Why Companion Planting Doesn’t Work. A Provocative Look At The Dangers And Inconsistencies Of Companion Planting & The Evidence That It Might Not Work.

I know that some companion planting ideas work. They make sense and I’ve seen evidence that they work. But in this article, which many may disagree with, I want to look at the evidence that it doesn’t always work as well as some say. And I accept that some people will find my ideas provocative, but unless we’re open minded and look for evidence, for and against any idea, we are not being thorough in our investigation. And believing something just because of confirmation bias, because it sounds attractive, or that someone else has written, without evidence, doesn’t make us good gardeners.

Why Companion Planting Doesn’t Work. Farmers now grow rape with buckwheat and it clearly works.
Oil seed rape grown with buckwheat companion planting

Below there is a list of arguments to companion planting and my response to them. The thing is there are pros and cons to every situation. And in this article I want to provoke you into thinking about plants and their requirements. I want to help you gain sense of perspective and reality when it comes to growing plants.

If you don’t agree with all I say, and I’m not infallible, please comment below and provide your evidence.

But first let’s start with an oft quoted companion planting suggestion. It’s the one suggesting marigolds should be planted around tomatoes, to keep them whitefly free.

Interestingly this one is backed up by science. Research shows strong evidence that this works in specific situations but less well in others. That might account for the mixed reports and garden shed stories. According to the research the active ingredient here is limomene.

Allelopathic plants: Some plants release chemicals that can inhibit the growth of other plants. For example, walnut trees release a chemical called juglone that can harm other plants, such as tomatoes and potatoes.

But who is planting walnuts and tomatoes together? No one does, and though this example is often cited, few others are ever mentioned. So why is it used as an argument for or against companion planting when it is barely relevant?

Plants with similar needs: Plants that have similar needs, such as water and nutrient requirements, can compete with each other. For example, we are often told not to plant tomatoes and cucumbers together, as they both have similar needs. But if we have well nourished soil why should that be a problem? There can be plenty of nutrition and water for both if we manage the crops properly. There are however other reasons not to grow them together, but I’ll cover that later.

Plants with different root depths: Similar to above we are often told that plants with different root depths can compete with each other for water and nutrients, so shouldn’t be grown together. For example, it is recommended that carrots and lettuce shouldn’t be grown together together, as carrots have deep roots, whilst lettuce has shallow roots. But in nature lots of plants with different rooting depths grow well together. Take woodlands where trees and spring flowers such as primroses and foxgloves coexist. Arguably they survive because they have different rooting requirements. And if we look at the seven layers that are typically considered essential to a food forest it seems to prove different rooting depths are fine.

Plants that attract pests: I read that companion planters believe some plants attract pests, and this stops them spreading to spreading to other plants. For example, they plant marigolds near tomatoes to stop the aphid going on the toms. However contrary to that there is also a recommendation not to plant marigolds near beans, as marigolds attract aphids and they’ll spread to the beans.. Surely both can’t be right? Companion plants either attract pests and keep them off of other crops or they attract them and they spread to crops.

And the other problem with this is that if we attract a pest and give it a cosy home, it will breed and spread further afield. That’s great for biodiversity but not for our vegetables! Life is never simple.

More Thoughts On Why Companion Planting Doesn’t Work.

A few more examples and the rationale for and against them.

Basil and rue: Basil and rue compete for water and nutrients so must not be grown together. Plants compete and we better get used to the idea.

I used to grow 10,000 tomatoes in greenhouses. If this competition argument is valid then they all competed and suffered because of it. The reality is they did complete but each had enough space and nutrients to crop well. If it didn’t work I would have had poor crops and gone broke!

Basil and brassica crops: Basil can apparently attract whiteflies to brassica crops, such as broccoli, cauliflower, and cabbage, so shouldn’t be grown together. In my experience the brassicas will do this on their own. They don’t need help!

Basil and tomatoes: Basil can inhibit the growth of tomatoes. Really? Why, how and where’s the evidence?

Mint family plants: Mint family plants, such as basil, mint, catnip, and lavender, can cross-pollinate and produce unwanted hybrids.

Lettuce survive all winter in my unheated greenhouse
Lettuce survive all winter in my unheated greenhouse

Can they? But arguably many plants such as sweetcorn can hybridise, but few seed companies advise against them being grown together. And even if they do hybridise we don’t have to grow the resulting plants.

Tomatoes and cucumbers: As discussed earlier it is often said that tomatoes and cucumbers have similar needs, such as water and nutrients, and therefore compete with each other. This misunderstands both crops. They actually have very different needs. Cues prefer a higher humidity than toms. And if toms experience a high humidity it prevents self pollination. They are best grown apart but not for the reasons of competition.

Carrots and lettuce: Carrots have deep roots, while lettuce has shallow roots, and they can compete with each other for water and nutrients. That’s a spurious argument if we irrigate. And if it is true how close together would create the issue? What spacing do they need before the alleged impact of growing together is eliminated. All the advice given is very general and lacks the necessary details if it is to make any sense to most gardeners.

Learning From History

We can learn a lot from older gardeners and from old gardening books. But we shouldn’t just repeat their ideas just because they have been repeated for years. We need to make the effort to understand the stories, and advice passed down through the generations, because only by understanding it can we make sense of it.

I’m not saying every companion planting idea is nonsense. Some clearly do work and I’ve provided evidence and the companion planting research where appropriate. But we have to be careful not to be lulled into the myth, spam and downright mendacious blog posts that push this stuff whilst they spam loads of adverts on their websites and blogs as we read it!

Supporting Research:


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Tag: Companion Planting Doesn’t Work

2 thoughts on “Why Companion Planting Doesn’t Work (Always)

  1. Marion Roberts says:

    Stefan, although I have an allotment, I grow maybe 60% herbaceous perennials, my favourite plant group. I have a book, ‘Perennials and their garden habitats’ by Richard Hansen & Friedrich Stahl, translated by Richard Ward. Very briefly, if plants of different root lengths are grown together, to mimic conditions found in nature, it will work, as each has their own soil space, therefore no competition. The ‘three sisters’ rule of planting also confirms this logic. Just a thought

    1. Stefan Drew says:

      Agreed. Simple examples that cut through to the reality.

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