Companion Planting: Truth or Myth?

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Robert Pavlis

I have wanted to do some posts on companion planting for quite some time. There is certainly a lot of nonsense out there about companion planting, but some of the advice seems to make sense. The problem with this topic is that it is vast and complex. So I have ignored it – until now.

This post is an overview of the topic. I will look at some general concepts and try to gain some basic understanding of companion planting. Future posts will look at specific combinations of plants.

Companion Planting: Truth or Myth?
Companion Planting: Truth or Myth?

Does Companion Planting Work?

If I plant some corn and climbing beans together, the corn provides structure for the beans to climb up and that is good for the beans. This example is quite clear and it certainly demonstrates that companion planting works.

Lettuce likes to grow cool and by mid summer it needs extra shade to stay cool or it bolts. Planting it behind larger tomato plants provides a cooler environment. Again a good example of companion planting that works.

Growing Great Tomaotes, by Robert Pavlis

It is kind of pointless to have a debate about whether or not companion planting works – I just gave two examples to show it does work. The problem is that just because it works in these two cases does not mean that the hundreds of other examples also work.

Planting marigolds to reduce root knot nematodes is a very common suggestion. It does work, provided that you match the right species of marigold with your crop, and you first grow the marigold and then in the same season immediately follow it with the crop, something only possible in warm climates. For this reason it does not work for most gardeners.

The only intelligent way to discuss companion planting is to consider each example on its own merits. So that should give me about a thousand future posts. 🙂

What Does the Science Say?

Most of the examples of companion planting promoted for gardeners have NOT been tested. The reality is that research money is tight and when it is available, it will be used for agricultural situations – not home gardens.

Most suggestions have no scientific basis. Many times it is just stuff made up to sell advertising, sell a book or promote an individuals idea. I had a look at one of the books that made this topic very popular; Carrots Love Tomatoes by Louise Riotte. It contains absolutely no scientific evidence for the claims and most combinations don’t even have logical explanations as to why they work – they just do. And yet those claims have been repeated millions of times as if they are fact.

In other cases, claims are based on anecdotal information which is not worth the screen it is displayed on.

What is Companion Planting?

I borrowed a couple of books on the topic from the library to see what authors are saying. I won’t mention the titles because neither one is worth reading. They covered such topics as composting, plant diversity, soil improvement, square foot gardening, vermicomposting, green manures, gardening by the moon, you get the idea. Anything that is remotely connected to organic gardening seems to qualify as companion planting.

Microbe Science for Gardeners Book, by Robert Pavlis

And that is one of the problems with this topic. There is no single accepted definition. If we can’t agree on what it is, how can we debate its existence?

I’ve taken a definition from the North American Permaculture Magazine, and modified it to include the case where only one plant receives a benefit;  “a type of polyculture, where two or more plant types are grown together because at least one of them shows improved growth due to the presence of the others.”

The key points of the definition are these:

1) Two or more types of plants are grown together.

2) At least one plant grows better because of its neighbors.

This definition narrows the scope and excludes things like growing a cover crop for a better future yield, intercropping to maximize use of space, and using raised beds.

Good Companions and Bad Companions

Black Walnut
Black Walnut

Apparently some plants prevent others from growing properly. Walnuts and tomatoes are a classic example of this. Tomatoes don’t grow under walnut trees – or so they say. Fennel does not seem to get along with anything, except dill – who know?

The above definition talks about the good companions with no mention of the bad. None of the definitions I looked at included the bad ones, but everybody goes on to discuss them as if they are part of the topic. Should this be called opponent planting? I like the terms good companions and bad companions, as in good bugs and bad bugs.

There is also very little scientific evidence supporting the idea of bad companions. Some people talk about allelopathy, but most of the support for this is demonstrated in the lab and not in the field. There are also other kinds of ‘bad’. For example, some plants are hogs when it comes to nutrient and water reserves. Think of the soil around Eastern White Cedars (Arborvitae); I think they would be bad companions for most plants.

