19. Crataegus uniflora Münchh. (one-flowered hawthorn)

C. parvifolia Aiton

C. trianthophora Sarg.

Pl. 528 c–e; Map 2430

Plants relatively small shrubs, 0.4–1.5 m tall, the larger stems unarmed or with unbranched thorns, the bark gray, relatively smooth to finely roughened, occasionally splitting longitudinally for short stretches, exposing a yellowish brown underlayer. Branchlets with scattered (rarely sparse or absent) thorns, these 2–5(–8) cm long, slender, straight or nearly so at maturity, black or nearly so at first year. Twigs brown at second year, densely and finely hairy when young, becoming glabrous and gray with age. Petioles absent or to 2(–6) mm long, then densely hairy, nonglandular or with a few long, slender, stalked glands on the upper surface. Leaf blades 1.5–4.0(–5.0) cm long, 1/2 as wide as long or usually wider, oblanceolate to spatulate, elliptic, or obovate in outline, usually symmetric in outline, angled or slightly tapered at the base, the tip rounded to bluntly or broadly (but sharply) pointed, unlobed, the margins finely to relatively coarsely bluntly toothed or scalloped, mostly at or above the midpoint, the teeth often gland-tipped when young, the glands often shed with age, the texture somewhat leathery, the upper surface moderately to densely pubescent with short stiff hairs when young, becoming sparsely hairy to glabrous with age, usually somewhat shiny, the undersurface densely pubescent with softer hairs along the main veins, more sparsely hairy between the veins, the secondary veins 2–4(–5) per side, the ultimate branches extending to the teeth. Inflorescences of solitary flowers or less commonly small clusters of 2 or 3(4) flowers, the stalks densely hairy and occasionally with 1 or 2 slender, stalked glands, the bractlets mostly membranous, small, linear to narrowly elliptic, with small gland-tipped teeth, shed early. Flowers 10–15 mm in diameter, the hypanthium densely woolly. Sepals 5–8 mm long, persistent and conspicuous at fruiting, lanceolate-triangular (2–4 mm wide), the margins strongly toothed and/or with slender lobes, the teeth and lobes gland-tipped, sparsely to moderately hairy on the inner surface, moderately to densely hairy on the outer surface. Petals 5–7 mm long, more or less circular, white. Stamens 20, the anthers pale yellow. Styles 5. Fruits 8–10 mm long, 8–10 mm wide, globose or subglobose, sometimes with a low, poorly developed, raised collar at the tip, the surface yellowish green, yellow, yellowish orange, or occasionally reddish orange at maturity, woolly during development, but usually becoming more or less glabrous with age, not shiny. Nutlets 4 or 5, the lateral faces not pitted. 2n=51; triploid or tetraploid (by flow cytometry; see Talent and Dickinson [2005]). Early May–late June.

Scattered in the Ozark Division north locally to Callaway and St. Louis Counties; also a single historical collection from the Crowley’s Ridge Section of the Mississippi Lowlands Division in Dunklin County (eastern [mostly southeastern] U.S. west to Missouri, Oklahoma, and Texas; Mexico). Mesic to dry upland forests, glades, banks of streams, and ledges and tops of bluffs; occasionally also pastures, fence rows, and roadsides.

Crataegus uniflora is somewhat variable in plant size, leaf shape, number of flowers per inflorescence (usually one), and anther color. Crataegus trianthophora (described from Carter County, Missouri), for instance, nearly always has inflorescences with more than one flower. Lance (2011) suggested the recognition of a second variety, var. brittonii (Eggl.) R.W. Lance, based on somewhat taller plants in the southeastern United States with larger, more noticeably lobed leaves. However, Phipps and Dvorsky (2006) presented evidence in support of maintaining this taxon as a separate species, C. brittonii Eggl.

Crataegus uniflora is distinctive among Missouri hawthorns, particularly in its small habit, slender spines, and large sepals. However, a putative hybrid with C. calpodendron known as C. ×vailiae shares some of these features. It is a larger shrub with up to 8-flowered inflorescences. It is discussed further in the treatment of ser. Macracanthae.