Koenigia polystachya

Koenigia polystachya is a species of flowering plant in the knotweed family, known by the common names Himalayan knotweed and cultivated knotweed. It has several regularly used synonyms, including "Polygonum polystachyum", "Aconogonon polystachyum" and "Persicaria wallichii". 
"Koenigia polystachya" is native to Asia (southwestern China, Indian Subcontinent, Myanmar, Afghanistan) and it is planted many places as an ornamental. It has the capacity to become an invasive species and has established itself in some parts of North America, primarily the Pacific Coast of the United States and Canada. It is an invasive weed in the mountains of Sri Lanka.
"Koenigia polystachya" is a rhizomatous perennial herb producing thick, hollow, erect stems easily exceeding one meter (40 inches) in length. Via its rhizomes it can form dense colonies. It may also spread asexually if sections of the stem containing rooting nodes are separated and moved to a new area; chopping the plant into small pieces does not necessarily keep it from growing. The hairy leaves are up to 20 centimeters (8 inches) long. The branching inflorescence is an array of lacy clusters of many small white flowers.
Called "thothnay" in Sikkim and Darjeeling areas of India, the pleasantly sour-tasting edible shoots and stems are used as vegetable or used to make piquant condiments and accompaniments for meals.