Description from Flora of China
Annual, biennial or perennial herbs, caulescent, with a taproot or woody branching caudex, occasionally with rhizomes. Stems one to several, simple or much branched. Leaves alternate, basal rosette leaves largest, decreasing in size upward, entire or toothed, often lyrate below, shortly petiolate below to subsessile above; stipules absent. Flowers perfect, zygomorphic to sometimes actinomorphic, forming a spicate raceme, not leafy, opening near sunset or near sunrise. Floral tube distinct, cylindric, deciduous soon after anthesis. Sepals (3 or)4, reflexed, green or yellowish. Petals (3 or)4, white, fading to reddish, rarely yellow, usually abruptly clawed. Stamens (6 or)8. Anthers versatile; pollen shed singly. Ovary with (3 or)4 locules, with 1(or 2) ovules per locule; stigma divided into (3 or)4 short linear lobes, receptive all around, and subtended by a ± conspicuous ringlike indusium. Fruit an indehiscent nutlike capsule with hard walls, broadly fusiform to subcylindric, terete to sharply (3 or)4-angled, sessile or basal portion sterile and stipelike. Seeds (1 or)2-4 per capsule, irregularly ovoid. 2n = 14, 28, 42, 56.
Two other species are known from cultivation. Gaura lindheimeri Engelmann & A. Gray is native to black-soil prairies of SC North America and is distinguished in part by its relatively large flowers (petals 1-1.5 cm), opening near sunrise, and sepals with long, erect hairs. It is commonly cultivated for its attractive flowers but is not known to be naturalized outside of its indigenous distribution; it is found in Hebei, Hong Kong, Jiangxi, and Zhejiang in China. Gaura biennis Linnaeus, native to a large area of C North America, has been in cultivation since ca. 1750; it is known from Yunnan in China but does not appear to be naturalized.
Recent molecular data demonstrate that Gaura, together with the unispecific genus Stenosiphon Spach, although comprising a monophyletic group, is embedded within the diverse genus Oenothera, and is best treated as part of that genus. The morphological characters used to delimit the genus-nutlike, indehiscent capsules with 1-4 seeds, flowers mostly zygomorphic, stigma lobes short-still delimit the group but now are viewed as specializations derived within Oenothera.
One of the earliest names in Onagraceae based on Chinese material was Gaura chinensis Loureiro (Fl. Cochinch. 1: 225. 1790). However, Merrill (Trans. Amer. Phil. Soc., n.s., 24(2): 39, 290. 1935) considered this to be one of Loureiro’s "grave errors" and, using Loureiro’s description, re-identified this taxon as a species of Haloragis J. R. Forster & G. Forster. Orchard (Bull. Auckland Inst. Mus. 10: 1-299. 1975) subsequently revised that group of Haloragaceae, and the species is currently treated as Gonocarpus chinensis (Loureiro) Orchard (see the following page).
Twenty-one species: C and E North America to C Mexico; one species (naturalized) in China.
(Authors: Chen Jiarui (陈家瑞 Chen Chia-jui); Peter C. Hoch, Warren L. Wagner)