Qing Dai

Baphicacanthus cusia (syn. Strobilanthes cusia), Assam indigo or Chinese rain bell, is a perennial herb in the Acanthaceae family, growing 0.5-1 m tall with quadrangular, swollen-jointed stems that often root at the nodes. The leaves are opposite, elliptic to lanceolate, 10-25 cm long, with serrate margins and a pointed apex; they turn dark blue-black on bruising or drying due to indican oxidation. Flowers are tubular, pale violet to pink, borne in short terminal or axillary spikes subtended by leafy bracts. The fruit is a small club-shaped capsule containing four seeds. Qing Dai is the dark blue-violet pigment-rich powder prepared by fermenting and lime-treating the leaves and stems, then collecting the floating indigotin-rich froth (POWO; Wikipedia).