Property owners in Bristol, Merseyside and Greater London need to be mindful of the threat of Japanese knotweed, as that’s where most instances are reported.
Analysis by invasive plant specialist Environet found that Bristol is by far the worst affected location in England with 5.18 incidences per km², followed by Merseyside with 2.49 per km² and Greater London with 2.29 per km².
There are almost 60,000 known infestations of Japanese knotweed across England, while it’s thought 5% of homes are affected.
Emily Grant, director of Environet, said: “The best way to protect your property is to be aware of the risks and ensure you can identify knotweed at different times of the year.
“If you do have knotweed on your property, it’s wise to act immediately to treat or remove it before it has the chance to spread further. With professional assistance knotweed can be dealt with and the value of a property largely restored.
“If you’re buying or selling a property, consider instructing a Japanese knotweed survey to check for any sign of the plant on the property and its surroundings.”
The origin of knotweed sightings near ports such as Bristol and Liverpool is believed to be linked to historical shipping and trading practices lacking in biosecurity.
In the 19th and 20th centuries, ships returning to the UK from various parts of the world often carried ballast, including soil from foreign ports containing non-native plant material, which was then discarded to make room for the next cargo.
In traditionally industrial areas such as Greater Manchester, large quantities of soil were typically moved around through mining and other activities, aiding the plant’s spread.
Over the next few weeks knotweed will start to die back, as temperatures cool and light levels drop, sending the plant into its dormant winter phase. Its green leaves will die and fall from the branches, the stems will turn brown and brittle, and after the first few frosts the plant will have completely died back, leaving only the dead canes remaining.
However, the plant will re-emerge next spring, so autumn and winter is the ideal time to excavate it when the garden is not in use.
