The worst postcode for Japanese knotweed has been revealed – with one new homeowner claiming a hidden infestation will cost him £15,000.Whitehall, a small neighbourhood in Darwen, Lancashire, with a population of 9,000, has the highest number of recorded infestations in the country, new data shows. The fast-growing invasive plant spreads through an underground network of roots and can wreak havoc on gardens, property values and house sales.A total of 92 cases were logged in BB3 2, which sits in Blackburn with Darwen – itself named the worst-affected borough with 2,135 cases, or 40 per square mile.Nathan Hannaway, 33, lives with his partner and their two children in the worst-hit postcode, where he claims knotweed was concealed in the garden of their £430,000 dream home.The family stretched their budget to buy the five-bedroom Victorian semi in the leafy suburb in August last year.They moved from a two-bedroom house in Bury and hoped to transform the overgrown garden and create more space to expand their family.But soon after moving in, a neighbour asked Mr Hannaway whether Japanese knotweed was discussed during the sale. Nathan Hannaway, 33, stretched his budget to buy his dream home near Blackburn last year, only to discover the garden is riddled with a severe case of Japanese Knotweed Blackburn Old Cemetery, which suffers from a serious Japanese Knotweed infestation Mr Hannaway had planned to transform his new garden (pictured), until he realised it was plagued by the invasive plant Japanese Knotweed visible in the area named the worst postcode for the plant