A common quote on the internet is, “It’s best to use strongly scented plants for companion planting because they confuse pests looking for their host plant.” This is a commonly voiced opinion, and it usually follows with a list of fragrant plants that make good companion plants, such as marigolds, onions, mint, chives, lavender, wormwood, and many other herbs.
This seems to make perfect sense. Insects looking to lay eggs on a cabbage might be confused when they smell a marigold and leave the area thinking there are no cabbages.
Read any of the thousands of websites and books about companion planting and they all agree; that strongly scented plants are your best option. The only problem is that none of these suggestions are based on science. ………. The facts are going to surprise you.
Companion Plants for Cabbage
If you have ever tried to grow cabbage you know that a number of insects like to lay their eggs on this plant, and the hatched larvae love to eat it. It’s a perfect subject for testing companion plants.
A research project at the Horticulture Research International, Wellesbourne, UK, planted cabbage in a field experiment, and surrounded the cabbage with 24 different kinds of plants including garden bedding plants, weeds, aromatic plants, and popular companion plants. Of these, 20 plants did result in fewer eggs being laid by the cabbage root fly.
The surprising find was that “the aromatic plants; curry, marigold, mint, onion, sage and thyme were not more effective at confusing the root fly than non-aromatic plants.”
Similar findings were found when they tested onions and the onion fly.
Green Leaves are More Important Than Scent
This same group of scientists has carried out a number of other experiments and has concluded that scent has nothing to do with a plant’s ability to deter insects. The most important characteristic is green leaves.
When plants with reddish leaves were tested, they performed poorly compared to similar varieties with green leaves.
The scent seems to play no part in how these insects select the leaf for handling. The insects simply land on a green leaf and then test it to see if it is a suitable host for egg-laying. If not, they leave and try another leaf.
They are not attracted to the color brown and as a result, don’t land on brown leaves or the bare ground beside a host plant.
It is leaf color and odor that makes a good companion plant for the purpose of confusing herbivorous insects.
Will Paper Work as a Companion Plant?
In another, very elegant experiment various crucifers, including cabbage, cauliflower, Brussels sprouts, swedes (rutabaga), and turnips, were grown in clover. The clover acted as a companion plant, decreasing the egg-laying of eight pest species from four different insect orders.
In another test area, the clover was replaced with plant models made with green paper. These worked just as well as the clover.
Since the paper produces none of the chemicals that the clover might produce, it clearly shows that it is the green leaves and not any plant chemicals that are confusing the insects.
The Appropriate/Inappropriate Landing Theory
The above-described work has led Finch and Collier to propose the Appropriate/Inappropriate Landing Theory, which in essence says that herbivorous insects find the general location of host plants by smell. As they get close, they land on plants based on leaf color, shape, and size, not smell. They then decide if the leaf is appropriate, ie the right kind of host plant, or inappropriate, probably using smell and taste. If it is a host plant, they continue with egg laying. If it is inappropriate, they leave and try again on the same plant, or a nearby plant or leave the area.
To Weed or Not to Weed?
Since weeds are mostly green, they make good companion plants. Unfortunately, most gardeners remove the weeds and expose bare soil or mulch, neither of which is green. This has the effect of making it much easier for insects to find the host plant since there are only green things left.
From the point of view of reducing insect eggs on host plants, it would be better to leave the weeds. The problem with this is that the weeds also compete with the host crop, reducing production. Weeds do however improve soil health.
Intercropping, growing two crops together may be a good alternative.
What is a Scented Plant?
Humans tend to think that all life forms are similar to ourselves. We smell a marigold and call it scented and then go on to assume every other life form thinks it’s scented too. That is one reason myths about scented plants are common.
But other life forms may not even detect the chemicals we sense from the marigold. And even if they can detect them, they may be interpreted differently. Dung beetles think manure smells great but you don’t see humans flocking to it.
Dogs smell all kinds of things we can’t. Insect pheromones – the sex fragrance of insects – cause male insects to fly long distances but are undetected by us.
Why is it then that we think insects will be affected by plants that we find fragrant?
Are Strongly Scented Plants the Best Companion Plants?
Most discussions of companion plants talk about this subject as if there is one thing to consider. In my previous post, Companion Planting – Truth or Myth?, I pointed out that companion plants can be used in many ways. It is not all about preventing insect damage. So to answer the question in the subtitle, we have to know the goals for the planting.
