Description from Flora of China
Hypericum perforatum var. confertiflorum Debeaux; H. perforatum var. microphyllum H. Léveillé (1908), not Candolle (1815).
Herbs, perennial, 20-60(-100) cm tall, erect from creeping and rooting base; stems numerous to few, much branched especially distally. Stems 2-lined, with few black glands on lines. Leaves sessile to subsessile; blade ± narrowly elliptic to ± narrowly oblong or linear, (0.7-)1-2.5(-3) cm × 3-7(-15) mm; thickly papery, abaxially paler; laminar glands pale, scattered and sometimes black, few, dots; intramarginal glands black, spaced, interspersed with small dense pale ones; main lateral veins ca. 2-paired, tertiary reticulation lax or scarcely visible; base subcordate-amplexicaul to rather narrowly cuneate, margin entire, plane or ± recurved, apex obtuse. Inflorescence 3- to numerous-flowered, from 1-3 nodes; flowering branches curved-ascending from up to 15 or sometimes more nodes below, the whole nearly flat-topped to broadly pyramidal or cylindric; bracts and bracteoles to 4(-7) mm, narrowly lanceolate to linear, margin entire. Flowers 1.5-2.5(-3) cm in diam., stellate; buds narrowly ovoid, apex acute. Sepals free, erect in bud, recurved in fruit, narrowly oblong or lanceolate to linear, equal, 3-4(-5) × 1-1.2 mm; laminar glands pale and often a few black, in 2(-4) rows, streaks (basally) to dots; intramarginal glands black, few, or absent; margin entire, apex acute to acuminate with tip somewhat glandular; veins 3(-5). Petals golden yellow, oblong to oblong-elliptic, (0.8-)1.2-1.5 cm × 5-6 mm, 3-4 × as long as sepals, asymmetric; laminar glands black or pale, dots to lines, or often absent; intramarginal glands black or pale, distal, in sinuses when present; margin distally ± crenate. Stamens 40-60, apparently 3-fascicled, longest 6-8 mm, 0.5-0.7 × as long as petals. Ovary narrowly ovoid to ovoid-ellipsoid; styles 3, 4.5-6 mm, 1.5-2 × as long as ovary, broadly to rather narrowly spreading. Capsule ovoid-conic to ovoid, 3-6.5 × 3-5 mm, 1-1.5 × as long as sepals; valves with abaxial vittae and lateral yellowish, elongate or short vesicles. Seeds dark brown, ca. 1 mm; testa finely linear-foveolate. Fl. Jun-Sep, fr. Jul-Oct. 2n = 32, 48.
Hypericum perforatum is apparently an allotetraploid that would appear to have arisen from a cross between two diploid taxa, namely H. maculatum subsp. immaculatum (Balkan Peninsula) and H. attenuatum (W Siberia to China).
In both the SW (S Europe, Mediterranean) and SE (China) parts of its range, small-leaved forms of Hypericum perforatum evolved, and both have been named var. microphyllum (respectively by Candolle in 1815 and H. Léveillé in 1908). This more southern Chinese form (var. confertiflorum Debeaux), which intergrades morphologically with the larger-leaved more northern Chinese form, has small, narrow leaves, with dense, smallish partial inflorescences remaining distinct on elongate, widely ascending branches.
Hypericum perforatum has been subdivided into four subspecies, of which two occur in China.
Open woodlands, meadows, grasslands, and steppes, riverbanks, stony and grassy slopes, roadsides, in dry or well-drained habitats; 100-2800 m. Gansu, Guizhou, Hebei, Henan, Hubei, Hunan, Jiangsu, Jiangxi, Shaanxi, Shandong, Shanxi, Sichuan, Xinjiang, Yunnan [NW India, Kazakhstan, Kyrgyzstan, Mongolia, Russia; NW Africa, SW to C Asia, Atlantic islands, Europe (except extreme N); introduced into many parts of the world].