Invasive weeds: Guidance for the control of invasive weeds in or near fresh water

Japanese knotweed

Fact file

Japanese knotweed was first brought to Britain in the mid-nineteenth century as an ornamental garden plant. Since then it has caused serious problems in a range of habitats - particularly roadsides, riverbanks and derelict land - by displacing native flora and even causing structural damage. There are three species of invasive knotweed in the UK: Japanese knotweed (Fallopia japonica); giant knotweed (Fallopia sachalinensis); and hybrid knotweed (Fallopia x bohemica), which is a cross between Japanese and giant knotweed. Japanese knotweed is the most widespread and troublesome bankside species, followed closely by hybrid knotweed, which has a similarly high regeneration capacity.

Only female plants are present in the UK. Japanese knotweed forms dense clumps with fleshy, red/green shoots, 2-3m tall, which have hollow green stems with red/purple flecks. Leaves are green, heart or  shieldshaped with a flat base, up to 120mm long. Creamy clusters of flowers are borne on the tips of most stems in late summer. The root system consists of rhizomes which are yellow when cut.

 

 

The underground rhizome system can extend at least 7m from the parent plant, and reach a depth of 3m or more. A piece of rhizome the size of a little finger nail can grow into a new plant. The crown, located at the base of the stem, will produce new plants. The stems die back in winter and take up to three years to decompose. Japanese knotweed crowns should neither be composted, nor removed from a site without a waste licence.

Source: Biological Records Centre

Control

Near water chemical control can be achieved with herbicides containing glyphosate. Spraying both the top and underside of leaves improves control.

Cutting should be done extremely carefully using a hand sythe to avoid spreading stem fragments. Flail mowing must not be carried out.

Digging out rhizomes and disposing of the spoil is an expensive option and often impracticable. The spoil can be removed from site as special waste, disposed of on-site at least 10m deep, or the material can be sieved through a 20mm mesh and the spoil re-used on site.

Spring Cut new growth. Dig out rhizomes or spray new growth with glyphosate. Excavate soil and dispose of on site or take to landfill as special waste, under licence.
Summer Cut new growth. Dig out rhizomes or spray with glyphosate.
Autumn Cut new growth. Dig out rhizomes and spray with glyphosate. This is the most effective time for glyphosate application.

 

Non-chemical control

Cutting

Use a simple scythe method of cutting to prevent stem fragmentation. Flail mowing or strimming must not be undertaken.

Continue cutting every 2-4 weeks to reduce both above and below-ground biomass.

Burn cut stems on site or remove to landfill (licence required).

Digging

Dig out soil around clump for up to 7m. Either sieve soil on site through 20mm mesh and re-use soil on site, or remove soil to landfill (licence required). Burn rhizome and stem fragments on site or bury 10m deep or dispose in landfill (licence required).

Pulling

Uproot stems by pulling from the base - best done from  June onwards. Burn rhizome and stem fragments on site or bury 10m deep or dispose in landfill (licence required).

Grazing

Grazing of shoots by horses, sheep and goats keeps the plant in check, provided previous dead growth is removed.

CONTACT THE ENVIRONMENT AGENCY FOR DISPOSAL ADVICE

Chemical control

Near water

In general

Glyphosate

Apply as soon as shoots appear, but best effects are when shoots are more than 1.5m tall, in August or September. Can be applied by stem injection using a 1 in 10 dilution.

2,4-D amine

Apply in May and then again in August /September for best results.

  Plants respond best when actively growing and when previous years growth has been removed. Herbicides  can be applied using tractormounted, knapsack longlance, or CDA applicators. Application in sensitive areas is best achieved by stem injection or weed wiper.

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