As far as insect damage goes, it seems fairly clear that scent has nothing to do with preventing herbivorous insect damage, at least not on crucifers. Any plant with appropriate large green leaves, including weeds, will reduce egg laying on plants.
What about other types of bugs like aphids? They may behave differently.
One thing that is interesting here is the fact that so many sources claim that scent is a big part of this when it’s not. It illustrates how popular beliefs and anecdotal reports can confuse gardeners. It also makes me wonder how the books on companion planting got their lists of “the best plants”? Clearly, they are not based on facts.
Another key point about using companion plants for producing crops is that insect damage is only part of the picture. It is really all about yield, and none of the studies presented here addressed yield. In cases where yield is important, it may be better to remove weeds and use no companion plants. But we don’t know that, based on the current information.
The more I delve into companion planting, the more complex the picture gets.
Oh yeah, garlic. Thank you, Thomas, for your correction on what repels vampires. There could be some midnight gardeners who would need this information, lol.
Cindy
I was under the impression that the purpose of aromatic companion plants was more to attract beneficial insects, namely pollinators like bees, rather than to directly repel pests.
Since I’m growing both herbs and vegetables for myself anyway, using herbs as companion plants for my vegetables serves as a form of intercropping, allowing me to get more edible plants crammed into my limited amount of space. They also serve to choke out weeds and act like a living mulch.
the insect repllent thing may be largely a myth, but companion planting still has its uses.
Bees are attracted to flowers. I doubt they are attracted to aromatic plants since they can’t get any food from them if they are not flowering.
I thought French marigolds were best for the sub soil pest. Like bad nematodes, cut worms ect. I use sweet basil around Roma and other nighshades. Seems to work. But I notice once I see white butterflies/moths. Then I have issues. Nothing like an old badminton racket to fix that.
For most people, marigolds do NOT work for root knot nematodes.
https://www.gardenmyths.com/nematodes-marigolds/
I have always believed that the people who write horoscopes write books on companion planting in their spare time. After reading this post I see no reason to change my mind. Thankyou.
Good response. For many years I bought into the companion planting effectiveness; not surprising given the plethora of sources endorsing it. But I have never found companion planting to be successful — just thought perhaps I wasn’t doing it perfectly.
Oh well, it’s why I read Pavlis!
I do agree with the examples you have given here but have you seen the white fly/marigold study from Newcastle University? This article from Science Daily: https://www.sciencedaily.com/releases/2019/03/190301160909.htm makes it sound like it is the scented limonene compound that deters the pest. Could it be that different insects use different mechanisms for finding host plants and sometimes scent does come in to play?
It is certainly possible that different insects behave differently.
Thank you for shedding light on this subject. I companion planted scented plants around my kale one year and had more cabbage looper larvae than when I planted it at the base of tomatoes and close to my runner beans. I have found that marigolds do work to an extent, but once the moth figures out where to go its pretty much ineffective.
Thank you sir for this information. You have changed the way I will garden in the future. I had always assumed that having no weeds was a worthy goal in a garden but now I will be leery of too much open dirt around my plantings. This is because I now understand, from what you told us, that it would make the plantings an easier target for the insect pests.
I had bought some marigold seeds but now I see that expecting them to ward off insect pests would be like trying toward off vampires with onions. I don’t think it would work.
Cindy
No, Cindy. It’s GARLIC that deters the vampires. Lol
I hope you don’t get tired of my thank you’s. Your information is so great!! Thank you so much…you’re up there with my favorite mentors and professors…I wish more people were better informed.
:))
You say it is the green leaves that are “confusing the insects”. That sounds like an extension of the myth that you are debunking. As the scientists point out in their theory, the insects are not confused. They are assessing likely places to lay eggs and one of the indicators of a likely place is green leaves. In our attempts to control Nature, we gardeners think we are cleverly confusing the unintelligent insects, when the bugs are just doing what they are genetically programmed to do which is to ensure the survival of their species. If planting clover, or fake green leaves, means less insect damage to our canbbages, that is maybe good, but we shouldn’t be thinking it’s because we, big brained Homo Sapiiens,
outwitted the bugs.
The solution is clearly to breed cabbages with tiny brown leaves.
hey,great we have russian red kale already, and raddichio and red cabbage, interplant those with greenleafy veg
Robert, thanks for picking out these gems of horticulture lore and researching any systematic studies.
I hope other really skilful gardeners weigh in with comments, like from Bionutrient Food Assoc.