Usually stout well branched subglabrous or finely puberulent annual herb 15-100 cm tall. Leaves narrowly elliptical to narrowly lanceolate, 1-10 cm long, 0.4-2.5 cm wide, narrowly cuneate at the base, the apex acuminate; submarginal vein not prominent; petioles 0.3-1.5 cm long. Sepals 4-6, deltoid, 1.5-4.5 mm long, 0.4-1.5 mm wide, puberulent, apiculate. Petals yellow, obovate, 1.8-2 mm long, 0.7-1.2 mm wide. Stamens as many as sepals; filaments 0.5-1.2 mm long; anthers 0.4-0.7 mm long, shedding pollen directly onto the stigma at anthesis. Pollen grains shed individually. Disc elevated about 0.5 mm, with a sunken glabrous or white hairy nectary opposite the base of each petal. Style 0.5-1.2 mm long; stigma globose. Capsule relatively thin-walled, puberulent, 10-28 mm long, 1-2 mm thick, terete, light brown, subsessile. Seeds in one or two rows in each locule of the capsule, diagonal, light brown with darker thin red-brown stripes, elongate-ovoid, 0.7-1.4 mm long, 0.3-0.4 mm thick, apiculate at one end; raphe inconspicuous; each column of seeds, which may include two rows, loosely enclosed in a column of spongy light brown endocarp from which the seeds fall easily, but which also separates into 1- or 2-seed units. 2n = 48. Flowers Jul-Jan; fruit Jul-Jan.

TAIPEI: Nankang, Leu 4. ILAN: Kungkuan to Tungkang, Peng 7018. TAOYUAN: Shanchuhu, Peng 13488; Yungan, Kao 8779. HSINCHU: Yulo River, Peng 12139. MIAOLI: Touwu, Peng 5415; Yunghsing, Lin 519. KAOHSIUNG: Liukuei, Peng 11203.

Asia, from the Amur River to Korea, throughout China to Yunnan, Kwangtung, and Hainan, North Vietnam; central and southern Japan to Taiwan. Often common in low moist places like rice paddies and at low and medium elevations.

Plants usually flower in the summer. Those persisting through winter often bear cleistogamous flowers that set smaller, narrowly elliptic-lanceolate fruits (ca. 4.5-6 mm long vs. (10-)15-28 mm long in the normal, summer-flowered plants). Their sepals are smaller, ca. 1-1.2 mm long, often coalesced and enclosing the inconspicuous, vestigial petals (sometimes entirely lacking). The following specimens are representative. TAIPEI: Nankang, December 12, 1988, Leu 15. HUALIEN: Meilun, January 18, 1984, Chen s.n. The distinctions between subsp. epilobioides, with subglabrous young leaves, glabrous disc, and seeds 1-1.4 mm long, and subsp. greaxtrexii, with puberulent young leaves, white-hairy disc, and seeds 0.7-1 mm long, do not seem to hold for populations from Taiwan and are not recognized here.