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Japanese Knotweed

Japanese Knotweed is an invasive plant that can live more than one year. 

We regularly get reports of Japanese Knotweed growing in Belfast, however we do not have a responsibility to deal with it. It is an offence to plant or encourage it to grow, but the responsibility for dealing with it rests with the landowner where the plant is growing. 

Japanese Knotweed can:

  • Seriously damage houses, buildings, hard surfaces and infrastructure growing through concrete, tarmac and other hard surfaces. Usually where weaknesses already exist.
  • Threaten native plants and animals by forming dense bushes and releasing a chemical substance to prevent other plants from growing.
  • Block routes used by wildlife.
  • Damage flood defence structures and reduce the capacity of channels to carry flood water.

Features of Japanese Knotweed

The plants produce red and purple shoots that appear early in spring. It has an appearance like an asparagus, but as the canes grow the plant takes its more characteristic appearance.

The mature canes are like bamboo, being hollow, and have a pattern of purple speckles. The plant can grow to over 3 metres in height. Flowering occurs in late summer and early autumn and consists of small cream or white flowers.

During the winter the leaves die to reveal orange and brown woody stems.

The underground stems can extend up to 7 meters horizontally from the parent plant and up to 3 meters in depth. They are thick and woody with a knotty appearance and when broken reveal a bright orange centre.

Managing Japanese Knotweed

As the landowner it is important to know how to deal with Japanese Knotweed on your property.

Before the Japanese knotweed become established, you should try to treat the plant with a herbicide treatment as soon as possible as it’s very difficult to control. 

For most residential cases we advise you not to cut the stem, crown or underground roots. Japanese knotweed spreads from crown, stem and underground root, and tiny cuts to the plant can produce a new plant. In most cases you can control the spread by not disturbing the plant, along with herbicide treatment.

Herbicide treatments can be bought from any garden or DIY stores, or you can seek professional help with the herbicide treatment. Repeated herbicide treatments over several years are normally needed for complete control of Japanese knotweed. You should continue monitoring the area and carry out treatment regularly to make sure no new shoots appear. 

Important information

  1. Remember it is illegal to dump Japanese knotweed waste in the countryside.
  2. It is illegal to plant or cause Japanese knotweed to grow.
  3. In Northern Ireland it is illegal to dispose of Japanese knotweed, or knotweed contaminated soil, at a landfill site without informing the landfill site that the waste material is Japanese knotweed.
  4. In Northern Ireland, Waste Transfer Notes are required by the licensed haulier to transport the material to the landfill site.
  5. Plant material should not be composted as it is ineffective and may result in further spread.
  6. If you intend to deep bury the dead Japanese knotweed material or dispose of it off-site for deep burial, it is recommended that you should only use glyphosate formulations. Other persistent herbicides may not be allowed for deep burial under various waste regulations and due to a potential risk of pollution of groundwater. It is recommended not to deep bury or bund Japanese knotweed onsite within 10 metres of the site boundary as a precaution.
  7. When planning works with Japanese knotweed, ensure you have built in biosecurity measures into your management plan. For example fencing off and creating a buffer around the area if it is a development site, warning signs, use of set haulage routes, covering loads, ensuring all staff working on the site are aware of the presence of knotweed, use of a set cleaning area etc.

If you require more information or guidance on Japanese Knotweed, please call 028 9027 0428.

Contact us
Environmental health
028 9027 0428
Public Health and Housing
Cecil Ward Building
4-10 Linenhall Street