Himalayan knot Japanese Knotweed
Himalayan Knotweed (Persicaria affinis) is another of those high altitude plants from elsewhere that does very nicely in the United Kingdom. Must be something about the climate.
This one is a garden favourite, spawning a handful of popular cultivars. It's hard to resist something that flowers from late spring right through into autumn (these pictures were taken at Wakehurst Place last weekend), followed by frost-bitten leaves turning red then chestnut brown. Right now it's still about the flowers.
You can find Persicaria affine growing naturally on rocky outcrops near the retreating glaciers of the Karakoram Mountains in West Pakistan. If you do find it, you'll be about 3.5 kilometres higher up than Kew Gardens. It will be, as Kew Director Sir Joseph Hooker remarked when he visited the Himalaya back in the late nineteenth century, 'hanging in rosy clumps from moist precipices'.
A more recent study of its ecology in the Karakoram Mountains found that Persicaria affine doesn't like to have its feet wet for long, at least in early summer, and would prefer a few more snow-free days than some of its fellow flora. On the other hand because it is not eaten by Himalayan sheep or goats it is very competitive in heavily grazed vegetation.
If taller shading plants are inhibited for some reason - grazing or soil preferences - Persicaria affine will thrive. So in a sunny garden setting, with a little weeding, it's a great little ground cover.
A few of the 150 or so species of Persicaria are weedy, some quite invasive, but this one seems to be tough and spreading without being worryingly so. And rest assured that although called Knotweed, it doesn't share the propensity of Japanese Knotweed (Fallopia japonica) to proliferate and persist.
Persicaria is named after the peach, Persica, a reference to the similarity of the leaf shape in some species. It used to be part of a much more broadly circumscribed Polygonum, so you might see the name Polygonum affine here and there.
More information: Kew profiles this plant on its website and there is detailed study of its ecology in the 1968 volume of the British Ecological Society's Journal of Ecology.