Companion Planting Ideas Have Been Around For Centuries, But Do They Work, What Does Science & Experience Tell Us?

The move to cut out the use of harmful pesticides in gardens and allotments has encouraged many people to investigate companion planting and the internet is full of companion planting charts, articles, videos and suggestions that many of us want to believe. Indeed the companion planting concept is compelling BUT there is very little verifiable research for much of it.

Thats not to say that companion planting ideas are all rubbish. Some of it clearly works. But, sadly, much more of it is myth, muck and mirrors than truth. Part of the problem is that because many of us want to believe in it, it has become a topic where charlatans and con merchants know it is easy to spin a web of deceit that will capture the unwary. In other cases it is repeated because people have believed what they have read and do keep repeating it as if it were true.

Lastly we have confirmation bias playing a part. This is where we see the tendency in people to search for, interpret, favour, and recall information in a way that confirms or supports one’s prior beliefs or values.

An Alternative To Companion Planting

Though in this article I’m going to show some examples of companion planting that works, there is also a need to consider alternatives. For example the leaf image on this page shows how Encarsia formosa wasps can be used to predate whitefly. No companion planting is needed for this method of pest control. But let’s get back to companion plants.

Companion Planting Logic

Encarsia formosa predated whitefly on tomato leaf from wiki images.
Encarsia formosa predated whitefly on tomato leaf

The world is complex and science reveals new, often incredible ideas, on a regular basis. However, as gardeners we have to apply some logic to each new idea we read. That’s not always easy, especially if we think like a human.

Let me give you an example. Look at a flower being visited by a pollinator. To us it might look like many yellow spring flowers. but the visiting insect isn’t like us. It has a superpower. It can see in ultraviolet. To the insect the flower isn’t plain yellow. It is a much complex and colourful flower complete with nectar guides, of colours we cannot see or even perceive with our limited visual ability.

And so it is with every other sense. We perceive the world in a totally different way. To us onions and garlic have strong and obvious smells. But is this any reason to suppose that a bee can even smell them?

Our big mistake is to assume that people, insects, pests and other organisms all perceive the world as we do.

Many studies have shown that the numbers of pest insects found on crop plants are reduced considerably when plant diversity is increased within the crop.

Warwick University

More Companion Planting Mistakes

When it comes to companion planting, the idea is that certain plants, when grown together, can have a beneficial impact on each other. In some cases this is true. But often, not for the reasons that we read online.

For example, planting sweetcorn alongside climbing beans with gourds at soil level is the basis to the Three Sisters. It’s a practice we’ve learnt from certain North American native tribes and it works for them. The plants work together with the sweetcorn providing structural support for the beans, which can climb them. This is a clear example of companion planting in action. But it’s not what most of us imagine as an example of companion planting (CP). We tend to think of things like growing basil with tomatoes because it allegedly improves the tomatoes flavour. Or growing marigolds to protect tomatoes from insect pests.

And whilst confirmation bias often convinces people that this works, the evidence is somewhat thinner for many CP idea we see promoted online.

Another example often promoted in American literature is about lettuce, which allegedly prefers cooler conditions and can struggle in the heat of mid-summer when it tends to bolt. By planting it behind larger tomato plants, which provide shade, we can create a cooler environment for the lettuce. This is apparently another practical demonstration of companion planting. To me this one is rubbish when viewed from a UK perspective. I grew lettuce for a living, half a million of them a year. Very few ever bolted, it was a rare exception. But I grew them in unshaded greenhouses and in the middle of a large unshaded field throughout the summer. They don’t really need shade, it’s a myth and I say that with decades of commercial experience where bolting was never an issue month after month, summer to winter. I’ve written an article of bolting elsewhere if you want to learn more about what causes bolting.

However, before we delve deeper into companion planting, it’s essential to clarify that while these examples work, or don’t work, it doesn’t mean that every suggested pairing is effective or ineffective. The realm of companion planting is vast, with hundreds of different combinations, some of which are beneficial, and others not so much.

The Science of Companion Planting

When it comes to companion planting, it’s crucial to recognize that most suggestions for gardeners haven’t been scientifically tested. Research funds primarily go towards agricultural and horticultural scenarios, not home gardens. As a result, many gardening companion planting suggestions lack scientific support.

An example of this is the popular book “Carrots Love Tomatoes” by Louise Riotte, which, despite its widespread acclaim, lacks scientific evidence for its claims. Additionally, many combinations in the book lack logical explanations for why they work. Therefore, it’s important to approach companion planting with a healthy dose of skepticism.

Confusion also reigns where the ideas are uncertain. For example I often hear that we should plant marigolds with our tomatoes. My first question when told this is which of the 50 species of marigold works .. or do they all work? Or do different species solve different pest problems? So far no one has ever shown me any evidence that answers any of these questions. That leaves me doubting all I’m told. But of course there might one one species of marigold that works for one pest problem there might even be research that supports it. And it could be that this then fails because gardeners use the wrong species of marigold! That would be ironic wouldn’t it?

