The species is native to East Asia and was introduced to the UK in the 1800s, where it has since spread out of control.
It can grow up to four inches a day and forms dense thickets which can kill native plant species, making it particularly harmful to the flora of the Norfolk Broads.
Norwich is the biggest hotspot for Japanese knotweed in 2025 so far (Image: Environet) Environet has been mapping the places with the highest rate of knotweed reports in Norfolk.
Norwich has had the highest number of reports by far, with an area around the city having 301 sightings of the invasive plant this year.
It trumps King’s Lynn, which has only had 71 reports of Japanese knotweed so far.
Fakenham has had 33 sightings, Great Yarmouth has had 28 and Dereham has had 25.
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Swaffham and Thetford have both seen 14 instances of the invasive plant reported, while Poringland and North Walsham have been invaded by the plant just 13 times.
Knotweed can grow up to three metres deep underground and spread up to seven metres horizontally, seeping through cracks in concrete, driveways and drains.
A DEFRA report in 2023 estimated the cost of tackling knotweed to the UK economy at nearly £250 million a year.