Since it was introduced to the UK in 1850, Japanese knotweed has gone from novel ornamental plant to rampant invasive species. Madeleine Finlay speaks to journalist Samanth Subramanian about the huge costs associated with finding it on a property, and Dr Sophie Hocking explains what the plant, and our attempts to control it, might be doing to the environment.. Help support our independent journalism at theguardian.com/sciencepod

There's a science fiction novel from the 1950s called The Day of the Triffids.

It's about an aggressive species of carnivorous plant which takes over the world.

You might not think a plant could be that terrifying in real life, But there's 1 species that strikes fear into anyone looking to buy somewhere to live.

This once admired ornamental plant is so hard to kill and grows so rampantly that homeowners can face being sued if they don't declare its presence when they sell a property.

But how did Japanese knotweed end up in the UK?

And what might our attempts to get rid of it be doing to the environment?

From The Guardian, I'm Madeline Finlay and this is Science Weekly.

Samantha Bramanian, you're an author and journalist and you recently wrote a long read for The Guardian called The War on Japanese Knotweed.

Now, that might sound quite extreme for a plant.

So tell me, what is it about this knotweed that makes it worth inciting a war over?

Well, the first thing to say about knotweed is that it looks like a relatively harmless plant.

It was imported into the UK in the 1800s, in the mid-1800s, and it was actually used as an ornamental fixture in a lot of British gardens.

It puts out these lovely little creamy flowers late in autumn, and otherwise has these big heart-shaped leaves and really a knotweed stand in a garden by itself.

But in that harmlessness is also hidden a kind of in eradicability or well, knotweed is indestructible almost.

And I think that is really the source of a lot of fear and anxiety that people have with knotweed, which is that once you've found a stand or a colony of knotweed, it becomes very difficult to remove it.

And it therefore just grows underground and overground until it takes over an entire patch of land.

This doesn't always happen, mind you, but it can happen, the chances are good.

And so the question of what happens to a garden with knotweed in it or indeed what happens to a patch of habitat with plenty of biodiversity, what happens to that when knotweed is introduced into the ecosystem, that's a really genuine botanical and biological fear.

And you said that it was imported over into the UK.

So where is this plant from originally and tell me a bit about the history of how it made its way here and really all over the world?

We usually refer to knotweed here in the UK as Japanese knotweed, so that sort of gives away the origin of the species, so to speak.

But it was imported into the UK by a German doctor.

He had been in Japan in the first half of the 19th century and he was habitually inclined to bring back species of plants that he found to be quite exotic.

And so this German doctor brought back cuttings of knotweed and installed them in his nursery in Leyden.

But they must have gone to plenty of other nurseries across England and Scotland and once they came into the country they were, as I said, you know, they were admired for how handsome and robust they looked.

In the late 19th century a trend in the UK called wild gardening, tried to position the ideal garden as something that wasn't meticulously curated, that allowed a certain amount of wildness into its appearance, and knotweed was the perfect plant for that.

And so what's the scale of its spread now?

It's difficult to quantify because there's been no major survey done of knotweed in the UK.

So for example, 1 statistic that people often cite is that 5% of homes across the UK have knotweed growing in them.

There's another study that I saw from 2021 that divided the island of Britain into nearly 3, 900 equal squares by area.

And knotweed had affected around 3, 100 of those squares.

So again, you can see that it's there across the island and it's quite extensive.

You can spot it in the Orkney Islands, in the Shetland Islands, all across Scotland, but predominantly you will find it in major cities.

And then in Wales and Southern England, so the Cornwall area.

But I went out to Wales to report a part of the story and knotweed is everywhere.

It's a genuinely huge problem out in the countryside.

We know that if you find knotweed on a property it can make a difference of tens of thousands of pounds to the value of that house.

And we're starting to see even successful lawsuits against sellers and surveyors as a result of finding knotweed.

There's a common misconception about knotweed, which is that it can burst through concrete and solid walls and things like that.

What it can do is if it happens upon a crack in a wall or if it happens upon a crack in tarmac or concrete it can sort of insinuate itself into that.

And then sometimes it can also mass up against a fence and eventually collapse it.

I've heard of cases of knotweed being found inside houses.

And even there, it's sort of wormed its way in through some crack or maybe through a pipe that's coming into the house from outside.