Companion Planting and Organic Gardening

It seems that many authors combine these two topics, as if one is dependent on the other. Or maybe it is that organic gardeners are more willing to believe all of the hype around companion planting?

One thing is clear – the two topics are not connected since you can practice one without the other. You can garden organically and not use or believe in companion planting, and vice versa.

The Benefits of Companion Planting

There are numerous ways in which one plant may benefit another and the following sections discuss a number of these. The examples given below are provided for illustrative purpose and not because there is necessarily scientific evidence to support them. The examples may be true or not, and some will be examined in more detail in future posts.

Physical Protection or Support

A large shrub provides wind protection which might allow tall delphiniums to flower without wind breaking the flower spikes. Is that a benefit for the plant or for humans?

The corn provides physical support for the bean to climb as does a shrub for clematis.

Trap Cropping

One plant is used to attract a pest, so that the pest leaves the other partner alone. Japanese beetles prefer Corylus avellana ‘Contorta’ over roses, at least in my garden. My roses were hardly ever attacked until my Contorta died.

I also grew Desmodium canadense, the showy tick-trefoil, and it was a Japanese beetle magnet.

We can agree that certain plants attract a specific insect more than others. But will placing such plants next to other plants protect them? Or do they attract more pests to your garden and the neighbors garden benefits? This is the case with Japanese beetle traps. They work because they attract and capture the beetle, but in a normal sized garden they bring in more beetles than they capture. I always recommend them to my neighbors.

Modify the Environment

Plant A can modify the environment for plant B. The lettuce and tomato are such a combination since the tomato provides shade. In the three sisters planting, the squash shades the ground so that fewer weeds grow, which is said to benefit the bean and corn.

It is easy to understand and accept as fact that plants modify the environment. However, extrapolating this to conclude that the companion plant grows better is false logic.

Attract Beneficials Including Pollinators

Many flowering plants attract pollinators to the garden which then go on to pollinate vegetable crops.

A big problem with this approach is that flowering plants also attract pests – they also want nectar. So is this a net benefit for partner plants?

Deter Pests

Ants loving a mint plant, by Robert Pavlis
Ants loving a mint plant, by Robert Pavlis

Certain plants will keep pests away, so if they are placed next to pest-attractive plants, they keep pests away from both plants. It is claimed that aphids and white cabbage butterflies hate the smell of mint. The same is claimed for keeping ants away but that is a myth.

Supplies Nutrients

The classic case here is growing legumes to provide nitrogen for their partners. This is a very common myth, but the reality is that as long as the legume is growing it adds very little nitrogen to the soil. There is no benefit to the partner plant. Once it dies and decomposes there is some benefit, but that does not meet our definition of companion planting.

Comfrey is commonly called a dynamic accumulator because its deep roots bring nutrients to the surface were other plants can use them. As discussed in Comfrey – Is it a Dynamic Accumulator? , this is simply not true.

Are there plant combination where one partner provides nutrients for the other?

Suppresses Weeds

If plant A suppresses weeds, then plant B grows better. Makes sense. But if plant A is not also a crop plant, is it not just another weed that you planted? It competes with B just like the weeds.

Improves Flavor

Is there a plant that you can grow beside your tomatoes to make them sweeter? Apparently planting basil and tomatoes together improves the flavor of both. Now that might be interesting, but given the general biology of plants and their limited ability to absorb complex flavor molecules through the air or soil – I am skeptical.

If a plant grows better because of a good companion, then it might produce more flavor molecules, or more sugar, making it sweeter.

Companion Planting Nonsense

In my wanderings It was easy to find some dumb comments.

  • “Herbs work especially well as companion plants. They multitask by attracting beneficial insects and repel pest insects”

Why is it that all herbs have this property? Herbs are defined by how we use them as in “a plant or plant part valued for its medicinal, savory, or aromatic qualities“. Would insect behavior not be determined by plant biology as opposed to how humans use the plant? Any such statement about “all herbs” is almost certainly false.

