JAPANESE KNOTWEED

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Japanese knotweed (Fallopia japonica) was brought over in the nineteenth century by Victorian plant collectors, who admired the heart shaped leaves and bamboo like stems.  Unfortunately today Japanese Knotweed is the most invasive plant in Britain, colonising most habitats and able to grow through walls, tarmac and concrete.

Japanese knotweed stems are green, with red or purple specks.  They form dense, cane like clumps that  can grow up to 3 metres tall.  The leaves are green and heart shaped, up to 12 mm long and with a flat base.  Creamy clusters of flowers are borne on tips of most stems during August till October.  The roots consist of rhizomes, which are yellow when cut.

Japanese knotweed is a highly vigorous plant with the ability to spread quickly from an enormous underground network of stems (rhizomes), which can be up to 3 meters below the plant and spread 7 meters across.

It can spread via fragments of roots and stem often with human assistance as plant material or as fragments in soil, and pieces of rhizome as small as 1cm can produce new plants and the cut green stems readily re grow. Fortunately, Japanese knotweed in this country is infertile and so cannot produce viable plants from seed.

Under the 1981 Wildlife & Countryside Act, made it an offence to plant or cause it to grow in the wild. Section 34 of the Environment Protection Act 1990 places a duty of care on all waste producers to ensure that any wastes are disposed of safely, namely that Japanese knotweed crowns should neither be composted nor removed from a site without a waste license.

  

Control

Do not cut down Japanese knotweed on site, as this will spread the plant.  Instead treat chemically with an herbicide to kill the stems and then burn them on site.

You can use either a glyphosate or trichopyr based herbicide.  Glyphosate (Roundup) is best used is when the plant is actively growing in the spring or at the end of the season in the autumn when the plant is dying down. Trichopyr (Icade) is an excellent chemical for control in areas not near trees and has the advantage of killing any surround grass.

You will need to repeat the application several times each season, and it can take up to five years to eradicate the plant entirely.

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Japanese Knotweed