This week we return to the knotweed family (Polygonaceae). The best-known of all the knotweeds is the infamous and highly invasive Japanese Knotweed (Reynoutria japonica). This ‘baddy’ was the subject of an earlier blog (click), where we mentioned its big brother (Giant Knotweed, Reynoutria sachalinensis). Both are ‘outlawed’ by UK legislation because they are invasive and considered as threats to native vegetation. I set out last week to look for the Giant Knotweed, using the database of the Botanical Society of the British Isles (BSBI) as my guide.
What I found was a different Knotweed. This one is similar to the Giant, but not very tall (1.5 metres). I noticed that its leaves are large like those of the Giant but the wrong shape. Not heart-shaped but narrowed at the base. And the flowers were distinctly fragrant. What could this one be?
Himalayan Knotweed in full flower in Glen App, next to the Water of App, Ayrshire. This is a shaded and wet location, October 2022. Photo: John Grace.
It took me some time, but using Stace’s 4th edition of New Flora of the British Isles and aided by a fine description from the Flora of China, I concluded this was the Himalayan Knotweed. It isn’t even in the same genus as the other Knotweeds, and to confuse me it has often been renamed and now it has been shifted unceremoniously to another genus. In Stace’s 1st edition it is Persicaria wallichii but in the 4th edition it is Koenigia polystachya. However, when I type the ‘old’ name into the BSBI’s database the clever software automatically makes the correction.
Roadside plants, near Glen App, South Aryshire. Photo: John Grace.
The plants I encountered were at Glen App, a location beside the road leading from Ballantrae to the Ferry Port of Cairnryan, near the point where the Ayrshire Coastal Path leaves the road and heads west to the sea. The first plants I saw were at the road edge, flowering profusely and catching my attention. Running parallel to the road is the Water of App, a small river that flows south to Finnart Bay. When I ventured down the slippery bank of the river there were many more plants, forming large patches. They were less than 2 metres tall. Those exposed to sunlight were distinctly pink-tinged.
This image shows the red pigmentation. Glen App, October 2022. Photo: John Grace.
Below, I’ve assembled some images of distinctive features. The leaves are quite large – I’ve chosen the largest I could find, most are smaller. Note the shape, the leaves are not heart-shaped like the Giant Knotweed. The attachment of leaves to the stem is typical of the family Polygonaceae but the stipules are especially large in this case. Note the red pigmentation of the leaf-stalk. The flowers are a few mm in diameter in spikes that are 3-6 cm long. In the more exposed habitats the flowers and their stalks are pink-tinged. The red pigmentation of the plant seems to depend on exposure to sunlight – the more open the habitat the more red is developed. With a hand lens you can see that the flowers have two types of petal (following botanical convention we should call them ‘tepals’), and the petals are not fused together. Three petals are broad and two are narrow (see the flower in the centre of the picture below). They have eight stamens with purple anthers and three styles.
A selection of images to show the general features of the plant. See the text above. Photos: John Grace.
The species comes from the Himalayan subalpine region, Pakistan and northwestern India through Nepal, Bhutan, Tibet, southwestern China and northern Myanmar. It has been deemed ‘invasive’ in some parts of its own native range.
The species was introduced as a garden ornamental to Britain and Belgium some time before the year 1900 and recorded in the wild in 1917. In Britain it has spread from its original introduction in southwestern England and it now scattered throughout the United Kingdom (see Conolly 1977 for older maps showing the spread of all the large invasive Knotweeds).
It is considered invasive in several countries, but in Britain this plant is not yet ‘outlawed’ like the Reynoutria species; it can still be obtained from specialist nurseries. One highly-regarded supplier describes the species with this alluring prose:
“Large, showy, invasive perennial with a frothy mass of honey scented white flowers that fade to pink over a long period from mid-summer into autumn. Best in moist soil in sun or part shade. Height and spread to around 2m x 1m. Although this plant is invasive the fragrance is amazing and bees, butterflies and other pollinators love the flowers. Great as a statement plant for the back of a border”.
The British distribution shows that the plant is not at all fussy about where it will grow. There is a preference for the west coast, but plenty of records from the east and many are inland. It seems to be gradually spreading.
British distribution Himalayan Knotweed, Koenigia polystachya (=Persicaria wallichii). From BSBI.
The best map I could find of the global distribution is from the EPPO data base and it is reproduced below (even this map seems incomplete in the native range). The species has become naturalised in North America and New Zealand.
Global distribution of Himalayan Knotweed Koenigia polystachya according to the European and Mediterranean Plant Protection Organization (EPPO), taken from their free-access web site https://gd.eppo.int/taxon/POLPS/distribution.
As for plant names, I have found one other English name: the Cultivated Knotweed. It isn’t at all poetic but presumably chosen by the horticultural industry to distinguish the plant from the Japanese Knotweed. There are many Latin synonyms and rather than list them I can refer you to the relevant page of the Royal Botanic Garden, Kew (click). There, you can find the names and also the names of the botanists who have worked on the taxonomy.
Reference
Conolly AP (1977) The distribution and history in the British Isles of some alien species of Polygonum and Reynoutria. Watsonia, 11: 291-311.
©John Grace
Botany in Scotland 
At last I have found information about Himalayan knotweed diolch 🐱
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