Invasive weed specialists have revealed the UK areas most at risk from Japanese knotweed this autumn.
Experts say the nightmare plant – which can grow up to 10cm (4in) per day and tear through brickwork and concrete – could soon spread to new gardens.
Homeowners in some of the UK’s wettest regions are most at risk, they warned, as heavy rain and flooding could carry fragments of knotweed into areas where it has never been seen before.
Analysis of Met Office rainfall data reveals a stark divide between the UK’s wettest and driest regions – and it’s bad news for those in the north.
South Scotland recorded the most rainfall between September and December last year, with a staggering 770.9mm falling over the four–month period.
It was followed by North Scotland, which recorded 679.1mm, while North West England and North Wales saw 573.2mm.
Experts warn that if similarly wet conditions return this autumn, flooding, riverbank erosion and powerful water flows could help invasive plants hitch a ride downstream.
Jennifer Holmes, from Environment Controls, said: ‘Following a flood, homeowners may discover unfamiliar, fast–growing plants during the next growing season.’
The map reveals areas at more moderate risk include South West England and South Wales, Northern Ireland and the North East of England.
Areas at the lowest risk include Central England, South East England and East Scotland.
Japanese knotweed can spread when pieces of its underground stem system, known as rhizomes, break away and are transported to new ground – with even a very small viable fragment capable of producing a new plant.
Himalayan balsam – another invasive species – poses a similar threat, with its seeds able to enter rivers, streams and drainage channels before being carried to new locations.
That means homeowners could spot unfamiliar fast–growing plants appearing in their gardens after a flood, even if they have never previously had a problem with invasive species.
‘Autumn and winter storms don’t just cause water damage,’ Ms Holmes said.
‘Flooding, riverbank erosion and high water flows can carry invasive plant seeds and root fragments downstream into places where they were not previously growing.’
The consequences can be costly, with established Japanese knotweed capable of exploiting weaknesses in paving, walls and drainage systems, as well as complicating property sales and construction projects.
Japanese knotweed is incredibly durable and fast–growing, and can seriously damage buildings and construction sites if left unchecked
The consequences of knotweed can be costly. Pictured: Knotweed has started to destabilise a 15–foot retaining wall at the edge of a property in Swansea
Treatment can also become more expensive as an infestation becomes established, while landowners may face civil claims if the plant spreads onto neighbouring property.
However, experts stress that rainfall alone does not determine the risk, which also depends on nearby or upstream infestations, proximity to waterways, local flooding and riverbank erosion, and the movement of contaminated soil.
Homeowners who spot suspicious growth following flooding are advised to photograph it and seek professional identification – rather than reaching for the garden tools.
‘Suspected Japanese knotweed should not be dug up, cut or strimmed because this can distribute viable plant fragments,’ Ms Holmes said.
The warning comes after climate scientists reported that El Niño conditions have developed in the tropical Pacific.
Although El Niño’s influence on UK weather is indirect and not guaranteed, it can increase the likelihood of milder, wetter and windier conditions during autumn and early winter.
Japanese knotweed is notorious for its huge roots which can stretch 3m (10ft) deep and towering bamboo–like stems which can tear through brickwork and concrete.
Left unchecked, it can quickly take over gardens and cause long–term damage. Across the UK, there are now over 100,000 infestations recorded annually.
Experts have previously warned that bouts of warm weather can also cause Japanese knotweed to spread more aggressively.
Late August and early September are the prime time for the invasive plant to flower – and for people to spot an infestation.
Key indicators to look out for in summer months are white to cream–coloured flowers which grow 6 to 15cm long, with a feathery look.
Heart–shaped leaves could also be a warning sign. These are broad with a pointed tip, growing alternately along the stem.
You should also look out for hollow, bamboo–like stems that are green, red or purple, which form dense thickets.
According to specialists, Japanese knotweed affects an estimated one in 20 homes and can cut property value by up to 15 per cent.
