Japan's Giant Knotweed Invaded England — Scientists Are Still Fighting It

追加: 2026-05-27

[00:00:00]In August 1850, a package arrived at Q Gardens in London. It came from the nursery of a German doctor named Philip France vonbold in Leiden, the Netherlands. Inside were 40 living plant specimens. The entry in Q's ancient inwards ledger, which still exists today, is brief and unremarkable.

[00:00:23]No one who wrote it suspected they were recording the beginning of one of the worst ecological and financial disasters in British history. The plant was called Falopia Japonica. It had hollow bamboolike stems, broad shovel-shaped leaves, and delicate cream flowers that bloomed late in summer when almost nothing else was flowering. Victorian gardeners loved it. Nurseries across Britain began propagating and selling it within years of its arrival. By the 1860s, it was being planted in parks along railway embankments and used to stabilize crumbling riverbanks. In 1879, the garden magazine described it as one of the most beautiful herbaceious plants in civilization.

[00:01:12]By 1951, the Royal Horicultural Society was still calling it a noble perennial best grown as an isolated specimen. What neither the magazine nor the society could see at the time was that the plant was already escaping. It was already in the rivers and the roadsides. It was already past the point where stopping it would be simple. It is now the most aggressive invasive plant in Britain. It covers almost every square kilometer of England. It grows through tarmac, concrete, and the foundations of houses.

[00:01:49]It can grow 10 cm in a single day and sends ryomes 2 m deep into the soil and 7 m horizontally from the main stem in every direction.

[00:02:01]Those underground ryomes are almost impossible to kill. A fragment the size of a fingernail, if buried in soil, can regenerate into a new plant. The Wildlife and Countryside Act 1981 made it a criminal offense to cause its spread. In Japan, where this plant is native, it barely registers as a problem. In Britain, it has no enemies at all. And for 170 years, nothing we have tried has been enough to stop it.

[00:02:32]This is the full story of how a single female plant sent to a Victorian garden in the summer of 1850 came to cost the British economy 166 million pounds every year. And why scientists are now deploying an insect the size of a grain of rice in a lastditch attempt to finish a fight that has been going on longer than most countries have had indoor plumbing.

[00:02:57]To understand the scale of what happened, you first need to understand what Japanese knotweed actually is in its home country. In Japan, fallopia japonica grows natively on volcanic slopes, particularly on the flanks of Mount Fuji and in the disturbed ground active volcanic terrain. The soil there is nutrient poor, heavily disturbed, and frequently buried under fresh lava and ash. Not we devolved to colonize those sites pushing through rocky ground regenerating rapidly after destruction and establishing itself before anything else could. It is in the ecological terminology a pioneer species built for survival in environments of extreme disturbance. In Japan it is kept in check by a community of intents and fungi that evolved alongside it over millions of years. More than 180 species of arropod natural enemies have been identified feeding on it in its native range. The plant and its predators exist in a balance so stable that notweed in Japan is considered ecologically unremarkable.

[00:04:08]It grows along roadsides and river banks in moderate stands and goes largely unnoticed. The moment it arrived in Britain, that balance disappeared entirely. There were no predators. There were no fungi adapted to attack it. The rich European soils were not volcanic rubble, but deep, moist, fertile ground.

[00:04:32]A plant that had evolved to fight its way through some of the harshest growing conditions on Earth was suddenly given perfect soil, abundant moisture, mild winters, and absolutely nothing to stop it. Vonabel had no way to know any of this. He was a serious scientist, one of the first European botonists to study Japanese plants in detail, and he genuinely believed he was introducing something beautiful and useful. He had seen notweed used in Japan as animal foder and as a plant for stabilizing disturbed ground. He sent it to Q as part of a larger consignment of specimens along with hostas, magnolia, and clemetus that are still in British gardens today and still causing no trouble whatsoever.

[00:05:23]What he did not send were the 180 species of insects that kept notweed in check at home.

[00:05:31]Stories like this rarely make the headlines. If you want to keep finding them, the slow disasters, the stranger solutions, the things happening just outside where most news cameras point, subscribe to this channel. New video every week. The Royal Botanic Gardens at Q distributed the plant to nurseries across Britain. Those nurseries sold it enthusiastically.

[00:05:57]Gardeners who found it becoming too vigorous did what Victorian gardeners typically did with unwanted plants. They threw it over the garden wall, dumped it on waste ground, or dropped it in the nearest stream. And here is where the biology of notweed becomes something close to a nightmare. Every European specimen is a clone of that single female plant sent to Q in 1850. The plant cannot reproduce by seed in Britain because it has no male counterpart, but it does not need one. A fragment of ryome carried downstream in a flood deposited on a riverbank will establish a new colony. A fragment on the blade of a digger moving between construction sites will start a new infestation wherever it is deposited.

[00:06:47]Notweed was naturalized in the wild by the late 1880s. It was first recorded growing outside cultivation in Maestig, South Wales in 1886.