However, I don’t totally rubbish companion planting. Some of it definitely works. For example where plants are grown to attract pollinators that also predate the insects that are pests on specific crops. Or crops that hide crops from predators.

An example of the latter is where UK Oil Seed Rape (OSR) is now often grown with buckwheat. The buckwheat hides the OSR from the flea beetles that otherwise devastate OSR seedlings. This means insecticides aren’t needed and the cost saved pays for the buckwheat seed. The magic part of this mix is that OSR tolerates cold weather and will survive frost and snow, but the buckwheat dies back as soon as it is hit by a light frost. The dense planting also has another effect. It reduces weed growth simply because it grows rapidly and out competes many weed species.

There are other examples of companion planting working extremely well in well researched and verifiable agricultural situations. I mention more of them towards the end of this article.

Defining Companion Planting

One challenge with companion planting is that there is no universally accepted definition of companion planting. However, one possible definition states that companion planting is “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.”

Key points of this definition include:

  1. Growing two or more types of plants together.
  2. At least one plant benefits from the presence of the other(s).

This definition helps narrow down the scope of companion planting, excluding practices such as using cover crops or intercropping, which have different purposes.

Good Companions and Bad Companions

While the definition talks about the benefits of good companions, it doesn’t mention the flip side: bad companions. In gardening, some plants can inhibit the growth of others. For instance, walnuts and tomatoes are often considered bad companions because tomatoes typically don’t grow well under walnut trees. But nor do most plants. the reason being that walnuts produce juglone (C10H6O3) which inhibits growth and hence competition from other plants. The process is called allelopathy.

The shade, lack of water, competition for nutrients etc is also going to make toms under trees a non runner .. but juglone is the main issue here.

Sadly, the concept of bad companions is less explored, and there is little scientific evidence to support it. Some mention allelopathy, where one plant inhibits the growth of others through chemical compounds, but this is often demonstrated in controlled lab settings and not necessarily in real-world scenarios.

Companion Planting and Organic Gardening

Companion planting is often linked to organic gardening, but the two are not mutually exclusive. You can practice organic gardening without embracing companion planting and vice versa.

Benefits of Companion Planting

Companion planting offers various ways in which one plant can benefit another. These benefits include:

  1. Physical Protection or Support: For example, tall shrubs can provide wind protection, allowing more delicate plants to flourish without damage. Tall plants also produce shade which can prevent moisture loss.
  2. Trap Cropping: One plant attracts pests away from another, serving as a decoy. However, the reverse is also true, this approach can sometimes attract more pests to the garden overall.
  3. Modifying the Environment: Some plants can alter the environment, such as providing shade or reducing weed growth.
  4. Attracting Beneficials: Certain flowering plants can attract pollinators that benefit nearby vegetable crops.
  5. Deterring Pests: Some plants can deter pests, keeping them away from other nearby plants.
  6. Supplying Nutrients: In cases like legumes adding nitrogen to the soil, one plant can provide essential nutrients for its companion. However, the timing and extent of these benefits can vary. Living mulches are an example of this practice.
  7. Weed Suppression: If one plant can suppress weeds, it may improve the growth of its companion. Yet, if the suppressing plant is not a crop itself, it could become a weed competing with other plants. The other issue here is that the companion plant is taking up the space that a crop plant could be using. So crop yield may drop.
  8. Improving Flavour: Some say companion planting can enhance the flavour of certain crops when grown together, although the scientific basis for this is often lacking. The basil and tomato companions is an example of this. But I’ve yet to see any explanation of how it is meant to work, or event that it does.

Companion Planting Nonsense

Amid the world of companion planting, you may come across statements that seem questionable or contradictory. For instance, the claim that all herbs work as companion plants, attracting beneficial insects and repelling pests, is highly doubtful. The idea that herbs inherently know which insects are beneficial and which are pests doesn’t hold up to scrutiny.

Similarly, the notion of planting a wilting plant next to dill to revive it seems more like folklore than sound gardening advice. And the concept that some plants can both deter and attract the same pests simultaneously raises doubts about the effectiveness of such strategies.

Should You Try Companion Planting?

While there are indeed instances where companion planting can be beneficial, it’s a complex practice with variable results. If you come across companion planting advice that lacks a logical explanation or references to scientific studies, it’s advisable to approach it with caution.

Companion planting can be fruitful when practiced with care and common sense in your garden. It’s essential to discern between well-supported combinations and unsubstantiated claims. In the upcoming posts, we will explore specific examples to determine which have enough scientific backing to be worthwhile.

