What is Japanese Knotweed?

Part of our Japanese Knotweed Removal Guide. Click to see more answers to your questions.

Japanese Knotweed (Fallopia Japonica) a member of the dock family was introduced to the UK back in the 19th century. With no natural predators and few effective competitors it spread from ornamental gardens and railway lines where it was planted for its screening qualities. The rapid spread throughout the country has led to it becoming an archetypal invasive plant species.  The plant has the potential to reach heights of 3m and can be identified by the broad, heart/shield shaped leaves. The hollow stems with raised nodes are reminiscent of bamboo, and the flowers are off white in colour ranging from 7 – 15cms in length.

The rapid growth of the plant will crowd out indigenous herbaceous species effectively taking over an area. The root system can extend to 7m from the visible surface growth and reach depths of 3m. On sites where repeated burial of tipped material has occurred roots can extend greater than 3m in depth. The plant has been known to find cracks in building, concrete, paths, etc. The strong growth can expand these cracks and ultimately cause heave to foundations and substructures. This can have a detrimental effect when trying to sell a property or securing a mortgage.

The new growth of the plant commences around spring shooting up a few centimetres a day and as the growth season lasts till early autumn the plant has the potential to grow a couple of metres over that period. Knotweed does not produce viable seeds and spreads via the rhizomes, the underground stem of the plant with a knotty brown appearance. This bud bearing material has the capacity to produce a new plant from a piece of rhizome the size of your fingernail.

The plant grows rapidly through the spring and summer then dies back in late autumn leaving large areas of dead brown stems which provide a haven for rats and can hugely increase erosion particularly of river banks.

Along with natural growth the plant can spread through waterways if the plant is close to one. Humans are a major factor too through improper treatment, moving of soil containing rhizomes and fly tipping. Anyone working in or around Knotweed must take care to check shoes and brush off any plant material before leaving the area. A single piece of rhizome can lay dormant for several years so the spread of Knotweed from one area to another may not be immediately apparent.