Description from Flora of China
Persicaria tinctoria (Aiton) Spach.
Herbs annual. Stems erect, 50-80 cm tall, usually branched. Petiole 5-10 mm; leaf blade green, dark blue-green when dry, ovate or broadly elliptic, 3-8 × 2-4 cm, abaxially sometimes appressed-pubescent along veins, adaxially glabrous, base broadly cuneate, margin shortly ciliate, apex obtuse or slightly acute; ocrea tubular, 1-1.5 cm, membranous, appressed pubescent, apex truncate, long ciliate. Inflorescence terminal or axillary, spicate, dense, 2-5 cm; bracts green, funnel-shaped, ciliate, each 3-5-flowered. Pedicels equal to bracts, slender. Perianth pinkish, 5-parted; tepals ovate, 2.5-3 mm. Stamens 6-8, included. Styles 3, connate at lower part. Achenes included in persistent perianth, brown, shiny, broadly ovoid, trigonous, 2-2.5 mm. Fl. Jun-Sep, fr. Aug-Oct.
This species is used medicinally and for dyeing.
Moist valleys, stream banks, widely cultivated; 200-1000 m. Widespread in China [Indo-China; cultivated and occasionally naturalized elsewhere].