[00:06:58]By 1900, it was appearing in London. By the midentth century, it was established across most of the country. Network Rail, which owns and manages Britain's railway infrastructure, received more than 11,000 complaints about Notweed between 2011 and 2019. Over 6,000 of those complaints were specifically about infestations spreading from railway land onto adjacent private property. The railway embankments that Victorian engineers helpfully planted with notweed for erosion control had become a national distribution network for the species, spreading it along every rail corridor in the country. on development sites. The presence of notweed now requires excavation and removal of contaminated soil which must be taken to licensed landfill.

[00:07:51]To prepare the site for the London 2012 Olympic Games, contractors spent £70 million removing notweed from the Olympic Park in East London. A single domestic property with a moderate infestation can cost between £2,800 and £8,000 to treat with no guarantee of success. DERA's estimate for the total cost of eradicating notweed from Britain is 1.56 billion and that estimate dates from 2003.

[00:08:25]Notweed has not been standing still since then. Property law has become entangled with notweed in ways that are genuinely difficult to untangle. Sellers in England and Wales are legally required to declare notweed on a property disclosure form before any sale can proceed. More than 1 and a half million homes are currently affected.

[00:08:47]Research by the invasive plant removal company Environet found that notweed is wiping an estimated 21.4 4 billion pounds from property values across Britain with affected homes typically losing around 5% of their value.

[00:09:03]Mortgage lenders increasingly refuse to lend on properties where notweed has not been professionally treated under a certified management plan backed by insurance. Almost a third of surveyed buyers said they would not purchase a property affected by notweed under any circumstances even after professional treatment. What began as an ornamental plant from a Victorian catalog is now a legal liability that can follow a homeowner through multiple property transactions.

[00:09:35]The obvious question which scientists began asking with some urgency in the early 2000s is why notweed keeps winning. The answer lies underground.

[00:09:47]Herbicides work on what they can reach.

[00:09:49]Glyphosate, the most effective chemical treatment currently available, kills the above ground growth. But the ryome system extends 2 m down and spreads 7 m in every direction. Any herbicide treatment that does not reach every fragment of the ryome will fail. And reaching every fragment through multiple growing seasons of repeated application is both expensive and not always possible. Along river banks and water courses, herbicide use is restricted by environmental regulations. On road veres and railway embankments, the scale of infestation makes systematic treatment practically impossible. The plant regrows from roots that the treatment never reached, and the cycle begins again. Herbicide campaigns, when they work at all, typically require four to five consecutive years of treatment to bring a single patch under management.

[00:10:48]not eradication, management.

[00:10:51]The goal for most land owners is not to eliminate the plant, but simply to prevent it spreading further while keeping the visible growth down. That is the best outcome most people can realistically hope for with current methods. Physical removal is even more expensive and carries its own risk.

[00:11:11]Digging out knotweed disturbs the soil.

[00:11:14]Soil disturbance spreads ryome fragments. Unless every fragment is physically removed from the site and taken to licensed landfill, excavation can turn a localized infestation into multiple new ones. Several well doumented cases exist of construction work spreading knotweed to previously clean sites. The harder you fight it mechanically, the more of it you can inadvertently create.

[00:11:42]In 2000, a program began at the Center for Agriculture and Biosciences International, known as CABI, that took a fundamentally different approach. Scientists flew to Japan and began systematically cataloging the natural enemies of fallopia japonica in its native range. They were looking not for something that could kill notweed outright, but for something that had evolved so precisely to target notweed that it would not harm any European plant species. The goal was classical biological control, the deliberate introduction of a natural predator from a plant's home range to suppress it in the invaded territory. It is the same approach used successfully against other invasive weeds worldwide.

[00:12:31]The challenge is ensuring the introduced organism cannot become new ecological problem in its own right.

[00:12:39]From more than 180 candidate species, the researchers identified one that passed every test. It is called aphalara and it is a celid, a small sapsucking insect related to aphids and white flies. An adult aphilara itadori is roughly 2 mm long. In Japan, it feeds exclusively on notweed and its close relatives. Host range testing across 87 plant species confirmed that only 1.52% of eggs were laid on plants outside the invasive notweed group. It was, as far as the scientists could determine, a specialist that could not survive without its host plant.

[00:13:26]In 2010, CABI received approval from the UK government for the first sanctioned release of a biological control agent against a weed in Britain. Celids were released at field sites in England and Wales. It was a historic moment in European conservation science and the beginning of a deeply frustrating chapter.

[00:13:50]The celids survived. They fed on notweed. But when winter arrived, most populations died. The strain released collected from the island of Kiushu in southern Japan was adapted to a milder climate than the British winter.

[00:14:07]Overwintering populations were sparse.