But the biggest kind of damage that it does to houses is really sort of the takeover of its garden.

And I think there's an interesting legal aspect or a legal sort of question that comes into all of this, which is really unquantifiable.

It's sort of an aesthetic, which is the ability to enjoy your garden, for example, in an aesthetic sense.

A number of lawsuits have raised this and kind of used that question in awarding damages or in regulating these lawsuits and the law of people suing when knotweed is found in gardens.

And even though that isn't quantifiable, you can't put a number to how much you enjoy a garden less when there's knotweed in it.

I think it's the kind of thing that leaves open a window for lawyers to really press for more money and I think courts have been quite ready to grant it.

I mean the biggest kind of award in damages and other payments that I heard about is £200, 000 on a £700, 000 house.

But what impact is Japanese nutweed having on the environment?

It's not a native plant, so it's an invasive non-native species.

The impacts to us as humans are very well known.

They have huge economic impacts, but it's also really negatively impacting biodiversity as well.

Dr Sophie Hocking, an ecologist and botanist at Swansea University, with a special interest in invasive plants.

We're currently in a climate crisis and also a biodiversity crisis.

Invasive species are 1 of the kind of top threats to biodiversity worldwide.

Japanese knotweed in particular, because it's such a competitive plant it can grow so rapidly and dominate native habitats, which means that eventually it kind of snuffs out native plants and just kind of out-competes native species.

It has better access to light because it can grow very tall, it can shade out other species and because of its perennial nature as well, at the end of the growing season where it starts to die back, it deposits a lot of organic matter onto the soil and that prevents any seedlings of any other species whether they're native or not to grow through in the new season.

I mean this is obviously a really fearsome plant but why is it so difficult to get rid of?

So it's all to do with these rhizomes under the ground.

So rhizomes are essentially very well adapted stems, they're stem tissue, and they grow under the ground.

And generally from an ecological perspective, their role is to kind of help a plant basically look for nutrients in its environment.

So the fact that it has this really dense kind of storage of biomass under the ground, that's where it kind of lives once it dies back in the winter.

So when you look at a Japanese knotweed plant, what you're seeing is the above ground biomass, right?

We see the stems, we see the leaves, we see the flowers.

But actually, the plant is a lot bigger than that.

And most of its biomass you can find under the ground in the form of those rhizomes.

The rhizomes act as a storage for carbohydrates.

So it's basically, we think of a battery.

They provide the plant with energy and that's how it can keep coming back year on year.

In those rhizomes as well, you have what we call a bud bank.

So when you, for example, spray Japanese knotweed or what you see above ground, or you might cut off the stems, you're killing 1 part of the plant or you're cutting off 1 bit of the plant, but it has multiple nodes that it can regrow from under the ground.

Usually what happens, especially in the UK, is that where you have Japanese knotweed invaded in a given habitat, if you think for instance where it grows along railway lines or riverbanks, any sort of disturbance within that soil system will mean that fragments of those rhizomes get washed away down the stream or kind of get spread along these linear kind of pathways and then wherever they're deposited that plant can take root and start growing again.

How has it evolved these incredible tactics to kind of stay alive despite all our best efforts?

I mean, what's its kind of natural environment and is there anything there that keeps it in check?

Japanese knotweed is from places like Japan and China, even parts of Russia as well.

So if you were to go out to 1 of these places, you'd see it growing in all sorts of different habitats and ecosystems.

But you know, this is a plant that can grow on the sides of active volcanoes.

So it's really, really well adapted to disturbance.

It really thrives in disturbed areas, which is why, partly why it's got these really interesting and impressive adaptations that make it such a good invasive species in the UK.

Throughout its native range there are some kind of forces that keep it in check, so like I said those natural disturbance regimes, so things like volcanoes for example, which is why we can't really kill it with fire in the UK.

There are other insects and invertebrates that will also feed off the plants, so different herbivores, but for the most part it is just really well adapted and even throughout its native range in certain urban areas it can also be an issue out there as well.

And this is of course where your research comes in because scientists like yourself have been working on how we keep this in check or how we remove it entirely And a lot of the methods for getting rid of Japanese knotweed are kind of questionable from an environmental perspective.

