Knotgrass, the herb of the wayside

Bian Xu, knotgrass, grows where nothing else grows: verges, courtyards, trampled ground, compacted and poor soils. A low, spreading annual, it lays its slender stems on the ground, with swollen nodes capped by a silvery membranous sheath. Its Latin name describes it: Polygonum, “many knees”, for its nodes, and aviculare, “of little birds”, whose favourite food is its seeds. The Chinese countryside calls it “flat bamboo”, for its knotty stems and narrow leaves. It grows in every Chinese province, is harvested in summer before full flowering, and appears as early as the Shen Nong Ben Cao Jing.

Water on the move, Heat carried along

In Traditional Chinese Medicine, Bian Xu is bitter and slightly cool, and enters only the Bladder: one flavour, one meridian, few plants are so narrowly defined. The bitter dries Dampness and makes things descend, the coolness clears Heat. It is traditionally used to clear Heat and let water flow: when Dampness and Heat lodge in the Lower Burner, the bitter makes them descend, the cool clears, and the passage opens again. The ancient texts put it in one phrase: it clears out the water held in the Bladder.

A herb of balance between Dampness and Heat

The tradition reserves it for situations where Dampness and Heat are roughly equal. When Heat dominates, dianthus (Qu Mai), which clears more strongly, is preferred; the two also form the founding pair of Ba Zheng San, the only classical formula where Bian Xu has a named role. The name “knotweed” covers several Chinese plants with entirely different uses, such as Hu Zhang or He Shou Wu: none can replace Bian Xu. It is a herb for excess, not deficiency, to be taken for as long as needed, and no longer.