Japanese Knotweed Fallopia japonica (previously named Polygonum cuspidatum) originates from East Asia (Japan, Taiwan, China and parts of Korea) and was first introduced into Britain in the 1850’s when a Dutch nurseryman sent the plant to Kew Gardens.
From here it has spread throughout the UK – initially from the sale and distribution of the plant to landscapers and gardeners but in more recent times it has spread in soils contaminated with small fragments of the rhizomes e.g. soil movement during building works and from fly tipping.
Japanese Knotweed is a hairless perennial producing an extensive rhizome system that can reach up to 7 metres from the plant down to a depth of 3 metres beneath the surface. It is this rhizome system that makes this plant so aggressive and difficult to control.
The stems rapidly grow to a height of 2 - 3 metres and are hollow, ‘bamboo-like’ and flecked with red pigment. In the winter the stems will die off but remain erect and new shoots will sprout from the rhizomes in the following spring. The large, fresh green leaves are oval-triangular, truncated at the base and pointed with a red leaf stalk. Numerous small greenish/white flowers approximately 3mm in size are produced late in the season on branched spikes.
Once established it can quickly dominate large areas squeezing out all other plant life.
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Japanese Knotweed is a dioecious plant – it has male and female organs on separate plants. In the UK we have only female plants so propagation by seed is not possible and therefore all our Japanese knotweed specimens are clones that have grown from rhizome fragments or from cuttings. We are now also aware that the rhizome fragments can spread by travelling down rivers and streams. Only a small piece of rhizome (less than 1gram) is needed to generate a new plant.
Japanese Knotweed is an extremely invasive and hardy plant. In its natural habitat in East Asia it can survive in harsh conditions in volcanic regions on poor, thin, acid soils with a pH of less than 4.0 and extremes of temperature, which in mountainous regions can fall below -35°C for long periods during the winter. It is therefore not surprising that it does so well in our comparatively gentle climate.
The British Government has been aware of the invasive problems caused by this species for some time; it is one of the weeds listed in Schedule 9 and subject to controls specified in section 14 of the Wildlife and Countryside Act of 1981. Whilst there is no statutory requirement under this act to remove Japanese Knotweed from your land, it is an offence to plant or cause the species to grow in the wild.
The plant, prunings and soil contaminated with it’s rhizomes are classified as controlled waste and must be disposed of at ‘licensed sites’ or rendered safe by burning.
Control Methods
Japanese Knotweed is not an easy weed to control, due to the extensive rhizome system that sustains the plant. Because there is no viable seed from UK plants we need to concentrate our efforts towards killing these underground rhizomes.
Chemical control is the best method of eradication, although this may take several years to fulfil. Choosing the most suitable chemical depends on the situation; in sensitive areas near water, trees and shrubs we should use non-residual translocated herbicides such as 2,4-D amine (e.g. ‘Depitox’) or glyphosate (e.g. ‘Roundup ProBio) taking care to avoid spray-drift onto the foliage of desirable plants nearby.Where there is a need to preserve grasses e.g. to prevent soil erosion, then 2,4-D amine is the best option.
InjectorDos Pro is a metered-dose injection applicator that provides an effective and safe way to treat weeds in places where spray applicators are unsuitable.
In the initial stages of chemical treatment, the plant may sprout new growth from it’s rhizomes up to 7 metres away from the original plant so one needs to be ready to re-treat the new shoots as soon as possible after they emerge and before they can extend the rhizome system further.
Cultural control of Japanese Knotweed is possible by cutting and burning the aerial growth and carefully excavating the soil to remove all traces of the rhizomes – but remember only one tiny piece left behind can cause the problem to recur. The environmental cost of removing soil to a depth of three metres and replacing it with fresh soil makes this exercise impractical.
Where it invades grassed areas, control of Japanese Knotweed can be achieved by regular mowing, which gradually weakens the rhizome system and eventually the shoots will stop emerging through the turf.