Now is best time to spray invasive knotweed
JAPANESE knotweed is an invasive shrubby perennial plant that was introduced to Ireland in the 19th century as a garden shrub.
It is now widespread throughout the country and is regarded as an undesirable invasive species.
The plant can grow to over 3m in height and has hollow bamboo-like stems that have a characteristic pattern of purple speckles.
The stems carry large heart shaped leaves during spring and summer. The plant flowers in July/August bearing small creamy white flowers.
During the winter the leaves die back and reveal orange/ brown woody erect stems which are characteristic of the plant during this period. If its growth is unimpeded the plant is found growing in the form of thickets.
Japanese knotweed plants have a strong root network with roots reaching up to 7 meters from a single plant and reaching up to 3 metres in depth.
The rooting system of Japanese knotweed is particularly powerful and can cause serious damage to property by its ability to grow through concrete, tarmac and other hard surfaces.
It can also cause particular damage to river banks both by its rooting system and also by causing bank erosion when the plant dies back during the winter.
During this winter die-back period the shrub leaves soil exposed to rainfall damage as no grass or weed plants can grow underneath the plant as it grows vigorously and outcompetes these native plants.
Where the plant is found along rivers this feature can cause collapse of river banks.
Existing Japanese knotweed plant spreads mostly by means of its root system to form thickets.
It can also spread easily to new sites due to tiny amounts of the plant’s roots, stems or crowns being easily capable of producing new plants.
Movement of these small plant parts to new areas can easily result in viable new plants forming new Japanese knotweed thickets. It is this ability to easily colonise new sites that makes Japanese knotweed such a virulent invasive.
Controlling spread is therefore dependent on preventing the spread of plant parts to other sites.
For in-situ control of existing thickets the only successful method is by chemical control by means of glyphosate application - with the optimum time for spraying being mid to late September.
It is unlikely that one treatment will be sufficient to eradicate the plant with further treatment being necessary the following year.
However spraying should not be carried out near watercourses and in any case current pesticide spraying regulations must be adhered to. For further details on Japanese knotweed and its control refer to http:// invasivespeciesireland.com.