He uncovered a nest of woody rhizomes that were as thick as wrists. They can stretch for twenty feet and are adapted to lie dormant in the ground for years at a time, in case the land above is destroyed by an eruption. “All of this knotweed is bits of von Siebold’s,” said Hathaway, as the plant towered over us. The nightmare is if she breeds. A single stem of Japanese knotweed can produce 190,000 flowers. As we talked on the hillside, tiny white seeds pattered onto my notebook and covered my hair. “If it suddenly decided to mutate and each of these grew into new knotweed,” said Hathaway, “then we’ve had it, I’m afraid.”

Sam Knight, “The Day of the Knotweed”