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Shrub or tree to 14 m, multistemmed. Bark grey, peeling to reveal reddish patches. Branchlets grey, glabrous or pubescent; four longitudinal striations may be visible when young. Leaves deciduous, opposite or subopposite, 2–9(–11) × 1–5 cm, shape variable, oblong, ovate, elliptic or lanceolate, papery, upper surface glabrous, lower surface glabrous or minutely pubescent, 3–10 secondary veins on each side of the midrib, apex acuminate; petiole 0.2–0.4 cm long and largely glabrous. Panicles terminal or axillary, pyramidal, 7–16(–30) cm long, densely flowered, sometimes with greyish brown pubescence. Flowers 6-merous; calyx tube cup-shaped, 0.2 cm long with (10–)12(–14) darkened veins or obscure ribs, pubescent or glabrous; epicalyx absent; petals white, pink or purple, 0.7–1 cm long (incl. stalk); stamens 15–30, dimorphic. Capsule globose to oblong, 0.6–11 × 0.4–0.6 cm with four to six valves. Flowering June to August, fruiting July to October (China). Ohwi 1965, Furtado & Srisuko 1969, Qing et al. 2007. Distribution CHINA: Anhui, Fujian, Guangdong, Guangxi, Hubei, Hunan, Jiangsu, Jiangxi, Qinghai, Sichuan, Zhejiang; JAPAN; PHILIPPINES; TAIWAN. Habitat Forest edges, streamsides, at low altitudes. USDA Hardiness Zone 7. Conservation status Not evaluated. Illustration Furtado & Srisuko 1969.
Along with Lagerstroemia limii, L. subcostata also is overshadowed by its relatives, but Dirr’s (1998) comment that he finds ‘minimal redeeming qualities’ in these ‘other’ species is perhaps a little harsh. The bark of L. subcostata is smooth and superficially grey or white but it can flake off to reveal reddish patches. As with L. fauriei the plant is usually multistemmed, with stiffly upright trunks bearing a very wide canopy. The flowers are small, white with a pink tinge, and do not make much show. There is a fine specimen at the US National Arboretum, with a very white trunk; this was grown from an offering in the Index Seminum of the Botanical Garden of Osaka City University, Japan, in 1961 (R. Olsen, pers. comm. 2007). At Quarryhill, grown from seed collected in Japan in 1989 (SOJA 353), L. subcostata has made trees up to 8 m tall. It is in cultivation elsewhere in the United States from other collections, possibly including those made by John L. Creech in Taiwan in 1967 (Aniśko 2006).