How exactly does the herb know which insect is beneficial and which is a pest? Why would it’s biology be able to attract one and not the other? Seems like nonsense to me.

  • “Plant valerian, lovage, and dill around any struggling plant to improve its health and vitality”

So if I have a plant that is wilting because of a lack of water I can plant dill beside it, and my plant will recover? And it also cures mildew? If dill is this effective it should be packaged as snake oil.

  • “Nasturtiums are excellent natural pest deterrents” and on a different website, “nasturtiums are a good trap cropping plant since they are a magnet for aphids”

Aphids are certainly a pest. How can one plant both deter aphids and attract them as a trap crop? It has to be one or the other – not both. It could also be neither.

It’s statements like this that must make you question everything about companion planting. Most articles about companion planting are poorly done, and include no explanations or references to support their position. Many just copy the same list over and over again, repeating the same nonsense.

Real Benefits of Companion Planting

Lets have a closer look at the definition, “a type of polyculture, where two or more plant types are grown together because at least one of them shows improved growth due to the presence of the others.” A very key point here is improved growth, but what does that really mean?

Consider the case of the bean climbing up the corn. Clearly the bean is growing better and is able to get its leaves up higher to catch more sun. One could conclude it has improved growth.

Do we care about improved growth? Or do we want a bigger bean harvest?

Corn uses a lot of water and nutrients, so its root system competes with the bean for resources. It also shades the lower leaves of the bean plant. What if the bean plant produces lots of growth but few beans? Is it still a good companion?

Companion planting in a vegetable garden should really be defined in terms of productivity. Does the plant pairing produce more food than when the plants are grown separately?

In an ornamental garden it makes sense to define improved growth in terms of appearance; more flowers, better leaf color or less disease.

Should You Try Companion Planting?

There are certainly some cases where plants benefit from having partners. And there are other cases that are nonsense. It’s next to impossible to tell one from the other.

One study that looked at several different vegetable crops in garden sized beds concluded that “companion planting can offer advantages over monoculture, but these are not achieved simply by combining certain compatible companion species. Crop densty, ratio, and relative planting times all affect the way that companion species interact with one another and their environment. ” In short – the topic is very complicated.

If the source of the suggestion does not give a plausible explanation as to why the combination works, don’t believe it. If it lacks references to scientific studies, it probably does not work.

Use common sense in the garden and ignore most companion planting information.

In future posts I will look at some specific examples to see if there is enough research to support their use.

References;

  1. Yield, pest density, and tomato flavor effects of companion planting in garden-scale studies incorporating tomato, basil, and Brussels sprout; http://orgprints.org/6614/

 

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

34 thoughts on “Companion Planting: Truth or Myth?”

  1. Hi, I was shocked to read what you said about fennel though I guess I shouldn’t be as I’m an extremely amateur gardener, throw a seed in the ground and hope it grows, it usually does. I threw in a lot of fennel seeds and had them flourishing alongside and twixt honeysuckle, potatoes, lots of different poppies, hollyhocks, lavender, aquilegia, love in the mist, and some almost like trees next to my climbing roses. I VERY much miss that beautiful garden as had to move. I live in Bournemouth, England by the way, don’t know if that makes a difference. Love your site.

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  2. Another book worth taking a look at is ‘Plant Partners’ by Jessica Walliser, which is well referenced and gives a scientific approach.

