Should I be worried about Knotweed?
Your queries answered
The lime-green leaves of the knotweed plant: eradication of this invasive species can be challenging and expensive. Photograph: Thinkstock
Q I moved into my three-bed semi-detached house five years ago. My (unattached) neighbour just told me that they have Knotweed and my property could be affected too. I don’t have a clue what this means or if it’s serious. What should I do?
A Knotweed is an invasive plant species introduced to Europe in the late 19th century initially as a garden plant. Their spread has escalated because they have no natural predators outside their native habitats. In Ireland there are four regulated types of Knotweed.
It should be noted that it is an offence under the Wildlife Act 1976 for a person to plant, disperse, allow or cause to disperse, spread or cause to grow any prohibited species. The four regulated Knotweed species are included on this list.
You indicate that the plant has been brought to your attention by your neighbour who believes that it is located within their property. You should initially seek professional advice from a Chartered Building Surveyor or gardening expert to confirm identification of the plant and implement a management plan in conjunction with your neighbour.
Identification of the plant in early spring is noted due to the red and purple shoots that grow from the ground. By mid-summer it can be distinguished from other plants by its tall hollow bamboo-like canes that are covered with purple speckles.
The lime green leaves are arranged in a zig-zag pattern along the canes and the leaves are shield shaped with a flat base and pointed tip. In winter the leaves will die back leaving a visible and distinct cane that can be identified.
The Society of Chartered Surveyors Ireland has produced an information paper to advise how best to manage and assess the risk of Knotweed. The assessment of Knotweed has been split into four separate categories which will allow a suitable management plan to be put in place. It is important that any management plan should cover the entire site and not just the visibly affected parts.
Once Knotweed has established itself on a site, it can cause damage to property (drains, paths and buildings). Its eradication/control can be challenging and expensive. The following advice should also be adhered to; you should not strim the Knotweed as this will create tiny pieces of the plant which will then spread and grow into new plants. You should also not attempt to dig up the plant, cut it back or attempt to compost the cuttings.
In terms of effective treatment of the plant there are a number of options available. These include;
– Excavating infested soil and removing it to an appropriately licensed waste management facility. All disposal in this instance should be carried out in accordance with the Waste Management Act.
– Knotweed can be excavated and then buried on site but it must be covered with five meters or more of overburden soil and a specialist root barrier membrane installed to fully or partial encapsulate the Knotweed-bound soil.