I reached the town of Maesteg, in Wales, where the weed was first reported in the wild, late in the afternoon. I had seen so much of it already that day, and in such calm profusion, that I was no longer sure what I was expecting to find in the place of its original escape. Since it breached the redbrick walls of the Royal Botanic Gardens at Kew, in West London, at some point during the 1850s, Japanese knotweed has colonized pretty much every corner of the British Isles, but nowhere with more assiduity than the wet valleys and clean towns of South Wales. Many local people, particularly from older generations, swear that the plant, with its reddish, hollow canes and bowing stems of green, ovate leaves, has been there all along.

Illustration by Matthew Richardson

There was a playing field low in the valley, next to the fast-flowing river that runs through the town. I parked the car and stood on a bridge above the water, looking for signs of the weed. John Storrie, a curator of the Cardiff Museum who died in 1901, was responsible for the early sighting in Maesteg. In The Flora of Cardiff, published in 1886, he described around sixty foreign plant species that had gained a foothold in the lower half of Wales. Storrie found most of these exotics growing on heaps of ballast that had been dumped along the coastline, the jetsam of docking ships, but he noted that Polygonum cuspidatum (one of many Latin names that existed for the weed before biologists settled on Fallopia japonica) was “very abundant on the cinder tips” near the town, among the mines and darkened hills about five miles inland.

The knotweed is native to Japan’s volcanic fumaroles. It was born to be inundated by ash and poisonous gases for years at a time. The Welsh valleys of the late nineteenth century were a home away from home. In the 1880s, Maesteg was emerging from half a century of primitive iron production and was about to sink into a hundred years of coal mining. Across Europe, plant nurseries advertised the hardy newcomer as fodder for cattle; its canes a good material for matchsticks; its underground stems, or rhizomes, an effective agent for stabilizing sand dunes. It was, they promised, “inextirpable.” No one knows whether it was originally planted near Maesteg with some purpose in mind or arrived from somewhere else — as it usually does — by means of a wandering root. In 1992, scientists showed that just 0.7 grams of knotweed rhizome, a fingernail, can spawn a new infestation.

The weed entered Britain in a box of forty Chinese and Japanese plants that was opened by the clerks at Kew Gardens on August 9, 1850. Specimen number 34, between a tree peony and a fan palm, was labeled Polygonum sieboldii. It was a simple shrub, known in Japan as itadori, or “pain puller,” and was used in the preparation of medicinal teas. The crate had been sent to London by Philipp Franz von Siebold, a Bavarian doctor and ethnologist who dedicated his life to opening the closed nation of Japan to the world and trying to make money from the subsequent rupture. Following the botanical custom, he offered his plants to Kew in the hope that the clerks would reciprocate and send him some Asian stock that he would be able to sell from his nursery on the outskirts of the Dutch city of Leiden. But Siebold’s box wasn’t up to snuff. “On account of the bad selection he is written to, telling him that only six of them are probably new to us,” reads the clerk’s note in the archives.

Of those six, plant number 34 was (and remains) sterile — or, more specifically, the weed does not produce male gametes. Dug up outside Nagasaki, most likely by one of Siebold’s Japanese students, she survived the six-month sea voyage to the Netherlands and subsequent transplantation in the nursery at Leiden with no other means of regeneration than her rhizomes. This means that the weed now present in more than 70 percent of the 3,859 ten-kilometer recording squares of the British Isles is a single female clone. It is the same story in most of mainland Europe. With the exception of a few massive matings with her cousin, the less-intrusive giant knotweed, itadori has conquered by what biologists call “vegetative spread” alone. People who make a living killing itadori, most of them men, often describe the weed as the largest female organism on the planet.