How to Deal With Japanese Knotweed

Japanese knotweed is one of the most problematic invasive plants in the UK. It can damage buildings, affect property values, and is difficult to eradicate. Here is what you need to know.

How to identify Japanese knotweed

• Spring: Red/purple shoots emerge from the ground, growing rapidly.
• Summer: Tall bamboo-like stems (up to 3m) with large heart-shaped leaves and distinctive zig-zag pattern.
• Late summer: Clusters of small creamy-white flowers.
• Winter: Stems die back to brown, hollow canes. The plant is dormant but the roots are still alive.

The leaves are shovel-shaped with a flat base and pointed tip. The stems are hollow with purple speckles and grow in dense clumps.

Why is it a problem?

• Grows extremely fast – up to 10cm per day in summer.
• Roots can extend 3m deep and 7m horizontally.
• Can grow through tarmac, concrete and building foundations.
• Spreads from tiny root fragments – even 1cm can regrow.
• Outcompetes native plants and damages ecosystems.
• Can affect mortgage applications and property sales.

Legal requirements

It is not illegal to have Japanese knotweed on your land, but:

• You must not allow it to spread to neighbouring properties or into the wild.
• Knotweed waste is classified as controlled waste and must be disposed of at licensed facilities.
• You cannot put it in your green bin or compost it.

Treatment options

Professional treatment: For most situations, professional treatment is recommended. Specialists use herbicide injection or spraying over multiple years. They can also provide management plans and insurance-backed guarantees that may be required for property sales.

DIY treatment: You can treat small infestations yourself with glyphosate-based weedkiller, but it takes 3-5 years of repeated applications. Inject or spray the herbicide when the plant is actively growing (late spring to early autumn). Do not cut it down first – the leaves need to absorb the chemical.

Excavation: Digging out knotweed is possible but expensive. All contaminated soil must be removed to a depth of at least 3m and disposed of as controlled waste.

What NOT to do

• Do not cut it down and leave it: It will regrow stronger.
• Do not strim or mow: This spreads fragments that can root.
• Do not compost: It will survive and spread.
• Do not move contaminated soil: This is illegal without proper disposal.
• Do not ignore it: It will spread and become harder to treat.

If you are buying a property

Japanese knotweed must be declared on property information forms. If present, ask for evidence of professional treatment and an insurance-backed guarantee. Some mortgage lenders will not lend on properties with untreated knotweed.