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  3. From Greer’s book, Companion Planting for the Kitchen Gardener.
    Very few references considering the size of the book.
    “SCIENTIFIC STUDIES UTILIZED”
    Amarawardana, Lakmali, Premaratne Bandara, Vijaya Kumar, Jan Pettersson, Velemir Ninkovic, Robert Glinwood. “Olfactory Response of Myzus persicae (Homoptera: Aphididae) to Volatiles from Leek and Chive: Potential for Intercropping with Sweet Pepper, Section.” Soil & Plant Science 57 (2007): 87-91.
    Asare-Bediako, E., A.A. Addo-Quaye and A. Mohammed. “Control of Diamondback Moth Plutella xylostella on Cabbage Brassica oleracea var capitata using Intercropping with Non-Host Crops.” American Journal of Food Technology 5 (2010): 269-274.
    Booij, C. J. H., J. Noorlander, J. Theunissen. “Intercropping Cabbage with Clover: Effects on Ground Beetles.” Biological Agriculture & Horticulture 15 (1997): 261-268.
    Fininsa, Chemeda. “Effect of Intercropping Bean with Maize on Bean Common Bacterial Blight and Rust Diseases,” International Journal of Pest Management 42 (1996): 51-54.
    “Geraniums and Begonias: New Research on Old Garden Favorites.” Agricultural Research (2010): 18-19.
    Hooks, C. R. R., Hinds, J., Zobel, E. and Patton, T., “Impact of Crimson Clover Dying Mulch on Two Eggplant Insect Herbivores.” Journal of Applied Entomology 137 (2013): 170-180.
    Jankowska, Beata. “Effect of Intercropping White Cabbage with French Marigold and Pot Marigold on Diamondback Moth.” 73 (2010): 107-117.
    Jankowska, Beata, Elz.bieta Je˛drszczyk, Małgorzata Poniedziałek. “Effect of Intercropping Carrot Daucus carota L. with French Marigold Tagetes patula nana L. and Pot Marigold Calendula officinalis L. on the Occurrence of Some Pests and Quality of Carrot Yield.” Acta Agrobotanica 65 (2012): 133-138.
    Karlidag, Huseyin and Ertan Yildirim. “Strawberry Intercropping with Vegetables for Proper Utilization of Space and Resources.” Journal of Sustainable Agriculture 33 (2009): 107-116.
    Moreau, Tara L., P. R. Warman, J. Hoyle. “An Evaluation of Companion Planting and Botanical Extracts as Alternative Pest Controls for the Colorado Potato Beetle.” Biological Agriculture & Horticulture 23 (2006): 351-370.
    Schultz, Brian, Cruz Phillips, Peter Rosset and John Vandemeer. “An Experiment in Intercropping Cucumbers and Tomatoes in Southern Michigan.” Scientia Horticulturae, 18 (1982): 1-8.
    Uvah, I. I. I. and Coaker, T. H., “Effect of Mixed Cropping on some Insect Pests of Carrots and Onions.” Entomologia Experimentalis et Applicata 36 (1984): 159-167.
    Yildirm, Ertan and Ismail Guvenc. “Intercropping Based on Cauliflower: More Productive, Profitable and Highly Sustainable.” Journal of Sustainable Agriculture 23 (2006): 29-44.
    Yu, Jing Quan. “Allelopathic Suppression of Pseudomonas solanacearum Infection of Tomato (Lycopersicon esculentum) in a Tomato-Chinese Chive (Allium tuberosum) Intercropping System.” Journal of Chemical Ecology 25 (1999): 2409-2417.
    Wanga, Koon-Hui, Cerruti R Hooksa, and Antoon Ploegb. “Protecting Crops from Nematode Pests:Using Marigold as an Alternative to Chemical Nematicides” Department of Plant and Environmental Protection Sciences, University of Hawaii at Mänoa Department of Nematology, University of California, Riverside Plant Disease, (2007): 1-6.
    Flores-Sanchez, D., A. Pastor, E. A. Lantinga, W. A. H. Rossing, M. J. Kropff. “Exploring Maize-Legume Intercropping Systems in Southwest Mexico.” Agroecology and Sustainable Food Systems 37 (2013): 739-761.
    Makinde, Eyitayo A., Olukemi T. Ayoola, Esther A. Makinde. “Intercropping Leafy Greens and Maize on Weed Infestation, Crop Development, and Yield.” International Journal of Vegetable Science 15 (2009): 402-411.

