Japanese Knotweed

Question 1

What is Japanese knotweed and why is it an issue within the built environment?

Answer 1

• Japanese knotweed is perennial plant which has an invasive root system and is capable of rapid growth. It forms thick colonies of above-ground stems and the root system can extend 7 metres horizontally and 3 metres vertically.
• The root system and above ground stems can disrupt drainage runs, patios, drives or damage walls and buildings with shallow foundations such as garages or conservatories.

Question 2

How do you identify Japanese knotweed?

Answer 2

Japanese knotweed differs in appearance between the growing season (March – October) and winter (November – February).
In the growing season, the Japanese knotweed has the following features:
- Shield-shaped green leaves.
- Alternate leaves of a ‘zig-zag’ leaf stem.
- Clusters of small white flowers.
- Main stems are hollow and bamboo-like.
- Main stems are green with purple speckles.
- Tightly-packed stands (clumps) of knotweed.
In winter, the Japanese knotweed has the following features:
- Leaves and flowers fall off.
- Main stems peel and become brittle and pale.

Question 3

How do you treat Japanese knotweed?

Answer 3

• Japanese knotweed infested-soils can be excavated completely although this may require excavation to 3m vertically and 7m horizontally depending on the severity of the growth. The soil can be buried on-site (with at least 5 metres of overburden) or removed to a licensed waste-management facility.
• Soil screening whereby soil is excavated, rhizomes are removed via hand or machinery and the soil is re-used.
• Encapsulation whereby Japanese knotweed root system is encapsulated with a specialist root barrier membrane.
• Biological control is being trialled whereby a sap-sucking insect is introduced to control growth.
• Chemical control which involves application of herbicides over several growing seasons although this method can takes at least 3 years to be effective.

Question 4

What legislation is relevant to Japanese knotweed?

Answer 4

• There are two main pieces of legislation relevant to Japanese Knotweed.
• Environmental Protection Act 1990 whereby any soil or plant material contaminated with Japanese knotweed needs to be classified as ‘controlled waste’.
• Wildlife and Countryside Act 1981 whereby it is illegal to grow Japanese knotweed in the wild.