Information on Himalayan Knotweed

Common Name: Himalayan Knotweed
Scientific Name: Persicaria wallichii
Irish Name: Glúineach spíceach
Family Group: Polygonaceae
Distribution: View Map (Courtesy of the BSBI)
Flowering Period


Click for list of all flowering by month
Himalayan Knotweed could sometimes be confused with:

Knotweed, Lesser, Knotweed, Japanese,

Himalayan Knotweed bears white or pale pink flowers (4mm across) in leafy panicles. These flowers have 5 unequal tepals and 3 stigmas. The upper leaves of the shrub are lanceolate-ovate whereas the lower leaves are cordate. They have red veins and margins and the hairless plant stems are also tinged red. There are hairy stipules on the stem. The flowers bloom in August and September.   

This is a tall, non-native, perennial plant species which grows to about 1.5 metres, particularly on roadsides, railways and besides streams, spreading on rhizomes. In winter, the plants die back, leaving brittle brown stems. Scattered throughout, Himalayan Knotweed is found more often in the north-west than the rest of Ireland. It belongs to the Polygonaceae family.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              

I first saw this species in 2015 on a walk in Co Wexford with Paula O’Meara, Joint Vice-county Recorder for that county. She kindly identified it for me and I photographed it at that time.  

If you are satisfied you have correctly identified this plant, please submit your sighting to the National Biodiversity Data Centre

Knotweed, Himalayan
Knotweed, Himalayan

Himalayan Knotweed is a garden escape, having been originally introduced from Asia. According to the National Biodiversity Data Centre’s analysis of trends within our native and non-native species, Himalayan Knotweed – which has an invasive species classification of ‘Medium Impact species’ – ‘is known to form dense stands that outcompete native species. The plants die back in the winter which can leave areas vulnerable to erosion. Cutting of the plant without proper site management could cause the plant to spread further’.