The Victorians brought Japanese knotweed to Britain as an ornamental garden plant, but they underestimated how vigorous it is. Alan Titchmarsh fights his way through dense Japanese knotweed in a graveyard in Swansea. It is the fastest-growing plant in Britain so, despite dying back in winter, it will be back in the spring when it can grow by as much as an inch a day. By the summer it can be 10 feet tall and completely smothering anything that tries to grow below it. This is why Swansea has appointed Britain's first knotweed-control team, battling with machetes and powerful weed-killers. But the problem is that knotweed grows through underground runners and a new plant can grow from a mere fragment of one of those runners. Not even concrete can stop it. It has been estimated that to eliminate it entirely would cost over a billion pounds. Luckily, our particular strain can not produce seeds so there is a limit to how fast it can spread. But if a mutation or hybrid created fertile plants, then Britain would be smothered in it.