Module 5 Japanese Knotweed Introduction to the UK Japanese Knotweed is one of the most problematic invasive species in the UK. It originated from Asia, specifically from China, Japan and Korea. It is now categorised as an invasive plant in North America and Europe. The plant was first introduced by the Victorians as an ornamental plant and as cattle fodder. It was first recorded from Kew Gardens from the early 1800s. The first recorded site in the wild was in 1886, in Maesteg in Wales, and it was first recorded in Swansea (Glamorgan) in 1902. The first official complaint about the plant was made in Swansea in 1970; and today it is a problem in all counties in the UK. In Europe only the female plants are found therefore the seeds are non-viable, every plant as a result is a clone, making it the world’s biggest female biomass. Japanese knotweed is a rhizomatous perennial (spreads by underground roots). Identification: Japanese Knotweed grows rapidly on thick reddish brown single hollow stems. The stems have purple specks running through them and visible joints (nodes) up their length. When the plant is mature it grows simple branches. The dark heart-shaped green leaves grow out alternately giving a zig-zag effect, and can grow up to 8-10 inches long and 3-6 inches wide with a pointed tip. The plant flowers in the summer with small green and white flowers on a loose panicle. The reason for the plant’s rapid growth is due to the fact it thrives on disturbance. When the plant is cut the discarded off-cuts (nodes or fragments of rhizome) can colonise within 6 days.
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Module 5 Japanese Knotweed
Introduction to the UK
Japanese Knotweed is one of the most problematic invasive species in the UK.
It originated from Asia, specifically from China, Japan and Korea. It is now categorised as an invasive
plant in North America and Europe. The plant was first introduced by the Victorians as an ornamental
plant and as cattle fodder. It was first recorded from Kew Gardens from the early 1800s. The first
recorded site in the wild was in 1886, in Maesteg in Wales, and it was first recorded in Swansea
(Glamorgan) in 1902.
The first official complaint about the plant was made in Swansea in 1970; and today it is a problem in all
counties in the UK. In Europe only the female plants are found therefore the seeds are non-viable, every
plant as a result is a clone, making it the world’s biggest female biomass. Japanese knotweed is a
rhizomatous perennial (spreads by underground roots).
Identification:
Japanese Knotweed grows rapidly on thick reddish brown single hollow stems. The stems have purple
specks running through them and visible joints (nodes) up their length. When the plant is mature it
grows simple branches. The dark heart-shaped green leaves grow out alternately giving a zig-zag effect,
and can grow up to 8-10 inches long and 3-6 inches wide with a pointed tip. The plant flowers in the
summer with small green and white flowers on a loose panicle.
The reason for the plant’s rapid growth is due to the fact it thrives on disturbance. When the plant is cut
the discarded off-cuts (nodes or fragments of rhizome) can colonise within 6 days.
Figure 1 Flowers and Leaf of the Japanese Knotweed
Figure 2. A) In early Spring red/purple shoots appear B) Grow rapidly forming tall, hollow canes
C) Purple-speckled stems D) The core of a rhizome is dark orange/brown.
Figure 3 Picture illustrates that a Japanese knotweed can grow up to 2m or 7ft lateral growth. It also
shows the zig-zag leaf formation.
Figure 4 Leaf Shape - there are a couple of other similar species which are less common but still
invasive.
Figure 5 Root of the Japanese Knotweed.
Figure 6 Over winter dead knotweed stems and canes - can be undetected if cut down.
Figure 7 Growth of knotweed throughout the year
Benefits of the Japanese Knotweed
Although it is an invasive specie there are some benefits of the plant:
1. The flowers provide a source of nectar for the bees all summer long.
2. Presence in riparian habitats allows for cover for mammals such as badgers and otters, and also
for nesting birds.
3. Helps to stabilize slopes.
4. It is a source of food and medicine.
5. Used as a dye.
6. Can increase the diversity of habitats. (Pseudo-woodland habitat - urban)
Disadvantages of Japanese Knotweed
There is proof that the Japanese knotweed does have social and environmental benefits, however the
negatives vastly out-weigh the positives!
1. Out-competes native flora by shading.
2. Can grow through tarmac and concrete
3. Rhizomes spread extensively underground
4. Very hard to eradicate - expensive and time consuming, sometimes taking years.
5. Waste containing the plant is ‘controlled waste’ and has to be disposed of correctly by law.
Figure 8 Japanese knotweed shown coming through the skirting boards inside a house
Figure 9 Japanese Knotweed growing through concrete on a road
Costs related to the Japanese Knotweed
Japanese Knotweed cost over £150 million a year to control and clear in the UK alone. DEFRA in 2010
stated that the cost of a national eradication program is estimated at £1.56 billion.
Legislation
· Wildlife and Countryside 1981 Schedule 9, Section 14 states it is an offence to deliberately
cause the plant to grow in the wild.
· The Environmental Protection Act 1990 is another broad ranging piece of legislation that
singles out Japanese Knotweed for a special mention. Under the Act it places a 'Duty of Care'
on the land owner and anyone they employ to dispose of soil or other material
contaminated with Japanese Knotweed. Contaminated soil is classed as ‘controlled waste’
which can only be taken to licensed landfill sites who must deal with it in an appropriate way
(e.g. deep burying). There are only a limited number of landfill sites willing to take this
material and they must be informed prior to the material being taken to them. A Waste
Transfer Note is needed and extra charges apply.
CASE LAW- EA vs PJ THOMAS,
2002
CASE LAW- EA vs. D. Reed,
Surrey 2004
CASE LAW- FLANNAGAN vs.
WIGAN MBC 1995
• EPA 1990, £2000, no • EPA 1990, £4000, no • Council found negligent
waste management
licence
• Wildlife and
Countryside Act 1981,
£1000, knotweed to
grow in wild
• Legal costs, £1576
waste management
licence, burning
demolition waste
• Legal costs, £4764.66
• Lit fire to burn out
knotweed on a track
– allowed JK to grow
into a garden.
• Required to:
– construct new wall
– treat knotweed
– pay all costs and
legal fees
Figure 10 Legal cases of Japanese Knotweed resulting in prosecution.
Other pieces of legislation also cover Japanese Knotweed such as:
● TOWN AND COUNTRY PLANNING ACT 1990 - Adversely affecting the amenity of the
neighbourhood (Chapter 11, Pt VIII, para 215-219)
● PREVENTION OF DAMAGE BY PESTS ACT 1949 - Keep land free from vermin (Section 4)