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  4. Growing beans on corn doesn’t work in the UK. The corn varieties that do reasonably well here don’t grow tall enough to support beans, which can easily grow twice as tall and more. Add in the third sister, squash, and you get three poor crops.
    Thank you for giving me some pointers for my own research (as scientific as I can make it but too small-scale to be definitive). I too have found that aromatic plants make no difference that I can see. And as for planting mint as a companion! How do you get rid of the mint afterwards?

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      • We’d like to know more about your success. How did you measure things? What were your results? How did you run your controls?

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        • Jude said that growing beans on corn doesn’t work in the UK and you just get 3 poor crops; I was disputing that assertion because I have observed it. I have no idea if the crop would have been slightly better or worse if done differently. But the fact is, it is possible to produce a respectable yield using Three Sisters in the UK.

          By the by, I had beans growing elsewhere on the same plot that year, which didn’t actually do so well. And I’ve used the same squash seed on a different plot and it was pretty poor in comparison. I haven’t got round to growing corn since. But there were too many other differing variables to understand what effect growing all 3 in the same plot had.

          Reply
  5. Wow. Thank you. Thought I was crazy for thinking all of these were bs.

    I have always liked the structural or shade ones (especially now that I live in southern CA and providing shade to some heat-sensitive plants is critical for much of the year). For the past few years I’ve been growing parsley and cilantro under tomatoes, radishes under tomatillos, and onions anywhere they’ll fit.

    This year I’m going to try to ‘companion plant’ some winter squash in my grass lawn to shade the grass so it doesn’t grow as much and also hopefully doesn’t dry out as quickly; hopefully the additional water and nutrient competition can be combated by focusing resources at the base of the squash plants. Mostly I’m just doing it because they take up *so* much room and I’d much rather have that room be in the lawn than in my carefully prepared, v limited raised bed space.

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  6. I have a pretty big garden here in the High Desert of California. Starting late July thru Sept we have a lot of problems with spider mites and white flies.

    I used to spray essential oils, rosemary based but I have so much work to do in the gardens that I just didn’t have time to keep up with it because it requires doing it every few days.

    I grow hydroponically (coir/perlite/vermiculite)…one day I was cleaning off one of the tables I keep my squash hydro buckets on.

    I took a couple of them which were dying because of the spider mite, white fly problem and put them down next to a huge French Lavender I have growing along the fence. My intention was to let them die and then throw them away.

    I kind of forgot about them being there, they were kind of hidden in the lavender.

    I had a chance to really look at them this week and guess what? No whiteflies or spider mites and the squash (bush type) has almost doubled its size with new leaves. I even shook the plants to see if the white flies would fly out, and I inspected for spider webs.

    Frankly I am astonished. Going from all crinkled up to looking like they were growing in the spring. So I am going to keep an eye on them, if this works thru out the next month I will be a happy camper indeed.

    I would also like to note that we are heavy into beetle/squash bug season and I also see none of them on these squash plants although at nite my screen door is covered with them.

    Reply
  7. Hi,

    Thanks for sharing some great information on this site.

    I am getting ready to plant new raised beds and I was doing some research on companion planting. After reading your post I came across this research from Iowa State University.

    https://www.farms.ag.iastate.edu/files/CompanionPlanting.pdf

    It does seem to indicate that there may be some benefits although this is a single planting under some adverse weather conditions. I will continue to look for additional published research.

    Reply
    • You always have to be careful about such studies.

      1) No indication it is not published – maybe there is a problem with it.
      2) A comment like ” which indicates any intercropping with a companion plant is more advantageous than none” is certainly incorrect. Some of the vegetables did not see improvement with some companions, and they did not test “all possible companions” so how can they reach this conclusion?
      3) No indication how they measured damage, and no statistics – so we don’t really know what the data shows.

      At best it shows there may be some benefit.

      Reply

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