Description from Flora of China
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.
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.
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.
Rice fields, ditches, marshes, grasslands, waste places; sea level to ca. 3000 m. 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)].