Japanese Knotweed Identification

Japanese Knotweed Identification

Extremely Invasive Plant

Japanese Knotweed is a highly invasive plant which causes untold damage to thousands of properties and sites each year in the UK. Fortunately it's relatively easy to identify and control.

Japanese Knotweed or given its scientific names Fallopia Japonica – Reynoutria Japonica and polygonum Cuspidatum is of the same family as rhubarb, sorrel and buckwheat. It was classified as an invasive weed in 1981   The Wildlife and countryside act 1981 contains legislation specifically to control the sale and spread of Japanese Knotweed. (See our section for more Japanese knotweed legislation.)

Donkey rhubarb, elephant ears, Japanese bamboo, American bamboo, Monkeyweed and fleeceflower are some of the other names given to a plant regarded as the most invasive plant in the UK.

Origins In Asia

The plant is originally from Korea, Japan and China where it was discovered in the late 18 century. Brought from Asia to Europe in the 19 th century it was imported into the UK in the middle of the 19 th century.

The myths about the introduction of Japanese Knotweed into the UK range from being introduced by the Victorians as a decorative plant, cheap feed for cattle even used as a method of anchoring sand dunes, supporting the soil levees on canal and railway embankments using the plants extensive root system to anchor the soil.

It was sold extensively by nurseries throughout the 19 th century. In the early 20 th century the full impact that Japanese Knotweed was having in areas where it was planted was being realised. In the absence of any natural insect or animal predators and no fungi or diseases the plant has thrived in the UK conditions and has spread quickly particularly along railways lines, canals and rivers. It is likely that on average there is a stand of Japanese Knotweed in every 10 square kilometres throughout the UK.

Knotweed Characteristics

The plant is known as an herbaceous perennial and is part of the same family as rhubarb and buckwheat.

What to Look For in the Spring and Throughout the Summer

First signs are small reddish purple asparagus like stems with bright green wrap around leaves emerging from the ground. The leaves unwrap and are described as heart or shield shaped with a smooth upper surface and reddish veins underneath. The stems are hollow but the plant rapidly grows up to a height of 2- 3 metres in a single season.

Mature plants can have leaves up to 10 inches long The stems are green with reddish spots on and can grow up to 50 mm in width with leaf stems appearing on alternate sides all the way along the main stem. The stem has a bamboo like appearance. The shade the leaf canopy produces makes it impossible for other plants to grow underneath or close to Japanese Knotweed...


Stems will continue to appear throughout the growing season producing colonies with copious leaf canopies. In late August clusters of creamy white flowers appear that produce seeds that are currently sterile and unable to produce new plants. Concern about the plant spreading by means of seed transfer is unfounded. All Japanese Knotweed plants currently in the UK are female and the plant can only spread by small pieces of its rhizome or root system. This method is very efficient at spreading the plant quickly and aggressively

While the development of the plant above ground is impressive the spread of the rhizome (root) below the soil surface is even more impressive. The rhizome of a small stand ( colony of a few stems ) of Japanese Knotweed can produce over 200 new shoots some appearing as far as 7 metres away from the parent plan and emerging from deep in the soil as far as 2- 3 metres down . The new stands form dense thickets which remain connected to the parent plant allowing all plants to share in the energy contained in the rhizome system.

This sharing of energy provides the new emerging shoots with enough power to break through tarmac, concrete and even break through joints in brickwork and pipes. New plants can emerge from a piece of rhizome as small as 2 cm. Fresh stems can also produce new shots and roots when buried. Japanese knotweed thrives where there is available water such as streams or damp areas with plenty of sunlight. Poor soil conditions do not prevent Knotweed developing. Japanese Knotweed can withstand heavy clay or sandy soils and a wide range of acidic and alkaline conditions.

The plant is also very tolerant to regular strimming, cutting and extremes of temperature. It will even survive burning with stems emerging a few days after the stand has burnt down. Do not spread cuttings around. It is classified as controlled waste and therefore needs to be handled and disposed of to a licensed site that accepts controlled waste. Ignoring these rules or fly tipping knotweed waste can lead to prosecution and heavy fines. (See Japanese Knotweed legislation) The simple act of cutting or disturbing the soil around the plant if not strictly managed could be classed as encouraging the Knotweed to grow. In many cases well intentioned cutting of Knotweed is the reason Knotweed has spread.

Disturbing the ground encourages the plant and stimulates rhizome activity. The rhizome appears as a brown knotted root and is a bright orange colour inside. The roots have the snapping characteristic similar to a carrot when broken. The rhizome sends down hundreds of fine white roots into the soil.


In the autumn the leaves gradually turn brown gradually falling to form a thick layer of slowly decomposing leaf mulch that prevents native seeds germinating.

Autumn - Winter

The plant withdraws all nutrients and moisture from the leaves and stem back into the rhizome .The leaves brown and drop off the stem forming a layer of mulch on the ground. The stems form unsightly brown hollow bamboo like brittle canes that decompose very slowly. The crown, rhizome and roots remain dormant throughout the winter easily surviving frozen conditions.

This destructive power to locate and exploit weaknesses in structures damages buildings, surfaces and its ability to spread to the detriment of all other flora and by consequence fauna makes Japanese knotweed a formidable plant. Large areas of land continue to be colonised by Japanese knotweed forcing natural flora and fauna out and producing serious environmental and commercial consequences.