Flora of North America species comparison

Crataegus uniflora is widespread in the southeastern United States from Long Island (New York) to eastern Texas; it is most abundant in the coastal plain and lower piedmont. A disjunction in the Mississippi embayment between trans-Mississippian and cis-Mississipian populations mirrors a similar pattern in C. spathulata (J. B. Phipps 1998).

Crataegus uniflora is usually easy to identify because of its small size, leaves small and coriaceous with lobes absent, numerous straight, fine thorns, inflorescences uniflorous or few-flowered, sepals glandular-pectinate and equal to or longer than the petals, anthers white to cream, and pomes yellowish to ruddy. Possible confusion with some members of ser. Lacrimatae is treated under the series discussion. Sterile plants might be confused with small, sterile ones of C. berberifolia (ser. Crus-galli) except for the smaller number of lateral veins on the leaves. An interesting feature of C. uniflora is the occasional deep lobing of extension-shoot leaves, even narrow ones, with veins to sinuses.

Crataegus uniflora is somewhat variable in plant size, leaf shape, number of flowers to an inflorescence (though commonly one), and anther color (usually ivory to cream). The typical form, which is common, has 1- or 2-flowered inflorescences, and is usually a small shrub with more or less spatulate to narrowly obovate leaves. Some particularly dwarf forms of this, on laterite soil in Georgia, have been observed with over 100 flowers when less than 3 dm.

Forms of Crataegus uniflora with 3–6-flowered inflorescences are also common and are represented by C. parvifolia and C. trianthophora. These are connected to the typical uniflorous kind by numerous intermediates and may be distinguished from the related C. brittonii by their smaller and narrower leaves with lobes absent.

Three anomalous variants, very similar or identical to Crataegus uniflora except for possessing small, more or less unlobed but glandular-serrate sepals, are known. These are: C. croomiana Sargent from northern Florida, C. bisulcata Ashe from North Carolina, and C. grossiserrata Ashe from Florida. The first, also best known, is further distinguished by pink anthers, the second by broader leaves, and the third by often orbiculate (unusual for ser. Parvifoliae) extension-shoot leaves with a tendency to deep lobing sometimes with veins to sinuses, and particularly strong and sharp leaf teeth. All suggest hybrid origin with ser. Lacrimatae.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Crataegus calpodendron is most common in the northern and Appalachian part of its range, where it is more likely to grow in the open (shrubs to 50 dm); in the southern part of its range, it is often in shaded habitats (trees to 70 dm). The distinctive vase-shaped branching pattern of well-grown northern C. calpodendron makes it easily distinguished even in winter, the lack of a central dominant trunk and the arching side trunks being characteristic. In open areas in the northern part of its range, mature leaves of C. calpodendron are often somewhat coriaceous and keeled along the midvein. In the south, the leaves remain thin until maturity. It is the second latest native hawthorn to flower, usually mid June (north) and early May (south, for example, central Alabama).

Vegetatively, distinction between Crataegus calpodendron and C. succulenta or C. macracantha is best supported by the more strongly impressed leaf venation, usually glabrous abaxial leaf surfaces, dark purple-brown, shiny, one-year twigs, and often longer and stouter thorns in the latter.

Crataegus calpodendron is variable in branching pattern, indumentum, and leaf and fruit shapes. Examination of a large number of specimens does not reveal discontinuities between leaf shapes, neither do these shapes correlate with the varieties hitherto recognized primarily on fruit characteristics. In general, C. calpodendron in the narrow sense, C. globosa, C. mollicula, and C. mollita represent forms with narrower, rhombic-elliptic, less deeply incised leaves with somewhat blunter lobes and teeth; C. hispidula, C. insperata, and C. obscura represent forms with nearly isodiametric leaves, the deepest and sharpest sinuses, and the biggest and sharpest teeth; C. scabera and C. spinulosa are intermediate. Smaller leaf and pome sizes are more common in drier or hotter terrain.

Crataegus ×rupicola Sargent (synonym of C. harveyana Sargent), from Missouri, represents a form with glands at the base of the leaf and on the petiole and perhaps is not pure calpodendron. Crataegus warneri Sargent from east-central Texas is provisionally placed here, although it could be a hybrid or hybrid derivative of C. calpodendron × C. crus-galli or an odd form of C. ashei with ten stamens and red anthers. Crataegus warneri has broadly ovate leaves 3.5–5 cm and appears to be somewhat xeromorphic; it is distinguishable from other forms of C. calpodendron with similar leaves only by its ten stamens and lack of pitted pyrenes. Crataegus ×simulata Sargent, from Missouri, is a probable hybrid of C. calpodendron with C. reverchonii var. palmeri (as C. palmeri) according to E. J. Palmer (1952). Crataegus ×vailiae is the most convincing hybrid of C. calpodendron (with C. uniflora; see Interserial Hybrids).

(Discussion copyrighted by Flora of North America; reprinted with permission.)