Tips for spotting and dealing with Japanese knotweed

Japanese knotweed.  PA Photo/thinkstockphotos
Japanese knotweed. PA Photo/thinkstockphotos

Japanese knotweed is a clump-forming perennial that produces stems that can grow up to a height of 3m 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.