Hypericum japonicum Thunb. ex Murray

Hypericum japonicum is very variable and, the variation being continuous, eight nodal variants have been recognized but not formally named. Five of these occur in China:

(i) Stems erect to decumbent; inflorescence from terminal node only; bracts linear to lanceolate; sepals narrow ("japonicum" in part). Lowland.

(ii) Stems erect to decumbent; inflorescence from more than one node, sometimes with other branches; bracts linear to lanceolate; sepals narrow ("japonicum" in part). Lowland.

(iii) Stems erect; inflorescence from terminal node; bracts leaflike; sepals narrowly oblong to narrowly elliptic ("maximowiczii"). ?Lowland.

(iv) Stems decumbent to procumbent; inflorescence from terminal node; bracts leaflike; sepals narrowly oblong to narrowly elliptic ("laxum"). Lowland to upland.

(v) Stems decumbent to prostrate; inflorescence from terminal node; bracts leaflike; sepals (at least outer) broadly elliptic to obovate ("calyculatum"). Upland to montane.

Herbs, annual, 2-45 cm, tall or long, erect to decumbent or prostrate and rooting at base or along stem; stems solitary or ± cespitose, unbranched below inflorescence or variously branched. Stem 4-lined, with scattered gland dots. Leaves sessile; blade usually ovate or ovate-triangular to oblong or elliptic, 0.2-1.8 cm × 1-10 mm; thickly papery, abaxially paler and sometimes glaucous; laminar and intramarginal glands dense; main lateral veins 1-7, basal, tertiary reticulation not visible; base cordate-amplexicaul to cuneate, not decurrent, margin plane, apex obtuse to rounded. Inflorescence 1- to ca. 30-flowered, terminal, sometimes with flowering branches from up to 3 nodes below; bracts and bracteoles lanceolate-subulate to leaflike. Flowers 4-8 mm in diam., stellate; buds cylindric-ellipsoid, apex ± obtuse. Sepals free, erect, narrowly oblong or rarely lanceolate to elliptic, subequal to unequal, 2-5.5 × 0.5-2 mm; laminar glands lines, distally dots; marginal glands absent; apex acute or obtuse to rounded; veins 3-5. Petals pale to bright yellow or orange, obovate to oblong or elliptic, 1.7-5 × 0.8-1.8 mm, 0.4-0.8 × as long as sepals; laminar glands absent; margin entire, eglandular. Stamens 5-30, irregular or in 5 obscure groups when few, longest 1.5-2.8 mm, 0.4-0.8 × as long as petals. Ovary ± broadly ovoid to subglobose; styles (2 or)3, 0.4-0.8(-1) mm, 0.4-0.6 × as long as ovary, spreading. Capsule cylindric to globose, (2-)2.5-6 × 1.3-2.8 mm, usually slightly shorter than to slightly exceeding sepals. Seeds straw-yellow, ca. 0.5 mm; testa finely linear-scalariform. Fl. Mar-Oct, fr. Apr-Nov. 2n = 16.

S. India, Ceylon, Punjab to Japan, S. Korea and S.E. China, south to New Guinea, S. & S.E. Australia, New Zealand, Hawaii.

Anhui, Fujian, Guangdong, Guangxi, Guizhou, Hainan, Hubei, Hunan, Jiangsu, Jiangxi, Liaoning, Shandong, Sichuan, Taiwan, Yunnan, Zhejiang [Bhutan, Cambodia, N and S India, Indonesia (Sumatra to Irian Jaya), Japan, S Korea, Laos, Malaysia, Myanmar, Nepal, Philippines, Sri Lanka, Thailand, Vietnam; SE Australia, Pacific islands (New Zealand)].

150-2600 m

Rice fields, ditches, marshes, grasslands, waste places; sea level to ca. 3000 m.

Brathys japonica (Thunberg) Wight; B. laxa Blume; Hypericum cavaleriei H. Léveillé; H. chinense Osbeck (1757), nom. utique rej., not Linnaeus (1759), nor Retzius (1788); H. japonicum var. calyculatum R. Keller; H. japonicum var. cavaleriei (H. Léveillé) Koidzumi; H. japonicum var. maximowiczii R. Keller; H. japonicum var. thunbergii (Franchet & Savatier) R. Keller; H. laxum (Blume) Koidzumi; H. nervatum Hance; H. thunbergii Franchet & Savatier; Sarothra japonica (Thunberg) Y. Kimura; S. laxa (Blume) Y. Kimura.

Hypericum japonicum, known as matted St. John's-wort, is an annual herbaceous flowering plant in the St. John's wort family Hypericaceae, in Hypericum sect. Trigynobrathys.