History of Japanese knotweed spread

The story of knotweed and its impact on Britain begins with an entry in the Inwards Books, ancient ledgers stored in the archives of Kew Gardens. Dated August 9th, 1850, the entry records the arrival of 40 knotweed plants from a nursery in Leiden, brought by German doctor Philipp Franz von Siebold. This marked the earliest known instance of knotweed in Britain, its introduction intertwined with the country's history of trade.

During the colonial, industrial, and post-industrial eras, foreign plants like knotweed, rubber, cinchona trees, wisteria, and rhododendrons were introduced to Britain through colonial connections and explorations. At the time, these plants were seen as beneficial and not recognized as potential invaders that could disrupt the native ecosystems. Knotweed, in particular, was touted for its ability to stabilize sand dunes, produce beautiful flowers, and serve as fodder for cattle. It even found favor in the wild gardening movement, recommended for its rugged charm. Little warning was given about its potential dangers.

As the 20th century unfolded, knotweed escaped the confines of gardens and spread rapidly across cities and countryside. It thrived in various environments, from riverbanks to neglected lots and industrial sites. Its presence became notorious in southeast Cornwall, where it earned the name "Hancock's Curse" after spreading from a private garden. Unknowingly, people unintentionally aided its spread by unknowingly transporting hidden rhizome fragments in soil. Knotweed became ubiquitous alongside roads, highways, shipyards, and railway tracks in areas of industrial activity.

A 1977 paper by the Botanical Society illustrated the invasion of knotweed through four maps. The first map from around 1900 showed a mostly knotweed-free Britain, except for a few dots in western Wales. By 1976, the fourth map revealed knotweed had spread extensively, reaching as far north as the Orkney and Shetland Islands. In 1981, the Wildlife and Countryside Act made it illegal to allow knotweed to grow in the wild, but its spread continued. A 2021 study divided the British Isles into squares and found that knotweed had encroached upon a significant number of them, displacing native plants.

While the ecological impact of knotweed remained largely unnoticed by the general public, its impact on the property market was undeniable. Philip Santo, a surveyor for Abbey National building society, encountered his first residential knotweed in 1995. However, the true concern emerged as property values soared. Between 1995 and 2016, average house prices in England and Wales tripled. Knotweed-infested properties were valued significantly lower, sometimes tens of thousands of pounds less than identical properties without knotweed. The exact threat it posed to property integrity was not fully understood, leaving questions about its encroachment and the potential impact on property values and lending.

Banks, unsettled by uncertainty, began refusing to finance homes affected by knotweed. Santander, a major mortgage lender, made it clear that they would not lend due to the unknown risk. Other banks followed suit, causing panic among the public. In 2012, the Royal Institution of Chartered Surveyors (RICS) released a protocol considering any knotweed within a seven-meter radius a threat to property values. Though cautious, this guideline impacted millions of homes overnight.

Despite these measures, the fundamental problem of effectively combating knotweed remained unsolved. Philip Santo, receiving numerous calls seeking guidance on assessing knotweed-infested properties, faced the challenge of providing answers without comprehensive literature or clear solutions.