So first of all, what are the different options that you have if you do want to tackle knotweed?

Historically, there's been a whole different range of methods suggested to be successful for managing Japanese knotweed.

These include various types of herbicides at different timings and different application rates, but also physiochemical methods as well.

So where we combine, say, herbicide application in 1 part of the season with a physical method later on that could be cutting, it could be mowing, it could be digging and encapsulation, as well as geomembrane covering.

So there's a whole range of different approaches that we could use and they all have varying degrees of efficacy and sometimes the best approach will depend on the situation at hand.

Some members of my research group at Swansea University published a very comprehensive study actually evaluating different control methods for Japanese knotweed in 2018 and what they found is that a single foliar application of glyphosate once a year was the most effective way of controlling Japanese nutweed and that's the approach now that's kind of supported by Welsh government as best practice.

And of course that's a chemical that can have an impact on the plants around it as well.

What about the environmental impact of doing that?

I guess you kind of have to balance getting rid of the nutweed versus the impact of the method that you're using to get rid of it.

And that's 1 of the key focuses of my research actually is what are the environmental impacts of these methods?

We know the best ways of controlling Japanese knotweed now, but what does that actually mean for the environment and our human health as well and how costly these different impacts are?

A paper I published recently this year kind of focused on that.

So we looked at a few different methods, all of which has been proposed to be viable ways of controlling Japanese knotweed in the UK and evaluated their impacts to ecology, to the wider environment, so what are the carbon emissions associated with these methods, as well as the impacts to human health, and then how costly these methods are to implement as well.

So Based on my research, the most sustainable way of controlling Japanese knotweed is the most simplest and most effective way.

So all that means is that essentially we know that the best way of controlling Japanese knotweed is annual glyphosate spraying and it turns out that that's actually the most sustainable approach as well.

It's not really the answer that many of us want to hear, but you know there are less materials and processes involved in that approach and all of that equates to less environmental impacts essentially.

Sophie, Speaking to you and to Samanth, I've gained a bit of respect for this plant.

I can't say that I'm still not, you know, slightly fearful and anxious about it, but it's certainly changed my perspective and I wonder has researching Japanese knotweed made you feel any differently about it?

You definitely do develop some sort of admiration for it because it's amazing.

Like if you think about the adaptations this plant has, how it's able to grow in kind of any habitat really, You can't help but be impressed.

It can kind of overcome a lot of different environmental pressures, which is in part why it's such an amazing invader and so successful at being an invasive plant.

It's really, really interesting to research this plant.

And yeah, like you said, you can't help but have some kind of respect for it.

Samanth, in May 2019 the Science and Technology Committee criticised the overly cautious approach we take to knotweed in the UK and they said that a significant industry is built around controlling Japanese knotweed and that mortgage lenders in other countries don't treat the plant with the same degree of caution.

So how worried should we really be about knotweed?

Well it's an interesting question because I spoke for this piece to plenty of other people in other countries and the impression I got first of all is that knotweed isn't as prevalent in homes in Sweden or France, for example, or in the Netherlands.

And so there are public budgets to deal with knotweed in those locations.

I think it's only in the UK that the question of private citizens having to pay for knotweed removal has come into play.

But the second question also is sort of a more philosophical and existential 1, which is that this plant has been brought over by human activity.

So human beings cultivated it, encouraged it to grow, and it has just done what its biological nature impels it to do, which is spread and grow and reproduce.

And really, it is the human activity of industrialization and commercialization that has enabled its spread in the UK and in other countries.

And so the question of what to do with knotweed is a commercial 1, it's true, but it's also a philosophical 1.

And on the philosophical front, I think there's no cut and dried answer.

Thanks again to Samanth Subramanian and Dr.

You can find a link to Samantha's fantastic long read about Japanese knotweed on the podcast webpage at theguardian.com and I would really recommend searching it out and taking a look.

The producer was Silas Gray, the sound design was by Tony Onuchukwu, and the executive producer is Ellie Burey.


Speaker 0: This is The Guardian.

Speaker 1: There's a science fiction novel from the 1950s called The Day of the Triffids. It's about an aggressive species of carnivorous plant which takes over the world. You might not think a plant could be that terrifying in real life, But there's 1 species that strikes fear into anyone looking to buy somewhere to live. Japanese knotweed. This once admired ornamental plant is so hard to kill and grows so rampantly that homeowners can face being sued if they don't declare its presence when they sell a property.

