Japanese knotweed: how to spot it and what to do about it

Earlier this week, invasive plant specialist Environet UK, revealed the number of Japanese knotweed hotspots across the county this spring.

Bournemouth had 21 infestations within a 4km radius, with 23 in Poole, 18 in Swanage, 22 in Weymouth and 24 in Dorchester.

The data was compiled using data from Environet UK's interactive online tracker, Exposed: The Japanese Knotweed Heatmap.

Here we look at why Japanese knotweed is so invasive, how to spot it and what to do if you have it where you live.

Japanese knotweed. Picture: Environet UK (Image: Environet)

What is Japanese knotweed?

Japanese knotweed first arrived in UK in 1850 in a box of plant specimens delivered to Kew Gardens.

Favoured for its rapid growth and pretty heart-shaped leaves, it was quickly adopted by gardeners and horticulturalists who were oblivious to its invasive nature.

Knotweed hibernates over winter but in March or April it begins to grow, with red or purple spear-like shoots emerging from the ground which quickly grow into lush green shrubs with pink-flecked stems and bamboo-like canes.

Japanese knotweed. Picture: Environet UK

How to spot Japanese knotweed

· Asparagus-like spears emerge from the ground in early spring and begin to sprout pale green leaves with distinctive pink veins

· In May the plant starts to grow rapidly. The stems harden into bamboo-like structures and the leaves, which grow in a zigzag pattern up the stem, are lush, green and heart-shaped

· By mid-summer the plant grows at a rate of around 10cm per day, with mature plants forming dense stands two or three metres tall

· In August the plant blooms, with small clusters of creamy white flowers appearing on the upper leaf axials.

Japanese knotweed. Picture: Environet UK

Will it affect my home?

According to Environet UK, the plant can pose serious problems if left unchecked, with the potential to grow up through cracks in concrete, tarmac driveways, pathways, drains and cavity walls.

Environet UK said: "The roots can grow as deep as three metres and spread up to seven metres horizontally.

"While serious damage to property is rare thanks to regulation which requires knotweed to be dealt with when a property is sold to a buyer using a mortgage or if it encroaches across a garden boundary, it commonly impacts use of the garden, causes legal disputes between neighbours and can impact a property’s value by around five per cent."

Japanese knotweed. Picture: Environet UK

What if I'm trying to sell?

The company said property sales can proceed as long as a professional treatment plan is in place with an insurance-backed guarantee to satisfy mortgage lenders

Nic Seal, founder and managing director of Environet, said: “Japanese knotweed tends to strike fear into the hearts of homeowners but as long as they’re aware of its presence and take action to remove it before it causes any serious damage or spreads to a neighbour’s property, there’s no reason to panic.

"By publishing the 2022 hotspots for Dorset we hope to raise awareness and encourage people in the area to be vigilant for signs of knotweed as the growing season takes off, so they can act quickly if needed.

"Anyone living near or moving to one of these hotspots would be wise to check their garden carefully, enter their postcode into Exposed to find out how many known occurrences are nearby and if in doubt, seek expert help.”