If you own land, let property or are about to develop a site, Japanese knotweed is worth taking seriously. It spreads quickly, it is difficult to kill, and its roots can exploit weaknesses in paving, drains and walls. Midsummer, when it is in full growth, is the easiest time to spot it.
What it looks like
- Dense clumps of hollow, bamboo like stems, often speckled with purple, growing up to two or three metres tall.
- Broad leaves shaped like a shield or heart, arranged in a zigzag along the stems.
- Clusters of small creamy white flowers in late summer.
- In winter, dead brown canes that remain standing, with new red shoots appearing in spring.
Why it matters
The plant spreads through underground rhizomes that can extend well beyond the visible growth. Small fragments can regrow, so careless digging or strimming makes the problem worse. Under section 14 of the Wildlife and Countryside Act 1981 it is an offence to plant it or otherwise cause it to grow in the wild, and, separately, soil containing knotweed is controlled waste under the Environmental Protection Act 1990 and must be handled and disposed of accordingly. On a development site, discovering it late can delay the programme and add significant cost.
What owners should do
- Survey the land, including boundaries, railway and river edges, and neglected corners. Check neighbouring land too.
- Do not disturb it. Do not cut, dig or move soil until a plan is in place.
- Get a management plan from a specialist. Treatment usually takes several seasons, and excavation and removal is costly.
- Keep records of treatment, so you can show buyers, tenants and lenders what has been done.
- Talk to neighbours where it crosses a boundary. Treating one side only rarely works.
For developers
A site investigation should include an ecological walkover before purchase or design. Knotweed found early can be planned into the programme. Knotweed found when the excavator arrives is much more expensive.
See our Building Surveys and Defect Diagnosis service, and how we dealt with the challenges of a contaminated former fuel station site.