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Japanese knotweed: Everything you need to know about the 'Asbo' plant

Does your garden contain the thug of the plant world?

Published 08/12/2015

The invasive species Japanese Knotweed
The invasive species Japanese Knotweed
The invasive Japanese knotweed has cost millions of pounds a year to control
Big concerns: Japanese knotweed can create difficulties when selling a home

Japanese knotweed is a plant so invasive it is like a species from a horror B-movie. It can force its way through concrete and roadways, block drainage channels and damage foundations and is notoriously difficult to eradicate.

The pernicious weed is capable of growing to eight feet or more in a single season and causing serious structural damage to buildings. It has been listed as one of the top 10 most unwanted species in Ireland.

Value of Belfast home falls by 40% as Japanese knotweed invades property  

Here is everything you need to know about the destructive plant:

- Japanese knotweed is native to eastern Asia and was introduced into Britain as an ornamental plant in the 19th century

- It is a clump-forming perennial that produces stems that can grow up to a height of 3metres or more, at a phenomenal rate.

- As well as growing fast, it's extremely invasive, reproduces very easily and is difficult to get rid of for good. The above-ground stems grow densely and have an unusual purple speckle, before turning brown and dying back in winter. Small flowers appear around this time of year, but the seeds aren't fertile.

- Japanese knotweed reproduces and spreads through its stem, crown and creeping underground stems (called rhizomes) - even a small piece can become a new plant. The plants are capable of breaking through tarmac and weak points in concrete and can damage buildings' foundations, drainage systems and walls. The plants can also increase the risk of soil erosion and flooding, among other problems.

- Japanese knotweed can even "play dead" - rhizomes can stay dormant underground for as long as 20 years before producing plants.

- If you're buying a property and the surveyor finds Japanese knotweed during the valuation, the mortgage provider may refuse to lend on the property, or may make a retention until satisfied the plant has been eradicated. Similarly, if you're selling a property with Japanese knotweed in the garden, you may have a big problem.

- If you're not selling, it isn't an offence to have Japanese knotweed on your land and you're not legally obliged to remove it, but you could be prosecuted if you allow it to spread to someone else's land.

- Eradicating Japanese knotweed can take several seasons - a specialist contractor should be able to ensure the plants don't come back, which is the tricky bit. However, the soil can contain rhizomes as far as 7m from each plant, making it extremely difficult to remove completely.

- Any contractor you use must be qualified to deal with Japanese knotweed. If taken off site, it must be disposed of by a licensed waste control operator at a licensed disposal site, because it, and any affected soil, is considered "controlled waste". Soil containing rhizomes must be buried at least 5m deep and covered with a root-barrier membrane, making it a major undertaking.

- The Department of Environment's Alien Invasive Species Team advises homeowners to contact the Property Care Association for guidance for dealing with Japanese knotweed in residential settings, even if the infestation is not on their own land.

- Incidences of the plant should be reported to Invasive Species Ireland

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