HIMALAYAN KNOTWEED (Persicaria wallichii)
Originating in the Indian Subcontinent, Himalayan Knotweed was first cultivated in the UK in the early 1900’s and first recorded in the wild in North Devon in 1917. Like Japanese Knotweed, it was introduced as an ornamental garden plant. Its arrival in Ireland is most likely as a garden plant and it may have been available from plant suppliers up until very recent times.
The plant is now very widespread across the country and enjoys the same growing conditions as Japanese Knotweed. Typically, Hilmalayan Knotweed emerges somewhat later in spring than Japanese Knotweed and also flowers later in the growing season.
It is a robust, rhizomatous, perennial, with thin bamboo like stems that can grow up to 1.8m in height. A very persistent and rapid grower, its upright stems and thin pointed leaves result in very dense coverage. It colonises roadsides, river banks, damp grasslands and waste ground, to the detriment of native flora. Reproduction is almost entirely vegetative, from either tiny fragments of rhizome, or small pieces of plant stem which contain as little as two stem nodes. Typically, it is spread by cutting machinery, passing traffic, flowing water and the deliberate dumping of garden waste.
A typical infestation of Himalayan Knotweed on waste ground - coming into flower in late summer
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