[00:14:10]Year after year, releases were made and monitored. And year after year, the celids failed to establish the self- sustaining populations that would actually begin suppressing the knotweed at any meaningful scale. In 2019, a CABI team went back to Japan specifically to find a better adapted population. They found it at Murakami in Nagata Prefecture, a region on Japan's northwestern coast with a climate marketkedly closer to Britain's own gray winters. At Murakami, they found notweed with severe leaf curling damage, the distinctive signature of heavy silid feeding pressure, and they collected the insects responsible. The Murakami strain, as it became known, underwent the same battery of host specificity tests as the original Kyushu population.

[00:15:03]It passed. DER approved its release in 2021.

[00:15:08]CABI released it at sites in England at both Japanese Notweed and Bohemian Notweed plots and began monthly monitoring. The 2022 and 2023 field results showed damage at both release sites. In the Bohemian notweed population, a celid carrying an overwintering morph was found in 2023.

[00:15:33]For the first time since the biocrol program began, insects were surviving British winters. New release sites were added in 2024.

[00:15:44]This is not a solved problem. It is the beginning of what may be a decadesl long process.

[00:15:52]Classical biological control does not work in months. It works over years, sometimes generations, as a predator population builds to a scale where it exerts meaningful pressure on its host.

[00:16:05]The researchers at Kabi are careful about this. They do not describe the silid as a solution. They describe it as a potential long-term management tool that could reduce the need for costly herbicide campaigns, ease the pressure on riverbank ecosystems where chemical treatment is restricted, and progressively weaken the plant across the country without the expense of excavation or the liability of spreading ryome fragments during mechanical removal. What makes notweed biologically remarkable also makes it a uniquely difficult target.

[00:16:41]The plant's ryome system is not just an underground stem. It is an energy reserve. During autumn, notweed pulls carbohydrates from its above ground growth back down into the ryome before the stems die back. That reserve fuels the explosive spring growth. The shoots can grow 10 cm per day and they can push through tarmac and crack mortar.

[00:17:06]Herbicides applied to the foliage only reach the ryome if the plant is actively transllocating nutrients when treated.

[00:17:14]The timing matters enormously.

[00:17:17]Treatment in the wrong part of the growing season has almost no effect on the roots. The celid by contrast feeds continuously throughout the growing season weakening the plant's ability to photosynthesize and limiting the carbohydrate reserves it stores underground.

[00:17:35]One female Aphalara itadori can lay up to 700 eggs during her lifetime and each egg produces a nymph that feeds on notweed sap. If the population establishes the cumulative feeding pressure across an entire growing season could progressively deplete the reserves that make notweed so difficult to kill.

[00:17:57]The broader story here is one that ecologists find painfully familiar.

[00:18:02]Notweed in Britain is not an anomaly. It is a case study in what happens when a species is introduced to a new ecosystem without the web of natural relationships that kept it in check at home. The gray squirrel brought from North America to British estates in 1876 is now driving red squirrels to functional extinction on mainland England. The American mink escaped from fur farms, devastated waterv populations across Britain over decades.

[00:18:34]Roodendran ponticum, another Victorian garden favorite, has smothered the understory of native woodlands across the Atlantic fringe of Britain and Ireland.

[00:18:44]All of them arrived the same way as something beautiful or useful introduced without the enemies that in their home ranges made them ecologically ordinary.

[00:18:55]The difference between those species and notweed is that notweed was actively propagated and distributed at industrial scale before anyone understood what it was. Nurseries sold it by mail order.

[00:19:09]Local councils planted it on roadsides.

[00:19:12]It was not just let in through a gap in the fence. It was invited, accommodated, and spread by human hands across a country it would spend the next century dismantling.

[00:19:25]Fonbolt is an instructive figure because he was not reckless. He was a serious scientist and an important one. His collections laid the foundation for European understanding of Japanese flora and many of the plants he introduced have caused no problems. whatsoever.

[00:19:43]Notweed was the exception. It happened to combine every characteristic that makes a plant dangerous in a new environment. Rapid vegetative spread, deep risomes, resistance to herbivvery, tolerance of disturbance, and a complete absence in its new range of the biological checks that controlled it at home. He could not have predicted it.

[00:20:08]The science that would have allowed him to predict it did not exist in 1850.

[00:20:13]What exists now is a small insect 2 mm long overwintering in test plots scattered across the English countryside.

[00:20:23]Scientists check the notweed leaves each month for the characteristic curling that means the sids are still there.

[00:20:30]They count the damage. They photograph the plants. They note the survival of insects that were never supposed to be here, feeding on a plant that was never supposed to be here either, and they wait. The knotweed pushing through the concrete of a driveway near you grew from a single female plant that arrived in a cardboard box 175 years ago. Every hollow stem, every ryome grinding through the foundations of someone's house is a clone of that same individual.

[00:21:03]And the insect that may finally bring it down is 2 mm long and traveled here from a foggy coastal prefecture in northwestern Japan because it happened to have a harder winter to endure.

[00:21:16]Whether that will be enough is something no one alive today will know for certain. Share your thoughts below.