Description from Flora of China
Hypericum japonicum Thunberg var. kainantense Masamune; H. japonicum var. lanceolatum Y. Kimura; Sarothra graminea (G. Forster) Y. Kimura; S. saginoides Y. Kimura.
Herbs, perennial or annual, 5-30 cm tall, erect to decumbent but not rooting; stems solitary or ± cespitose, unbranched below inflorescence or variously branched. Stems 4-lined, eglandular. Leaves sessile; blade lanceolate or rarely ovate-lanceolate to linear, 0.6-1.3[-2.5] cm × 1-5[-8] mm; thickly papery, abaxially paler and sometimes glaucous; laminar gland dots dense distally, sparser proximally; intramarginal glands dense; with or without one pair of basal main lateral veins, veins prominent abaxially, tertiary reticulation not visible; base cordate to rounded or sometimes cuneate and usually somewhat decurrent and forming a "V," margin plane or recurved, apex obtuse to rounded. Inflorescence 1-21-flowered, terminal, sometimes with subsidiary branches from up to 3 nodes below, triangular-lanceolate to linear; bracts and bracteoles triangular-lanceolate to linear. Flowers 5-8 mm in diam., stellate; buds ellipsoid, apex subacute. Sepals free, erect, lanceolate to narrowly elliptic, subequal to unequal, 2.8-5[-9] × [0.8-]1.5-2 mm; laminar glands lines, distally dots; marginal glands absent; apex acute to subacute; veins 3-5. Petals pale to bright yellow or orange, obovate to oblanceolate, 5-10 × 2-5 mm, ca. 1.3 × as long as sepals; laminar glands few, streaks, or absent; margin entire, eglandular. Stamens 30-40, irregular, longest 2.5-4 mm, 0.4-0.6 × as long as petals. Ovary narrowly ovoid-conic; styles 3, 0.7-1.8 mm, ca. 0.9 × as long as ovary, ± spreading. Capsule ovoid-conic, 2.5-8 × 1-3.5 mm, ca. 2 × as long as sepals. Seeds brown, ca. 0.5 mm; testa finely ribbed-scalariform. Fl. May-Oct, fr. Jul-Oct. 2n = 16, 14.
The description of Hypericum japonicum var. kainantense, from Hainan, appears to fit H. gramineum better than H. japonicum. One of the authors (Robson) has not examined the type specimen of this variety or the specimen on which the record of H. gramineum from "Chang Ngo Shan" (Zhang Nao Shan, Wanning, Hainan) is based, so the occurrence of this species in Hainan remains unconfirmed.
Wet but well-drained habitats; (1200-)1900-2700 m. ?Hainan, Taiwan (Xinzhu), S Yunnan [Bhutan, NE India, Papua New Guinea, Vietnam; Australia, Pacific islands (Hawaii, New Caledonia, New Zealand)].