identification of Japanese Knotweed

Identification of Japanese Knotweed


Bright green shield shaped leaves , Tall green canes, with purple speckles , Creamy white flowers in late summer , the images below show Japanese Knotweed at various stages

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Japanese Knotweed Identification

Japanese Knotweed rarely exceeds a height of 3M. It has shield-shaped leaves which are flat at the base and are carried on zig zagged stems, which are sturdy, purple spotted, hollow and bamboo - like with regular spaced nodes.
The flowers (only female in the UK and Ireland) which appear in late Summer or early Autumn,
are creamy white/colours and are formed in dropping clusters 8cm - 12cm in length.
In Spring, the emerging stems are green to red/purple with rolled leaves that uncurl as the shoots extend.
At the end of the year, the stems persist and the various shades of brown, sometimes with an orange tinge.
In external appearance, the rhizome is dark brown and slightly leathery.
It is brittle when fresh and snaps like a carrot and has a musty “antique store’ smell.
The interior is an orange/yellow colour, generally darker towards the centre. Lines often radiate from the centre.
At the base of the Japanese Knotweed stems, an enlarged crown develops from which shoots and rhizomes emerge. The crown is hardy and lumpy in appearance and before the growth seasons it can possess pink/red-smooth-shiny buds.
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Impacts of Japanese Knotweed

The underground rhizomes of the species can penetrate loose aggregate and grow through existing small cracks, openings or voids in alphabet/concrete. Like many plant species, once established, the underground structures of Japanese Knotweed rhizome can slowly increase in volume over time and ultimately impact built structures (e.g. drains and patios), causing damage by the heaving apart of materials by the growth of the rhizomes.
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Japanese Knotweed is a tall vigorous plant.

It is an invasive non-native plant pest and is considered to be one of the most problematic plant species in the UK and Ireland. The species was introduced to Britain in the mid 19th century as an ornamental plant for large gardens,prized due to its imposing size and sprays of creamy white flowers. By 1886 it was established in the wild and now it is widely distributed. As it is non native to the UK and Ireland, it is not exposed here to any of its natural enemies, such as insects, bacteria and fungi that feed and grow on and in it in its country of origin. The absence of these checks in conjunction with its highly invasive and competitive nature has facilitated the invasive spread of Japanese Knotweed throughout England, Wales, Scotland and Ireland
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rapid growth rate

The plants well documented rapid growth rate is in relation to the upward growth of shoots at the beginning of the growing season. The rate of lateral growth, via extension of its root system (rhizome), is not well understood. The rate of spread will be dependant on the density and composition of the soiled the presence of built structures. Dense soil will usually limit the spread of the plant. New shoots can emerge from spreading rhizomes. Rhizome extension is generally limited to a 4 Meter radius from visible above ground plants and a depth of 2 Meters. However, growth of 7M beyond these limits and a depth of 3 Meters for roots is achievable in favourable condition of soil, the distance of 7 Meters is considered a safe distance zone

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