Companion Planting: Tips and Strategies for a Thriving Garden

Diversify Your Plantings

Avoid monocultures, where a single type of plant is grown extensively in rows or blocks. Monocultures make it easy for pests and diseases to target their preferred plants and spread rapidly. Mix and match plant species and types to create a more resilient garden.

Utilise Tall Plants

Deploy taller plants like peas or sweet corn to provide partial shade for crops that tend to bolt, such as coriander, lettuce, and spinach. Light shade can help these sensitive plants thrive in hot weather, where during out is an issue. But they would be better in full sunlight with adequate moisture!

Harness the Power of Herbs

Scatter herbs throughout your garden and vegetable plot, as most of them boast strongly scented leaves that can help deter insects. The aromatic presence of herbs can act as a natural pest repellent.

Experiment with Intercropping

Practice intercropping by sowing fast-growing crops like lettuce or radishes between widely spaced rows of slower-growing crops like Brussels Sprouts or parsnips. This not only maximizes space but also hinders weed growth, as weeds compete for nutrients, light, and water, and can transmit diseases.

Of course whether this is true companion plating, or just better space utilisation, depends entirely on the definition of companion plating we decide to use.

Attract Beneficial Wildlife

Encourage the presence of beneficial creatures in your garden by planting insect-friendly or bird-friendly species either alongside your crops or in close proximity. These plants can draw natural predators like birds that consume slugs, hoverflies that prey on aphids, and bees that aid in pollination.

Beware Vigorous Companion Plants

Be cautious when planting companions like mint, which tend to grow rapidly and might overwhelm your crops. To control their growth, consider cultivating mint in containers or, as I do, designate areas of your garden. for example I grew mint to encourage pollinators around my plum trees. It’s nothing to do with plums as such, just a convenient place to grew the mint. We also grew beds of mint in the food forest as it encourages pollinators and swamps out weeds. I know some people say it becomes invasive, but in my experience when grown like this it isn’t very deep rooting and is easy to remove by pulling it at the end of the season when the stems are drying out. 90+% of the roots pull out this way. and the residue is easy to deal with when it regrows.

Agricultural Companion Planting That Works

Agricultural research indicates that companion planting can serve as an alternative food source for the natural enemies of pests. For example, the limiting factor that governs numbers of predators isn’t always the prey. It can be things like pollen that is essential in the fecundity of the predator.

Oil seed rape grown with buckwheat companion planting

An example are various syrphid species, whose larvae are voracious predators of aphids,. The adults must feed on pollen and nectar to breed. So providing these plants as companions to the crop being protected makes a huge difference.

It works because nectar is a carbohydrate source, providing energy, whilst the pollen supplies nutrients for egg production.

In English wheat fields in England research shows that over 95% of gravid female syrphids were found with pollen in their gut. As a result, the right flowering companion plants can increase the breeding potential and longevity of many parasitic species that predate pests.

There is a proviso with the above. The companion plants need to flower at the right time to ensure maximum prefatory when needed. Otherwise wee get predators with nothing to predate!

Companion plants can also be used as what are called banker plants. Banker plants are normally non-crop species that are deliberately infested with a non-pest insect that improves biological control. It does it by providing the natural enemies with alternative prey for times when there is an absence of pest species.

Research Evidence

Kostal, V. & S. Finch (1994). Influence of background on host-plant selection and subsequent oviposition by the cabbage root fly (Delia radicum). Entomologia Experimentalis et Applicata 70: 153-163.

Finch, S. & M. Kienegger (1997). A behavioural study to help clarify how undersowing with clover affects host-plant selection by pest insects of brassica crops. Entomologia Experimentalis et Applicata 84: 165-172.

Finch, S. & Collier, R.H. (2000). Host-plant selection by insects – a theory based on ‘appropriate/inappropriate landings’ by pest insects of cruciferous plants. Entomologia experimentalis et applicata 96, 91-102.

Finch, S. & Collier, R. H. (2003). Insects can see clearly now the weeds have gone. Biologist, 50 (3), 132-135 (PDF Document)

Finch, S., Billiald, H. & Collier, R.H. (2003). Companion planting – do aromatic plants disrupt host-plant finding by the cabbage root fly and the onion fly more effectively than non-aromatic plants? Entomologia Experimentalis et Applicata109: 183-195.

Morley, K., Finch, S. & Collier, R.H. (2005). Companion planting – behaviour of the cabbage root fly on host plants and non-host plants. Entomologia experimentalis et applicata, 117, 15-25.

Finch, S. & Collier, R.H. (2012). The influence of host and non-host companion plants on the behaviour of pest insects in field crops. Entomologia experimentalis et applicata, 142, 87-96.

Image Attribution: MycompCC BY-SA 4.0, via Wikimedia Commons
Tag: companion planting

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