ones they are.

Do Marigolds Repel Whitefly?

I saw this specific claim quite a bit; “French marigolds produce chemicals that repel whitefly”.

A simple search quickly tells you that whitefly is a known pest of marigolds. French marigolds do produce a chemical, called limonene, which attracts whitefly. This explains why marigolds can be used in a greenhouse setting to protect other crops like tomatoes; the whitefly is more attracted to the marigold.

How can whitefly be both a pest of marigolds and repel them at the same time? Why is common gardening advice so wrong?

Do Marigolds Repel Beans?

I bet that title got your attention.

A study intercropped marigolds with a green bean crop to see if they would repel Mexican bean beetles and it worked. Beans grown with marigolds had less beetles, but the marigolds also stunted the growth of the beans, resulting in a lower harvest of beans. Marigolds produce root exudates such as alpha-terthienyl, terpenes and thiophenes, that inhibit the growth of beans.

To be fair, some popular reports on companion planting do mention the fact that marigolds and beans should not be planted together.

Do Marigolds Repel Aphids?

I found numerous cases were marigolds attracted aphids and in some cases the marigolds were used as a trap crop for them. They clearly don’t repel aphids.

Consider this. A common claim by garden writers is that you can control aphids on plants with sticky yellow cards. The aphids are attracted to the yellow color on the cards, head for them, land on them and get stuck. But many marigolds are yellow! If one yellow attracts them, why would a different yellow repel them?

Keep in mind that most aphids don’t fly in their normal life cycle, so they won’t find either the cards or the marigolds. Flying aphids will find both.

Marigolds as a Trap Crop

A trap crop is a plant that is interplanted with a main crop. Its purpose is to attract insects away from the main crop.

Marigolds have been used commercially to attract the rape blossom beetle away from cauliflower.

Claims are made that Japanese beetles are attracted to marigolds, saving other plants from damage. It is true that Japanese beetles are attracted to marigolds; they are also attracted to some 300 other plants as well. In order for this trap crop to be beneficial, two things must happen.

• The beetles must prefer marigolds over the protected plants.
• The marigolds can’t be so attractive that more Japanese beetles enter the area, which is the problem with chemical traps.

There seems to be no supporting evidence that these two conditions are met. The second point is a common problem with many suggested trap crops.

Marigolds are neither a ‘most favored’, nor a ‘least favored’ food of Japanese beetles, so it is unlikely they work well as a trap crop for the Japanese beetle.

If you use marigolds as a trap crop, don’t plant them too close to the main crop.

Do Marigolds Attract Parasitoids?

Parasitoids are small insects whose immature stages develop either within or attached to the outside of other insects, referred to as hosts. Parasitoids eventually kill the host they feed on, as opposed to parasites like fleas and ticks, which typically feed upon hosts without killing them. Their presence can have a significant effect on pests.

When marigolds were grown along side sweet peppers, in the field, the number of parasitoids doubled. Marigolds planted adjacent to onions promote greater richness and abundance of parasitoids compared to onion monocrops. Marigolds also proved to be a good food source for the aphid parasitoid, Aphidius platensis, extending the life of these predators.

These parasitoids tend to be small and they don’t congregate in large numbers. Because of this, most gardeners are not even aware of their existence and they are rarely mentioned in discussions about companion planting. This may be the best use of marigolds in the garden.

Do Marigolds Attract Bees?

Marigolds are a good source of nectar and pollen for bees and their flower is attractive to them. Being annuals, marigolds also have a long season of bloom which makes them a better food source than perennials.

But ….. there is a problem. Many modern cultivars have been bred for large flower heads with many petals. In these plants, the organs carrying the pollen and nectar are no longer accessible to bees. This hybridizing has also reduced the mount of pollen and nectar to the point where marigolds are not included on some lists of bee friendly annuals. The pot marigold, calendula, is much better for bees.

Comments on social media claiming that marigolds attract bees are of little value if the species and cultivar of the plant are not specified. I suspect most lists of bee friendly plants that mention “marigolds” are actually talking about calendula, not tagetes.

Do Marigolds Work for Companion Planting?

There are some clear cases where it does work.

• If you grow marigolds with simple flowers that are close to the native species, it will attract pollinators, such as bees.
• Lots of pests like marigolds so in certain cases they will work as a trap crop.
• Marigolds do control nematodes in soil, but only in very special cases, and only if the plants are used correctly.

However, marigolds seem to be a poor choice for repelling pests, although there are probably some they do repel. Their strong scent seems to have little to do with this. Also, growing them near other plants may inhibit the growth of the other plants. I doubt beans are the only ones affected.

Most of the common advice given in gardening circles about companion planting should be ignored. It rarely mentions a species or cultivar of marigold or the pest involved, so the information is not of much value. If a specific case is discussed, it really needs a scientific reference before the claim should be believed; gardeners rarely do controls.