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Japanese Knotweed

Japanese knotweed (Fallopia japonica), is an invasive herbaceous perennial (a plant that can live more than one year).  Since it was introduced as an ornamental plant in the 19th Century from Japan, it has spread particularly along watercourses, transport routes and waste grounds.  Japanese knotweed can:

  • Threaten native plants and animals by forming dense thickets.  Block routes used by wildlife to disperse.
  • Riverside Japanese knotweed damages flood defense structures and reduces the capacity of channels to carry flood water. 
  • Japanese knotweed can also seriously damage buildings, hard surfaces and infrastructure in some cases.  Once established underneath or around the built environment, it can be particularly hard to control, in some cases growing through concrete and tarmac and other areas of hard-standing. 

Japanese Knotweed can cause concern with regard to potential impacts on property.  There is an increase in reports to the Council with regard to the presence of particular invasive species. 

Japanese knotweed is listed on Schedule 9 Part II of the Wildlife (Northern Ireland) Order 1985 (as amended).  As such, it is an offence for any person to plant or otherwise cause to grow, Japanese Knotweed, in the wild.  The natural spread of the plant from one area to another is not enforceable under this particular legislation.

The legislation does not give local Councils the power to force a landowner to take control of this species on their own land. 

Any breaches of the Wildlife Order (NI) 1985 are investigated by the PSNI.

 

Identification

The key features of the plant are summarised below:

  • Produces fleshy red tinged asparagus like shoots when it first breaks through the ground in an established stand.
  • Has large, heart or spade-shaped green leaves which are approximately the size of your hand.
  • Has leaves arranged in a zig-zag pattern along the stem
  • Grows up to 3 metres in height. 
  • Yellow / cream flowers in late summer - typically they start forming from late July onwards.
  • Hollow bamboo like stems which have distinctive ring like nodules at regular intervals along it.
  • Brown stem in winter once it has died back.
  • Extensive rhizonme (root) systems (up to 7m from the visible plant). 
  • Rhizomes are orange in colour.
  • Spread entirely via the movement of plant and rhizome fragments.

More information on management and control of this species can be found at the following websites:-

http://invasivespeciesireland.com/toolkit/invasive-plant-management/terrestrial-plants/japanese-knotweed/

http://www.property-care.org/homeowners/invasive-weed-control/

http://www.rics.org/uk/knowledge/professional-guidance/information-papers/japanese-knotweed-and-residential-property-1st-edition/

Action on Council Sites

Council are committed to the control of this species on Council sites.  Control methods used by the Council generally involve the use of chemicals which are used only by licensed members of staff. 

Chemical control

Herbicide active ingredient

Glyphosate

Apply when first signs of growth appear (April/May) and late in the season (mid to late September)

Treatment should continue twice a year until all signs of regrowth are eliminated. 

In general

Herbicides can be applied using a range of suitable applicators such as a knapsack sprayer.  Control is easier if dead winter stems are tidied over the winter months to assist with access before growth commences i.e. to prevent tripping on them or them interfering with your knapsack lance. It is advised to leave the cane in situ to reduce the risk of spread to other sites. Be careful to avoid spreading knotweed crowns when tidying dead canes. Application in sensitive vegetation areas is best achieved by stem injection or weed wiper.

Further details on control measure

  1. Remember that it is illegal to dump Japanese knotweed waste in the countryside.
  2. It is illegal to plant or otherwise cause Japanese knotweed to grow.  Hence you should be careful to ensure that you do not cause further spread.
  3. It is illegal to dispose of Japanese knotweed at a landfill site without informing the landfill site that the waste material is Japanese knotweed.
  4. Japanese knotweed can regenerate from very small fragments of rhizome (as little as 0.7 grams).
  5. Plant material should not be composted as it is ineffective and may result in further spread.
  6. Plants should be treated in the same season as they are identified. Try not to let stands of Japanese knotweed become established as this species is very difficult to control.  If it is a recent introduction it is best to tackle it quickly to prevent the rhizome system from fully establishing.
  7. Japanese knotweed is not an easy plant to control due to its extensive underground rhizome system. Therefore treatment often needs to be repeated until no regrowth is observed over several years for eradication to be achieved.  Continued monitoring of the treated areas should also be carried out to ensure that no new shoots appear.