But how did Japanese knotweed end up in the UK? Why is it so damaging? And what might our attempts to get rid of it be doing to the environment? From The Guardian, I'm Madeline Finlay and this is Science Weekly. Samantha Bramanian, you're an author and journalist and you recently wrote a long read for The Guardian called The War on Japanese Knotweed.

Now, that might sound quite extreme for a plant. So tell me, what is it about this knotweed that makes it worth inciting a war over?

Speaker 0: Well, the first thing to say about knotweed is that it looks like a relatively harmless plant. It was imported into the UK in the 1800s, in the mid-1800s, and it was actually used as an ornamental fixture in a lot of British gardens. It puts out these lovely little creamy flowers late in autumn, and otherwise has these big heart-shaped leaves and really a knotweed stand in a garden by itself. I think it's quite nice to look at. It's a pretty big plant.

But in that harmlessness is also hidden a kind of in eradicability or well, knotweed is indestructible almost. And I think that is really the source of a lot of fear and anxiety that people have with knotweed, which is that once you've found a stand or a colony of knotweed, it becomes very difficult to remove it. And it therefore just grows underground and overground until it takes over an entire patch of land. This doesn't always happen, mind you, but it can happen, the chances are good. And so the question of what happens to a garden with knotweed in it or indeed what happens to a patch of habitat with plenty of biodiversity, what happens to that when knotweed is introduced into the ecosystem, that's a really genuine botanical and biological fear.

Speaker 1: And you said that it was imported over into the UK. So where is this plant from originally and tell me a bit about the history of how it made its way here and really all over the world?

Speaker 0: We usually refer to knotweed here in the UK as Japanese knotweed, so that sort of gives away the origin of the species, so to speak. But it was imported into the UK by a German doctor. He had been in Japan in the first half of the 19th century and he was habitually inclined to bring back species of plants that he found to be quite exotic. And so this German doctor brought back cuttings of knotweed and installed them in his nursery in Leyden. But they must have gone to plenty of other nurseries across England and Scotland and once they came into the country they were, as I said, you know, they were admired for how handsome and robust they looked.

In the late 19th century a trend in the UK called wild gardening, tried to position the ideal garden as something that wasn't meticulously curated, that allowed a certain amount of wildness into its appearance, and knotweed was the perfect plant for that.

Speaker 1: And so what's the scale of its spread now?

Speaker 0: It's difficult to quantify because there's been no major survey done of knotweed in the UK. So for example, 1 statistic that people often cite is that 5% of homes across the UK have knotweed growing in them. There's another study that I saw from 2021 that divided the island of Britain into nearly 3, 900 equal squares by area. And knotweed had affected around 3, 100 of those squares. So again, you can see that it's there across the island and it's quite extensive.

You can spot it in the Orkney Islands, in the Shetland Islands, all across Scotland, but predominantly you will find it in major cities. So London, of course, is a huge hub. And then in Wales and Southern England, so the Cornwall area. But I went out to Wales to report a part of the story and knotweed is everywhere. It's a genuinely huge problem out in the countryside.

Speaker 1: We know that if you find knotweed on a property it can make a difference of tens of thousands of pounds to the value of that house. And we're starting to see even successful lawsuits against sellers and surveyors as a result of finding knotweed. But why? What exactly does it do?

Speaker 0: There's a common misconception about knotweed, which is that it can burst through concrete and solid walls and things like that. That actually isn't true. What it can do is if it happens upon a crack in a wall or if it happens upon a crack in tarmac or concrete it can sort of insinuate itself into that. And then sometimes it can also mass up against a fence and eventually collapse it. I've heard of cases of knotweed being found inside houses.

And even there, it's sort of wormed its way in through some crack or maybe through a pipe that's coming into the house from outside. But the biggest kind of damage that it does to houses is really sort of the takeover of its garden. And I think there's an interesting legal aspect or a legal sort of question that comes into all of this, which is really unquantifiable. It's sort of an aesthetic, which is the ability to enjoy your garden, for example, in an aesthetic sense. A number of lawsuits have raised this and kind of used that question in awarding damages or in regulating these lawsuits and the law of people suing when knotweed is found in gardens.

