Flora of North America species comparison

Crataegus phaenopyrum has a wide native range across the middle latitudes of the United States from Missouri to Maryland and southward but is not particularly abundant. In the northern and western states and Ontario, it is introduced. All alleged specimens for Alabama proved to be wrongly identified (J. B. Phipps 1998).

The flowering season of the Washington thorn is the latest of all North American hawthorns. The species is one of the most widely planted ornamentals in the genus, being elegantly treelike and possessing beautiful, remarkably rust-resistant foliage at all seasons as well as attractive flowers and fruit. Several cultivars are known.

Crataegus cordata (Miller) Aiton 1789 (= Mespilus cordata Miller 1757) was commonly used for C. phaenopyrum until the early 20th century (J. B. Phipps 1999).

(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.)