Japanese knotweed: What does it look like, and how can you get rid of it?

Japanese knotweed: What does it look like, and how can you get rid of it?

Japanese knotweed is among the invasive non-native species that could be costing the UK economy around £4 billion annually, according to a new study.

After the ash tree-killing fungus Hymenoscyphus fraxineus, Japanese knotweed is the second most expensive, costing the UK around £246.5 million a year, as reported by the Guardian.

The research suggests tackling invasive non-native species now costs double what it did a decade ago, up from £1.7bn in 2010.

Japanese knotweed is known to cause property damage, potentially depreciating the value of homes and even stopping other plants from growing in the garden.

So how can you spot the invasive plant, and what can you do to stop it?

What is Japanese knotweed?

Japanese knotweed is a type of invasive plant species notorious for its propensity to spread, causing damage to building structures and costing up to tens of thousands of pounds.

“In winter, the plant dies back to ground level but, by early summer, the bamboo-like stems emerge from rhizomes deep underground to shoot to more than 2.1m (7ft), suppressing all other plant growth,” explains the Royal Horticultural Society (RHS).

The weeds are almost impossible to remove by hand or eradicate with chemicals.

What damage does Japanese knotweed cause?

The weeds are known to block pipework and going on to clog sumps and drainage pits.

Water pipes and cabling are other underground infrastructures susceptible to damage in its wake.

Japanese knotweed is also known to have major environmental and legal implications.

In fact, specialists from across the UK have documented a huge portfolio, including evidence of major damage which has been caused by Japanese knotweed over the past 25 years.

What kills Japanese knotweed?

A weed killer, which is glyphosate-based, is considered one of the better options for getting rid of the weed.

Glyphosate is a systemic herbicide that takes several applications over up to four seasons to completely eradicate it.

Spraying or injecting the stems with other chemicals which are approved herbicides may also help but, again, the substance needs to be resprayed, taking up to three years to fully treat and kill the weed.

What does Japanese knotweed look like?

According to Warner Goodman, signs you may have Japanese knotweed include:

  • Zigzag stems

  • Lush green-colour leaves

  • Shield-shaped leaves with flat bases

  • Bamboo-style stems

  • Red-tinged shoots

  • Found in dense clumps

  • In July, it will sprout clusters of white flowers

  • Between September and November, it will leave brown stems once the leaves have died back

If you are concerned you may be at risk of Japanese knotweed you can use the Heatmap which shows sightings of the weed in your area highlighting the level of risk using a scale of yellow to red colouring.