And even though that isn't quantifiable, you can't put a number to how much you enjoy a garden less when there's knotweed in it. I think it's the kind of thing that leaves open a window for lawyers to really press for more money and I think courts have been quite ready to grant it. I mean the biggest kind of award in damages and other payments that I heard about is £200, 000 on a £700, 000 house.

Speaker 1: So that's what it can do to our homes. But what impact is Japanese nutweed having on the environment?

Speaker 2: It's not a native plant, so it's an invasive non-native species. The impacts to us as humans are very well known. They have huge economic impacts, but it's also really negatively impacting biodiversity as well.

Speaker 1: Dr Sophie Hocking, an ecologist and botanist at Swansea University, with a special interest in invasive plants.

Speaker 2: We're currently in a climate crisis and also a biodiversity crisis. Invasive species are 1 of the kind of top threats to biodiversity worldwide. Japanese knotweed in particular, because it's such a competitive plant it can grow so rapidly and dominate native habitats, which means that eventually it kind of snuffs out native plants and just kind of out-competes native species. It has better access to light because it can grow very tall, it can shade out other species and because of its perennial nature as well, at the end of the growing season where it starts to die back, it deposits a lot of organic matter onto the soil and that prevents any seedlings of any other species whether they're native or not to grow through in the new season.

Speaker 1: I mean this is obviously a really fearsome plant but why is it so difficult to get rid of?

Speaker 2: So it's all to do with these rhizomes under the ground. So rhizomes are essentially very well adapted stems, they're stem tissue, and they grow under the ground. And generally from an ecological perspective, their role is to kind of help a plant basically look for nutrients in its environment. So the fact that it has this really dense kind of storage of biomass under the ground, that's where it kind of lives once it dies back in the winter. So when you look at a Japanese knotweed plant, what you're seeing is the above ground biomass, right?

We see the stems, we see the leaves, we see the flowers. But actually, the plant is a lot bigger than that. And most of its biomass you can find under the ground in the form of those rhizomes. The rhizomes act as a storage for carbohydrates. So it's basically, we think of a battery.

That's essentially what rhizomes are. They act as a battery. They provide the plant with energy and that's how it can keep coming back year on year. In those rhizomes as well, you have what we call a bud bank. So it's a lot of different growth nodes.

So when you, for example, spray Japanese knotweed or what you see above ground, or you might cut off the stems, you're killing 1 part of the plant or you're cutting off 1 bit of the plant, but it has multiple nodes that it can regrow from under the ground. Usually what happens, especially in the UK, is that where you have Japanese knotweed invaded in a given habitat, if you think for instance where it grows along railway lines or riverbanks, any sort of disturbance within that soil system will mean that fragments of those rhizomes get washed away down the stream or kind of get spread along these linear kind of pathways and then wherever they're deposited that plant can take root and start growing again.

Speaker 1: How has it evolved these incredible tactics to kind of stay alive despite all our best efforts? I mean, what's its kind of natural environment and is there anything there that keeps it in check?

Speaker 2: Japanese knotweed is from places like Japan and China, even parts of Russia as well. That's its kind of native range. So if you were to go out to 1 of these places, you'd see it growing in all sorts of different habitats and ecosystems. But you know, this is a plant that can grow on the sides of active volcanoes. So it's really, really well adapted to disturbance.

It really thrives in disturbed areas, which is why, partly why it's got these really interesting and impressive adaptations that make it such a good invasive species in the UK. Throughout its native range there are some kind of forces that keep it in check, so like I said those natural disturbance regimes, so things like volcanoes for example, which is why we can't really kill it with fire in the UK. There are other insects and invertebrates that will also feed off the plants, so different herbivores, but for the most part it is just really well adapted and even throughout its native range in certain urban areas it can also be an issue out there as well.

Speaker 1: And this is of course where your research comes in because scientists like yourself have been working on how we keep this in check or how we remove it entirely And a lot of the methods for getting rid of Japanese knotweed are kind of questionable from an environmental perspective. So first of all, what are the different options that you have if you do want to tackle knotweed?

