Companion Planting: Truth or Myth?

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.

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.

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.

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

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

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/