Speaker 2: Historically, there's been a whole different range of methods suggested to be successful for managing Japanese knotweed. These include various types of herbicides at different timings and different application rates, but also physiochemical methods as well. So where we combine, say, herbicide application in 1 part of the season with a physical method later on that could be cutting, it could be mowing, it could be digging and encapsulation, as well as geomembrane covering. So there's a whole range of different approaches that we could use and they all have varying degrees of efficacy and sometimes the best approach will depend on the situation at hand. Some members of my research group at Swansea University published a very comprehensive study actually evaluating different control methods for Japanese knotweed in 2018 and what they found is that a single foliar application of glyphosate once a year was the most effective way of controlling Japanese nutweed and that's the approach now that's kind of supported by Welsh government as best practice.

Speaker 1: And of course that's a chemical that can have an impact on the plants around it as well. What about the environmental impact of doing that? I guess you kind of have to balance getting rid of the nutweed versus the impact of the method that you're using to get rid of it.

Speaker 2: And that's 1 of the key focuses of my research actually is what are the environmental impacts of these methods? We know the best ways of controlling Japanese knotweed now, but what does that actually mean for the environment and our human health as well and how costly these different impacts are? A paper I published recently this year kind of focused on that. So we looked at a few different methods, all of which has been proposed to be viable ways of controlling Japanese knotweed in the UK and evaluated their impacts to ecology, to the wider environment, so what are the carbon emissions associated with these methods, as well as the impacts to human health, and then how costly these methods are to implement as well.

Speaker 1: So what did you find?

Speaker 2: So Based on my research, the most sustainable way of controlling Japanese knotweed is the most simplest and most effective way. So all that means is that essentially we know that the best way of controlling Japanese knotweed is annual glyphosate spraying and it turns out that that's actually the most sustainable approach as well. It's not really the answer that many of us want to hear, but you know there are less materials and processes involved in that approach and all of that equates to less environmental impacts essentially.

Speaker 1: Sophie, Speaking to you and to Samanth, I've gained a bit of respect for this plant. I can't say that I'm still not, you know, slightly fearful and anxious about it, but it's certainly changed my perspective and I wonder has researching Japanese knotweed made you feel any differently about it?

Speaker 2: Oh yeah, 100%. You definitely do develop some sort of admiration for it because it's amazing. Like if you think about the adaptations this plant has, how it's able to grow in kind of any habitat really, You can't help but be impressed. It can kind of overcome a lot of different environmental pressures, which is in part why it's such an amazing invader and so successful at being an invasive plant. It's really, really interesting to research this plant.

And yeah, like you said, you can't help but have some kind of respect for it.

Speaker 1: Samanth, in May 2019 the Science and Technology Committee criticised the overly cautious approach we take to knotweed in the UK and they said that a significant industry is built around controlling Japanese knotweed and that mortgage lenders in other countries don't treat the plant with the same degree of caution. So how worried should we really be about knotweed?

Speaker 0: Well it's an interesting question because I spoke for this piece to plenty of other people in other countries and the impression I got first of all is that knotweed isn't as prevalent in homes in Sweden or France, for example, or in the Netherlands. It's largely in public areas. And so there are public budgets to deal with knotweed in those locations. I think it's only in the UK that the question of private citizens having to pay for knotweed removal has come into play. But the second question also is sort of a more philosophical and existential 1, which is that this plant has been brought over by human activity.

So human beings cultivated it, encouraged it to grow, and it has just done what its biological nature impels it to do, which is spread and grow and reproduce. And the knotweed itself bears no guilt. It isn't sort of a sentient being. It is merely trying to live. And really, it is the human activity of industrialization and commercialization that has enabled its spread in the UK and in other countries.

And so the question of what to do with knotweed is a commercial 1, it's true, but it's also a philosophical 1. And on the philosophical front, I think there's no cut and dried answer.

Speaker 1: Thanks again to Samanth Subramanian and Dr. Sophie Hocking. You can find a link to Samantha's fantastic long read about Japanese knotweed on the podcast webpage at theguardian.com and I would really recommend searching it out and taking a look. And that's all from us. The producer was Silas Gray, the sound design was by Tony Onuchukwu, and the executive producer is Ellie Burey.

We'll be back on Tuesday. See

Speaker 2: you

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Japanese knotweed: why is it so damaging and can it be stopped?