J. Elisha Mitchell Sci. Soc. 16: 73. 1900.
Shrubs or trees, multistemmed, 30–70 dm. Stems: twigs: new growth golden green to strongly reddish tinged, glabrous, 1-year old usually shiny, dark-brown, older gray; thorns on twigs 1-year old shiny, deep brown to blackish, often black-tipped, stout to slender, 2.5–6 cm. Leaves: petiole length 30–60% blade, sparsely to densely glandular; blade ovate, 3–6 (–9) cm, 70–100% mature size at anthesis, base subcordate, truncate, or rounded to cuneate, lobes 3–6 per side, sinuses moderately deep, lobe apex acute to acuminate, margins serrate, teeth numerous, small, larger ones conspicuously glandular, particularly at base, smaller ones inconspicuously gland-tipped, veins 4–7 per side, apex acute, adaxial surface densely appressed-scabrous-pubescent young, glabrescent, rarely glabrate. Inflorescences 5–12-flowered; branches glabrous; bracteoles usually absent (even young), sometimes few to several, often red-tinged, ± linear. Flowers 13–17 (–22) mm diam.; hypanthium glabrous; sepals 4–5 mm, margins subentire, abaxially glabrous; stamens 5–10, anthers pink to reddish purple; styles 3–5. Pomes bright to deep red, suborbicular to ellipsoid, 8–15 mm diam.; sepals erect-patent or erose; pyrenes 3–5, dorsally shallowly grooved. 2n = 68.
Phenology: Flowering Apr–Jun; fruiting Sep–Nov.
Habitat: Old fields, fencerows, brush, open woods, montane balds and rocky slopes
Elevation: 0–1600 m
N.B., Nfld. and Labr. (Nfld.), N.S., Ont., P.E.I., Que., Ala., Ark., Conn., Ill., Ind., Ky., Maine, Md., Mass., Mich., Minn., N.H., N.J., N.Y., N.C., Ohio, Pa., R.I., Tenn., Vt., Va., W.Va., Wis.
Crataegus macrosperma ranges from north-central Minnesota through the central Great Lakes region and the St. Lawrence Valley to southern Newfoundland south to Arkansas, and in the Appalachians, to northeastern Alabama. Recent observations show that the species had been cultivated at the Arnold Arboretum from an alleged southern Missouri provenance, which links with the discovery of specimens of wild C. macrosperma and C. schuettei from the Ozarks of northern Arkansas. These populations are relictual, and the southwestern limits of the species are poorly known.
Crataegus macrosperma is one of the earliest hawthorns to flower, preceded only by C. collina in the south, C. mollis in the Midwest, and C. chrysocarpa and allies in the north.
Crataegus macrosperma is most similar to C. fluviatilis, the latter differing only by the hairy inflorescences. It is similar also to C. wootoniana of New Mexico, the latter differing mainly from the roanensis form of C. macrosperma by its frailer nature, smaller flowers, and perhaps thinner, smaller, but proportionately wider leaves, as well as its disjunct distribution. Crataegus schuettei is another similar but usually stouter species with 20 stamens; C. flabellata, often confused with C. macrosperma in the literature, should be readily distinguished by its glandular to glandular-serrate sepal margins and its hairy inflorescences.
In Crataegus macrosperma, leaves vary in size, shape of base, number, depth and sharpness of lobes, and number of lateral veins. The fruit is usually more or less oblong; forms with suborbicular fruits are found nearly throughout. Recent typification (J. B. Phipps 2009) shows that some of the varieties were based on wrong material; critical modern study of their variation is lacking, and none are formally recognized here. Finally, the stamen number (5–10) of C. macrosperma is rather variable.
Suberect shrubby forms with small foliage (3–4 cm), cuneate leaf bases, and ellipsoid fruits represent Crataegus roanensis and are common to the Great Lakes region to Pennsylvania extending southward to the species southern limits, where they become large shrubs or small trees. Plants of similar range, stature, and leaf size, but with a more nearly truncate leaf base and suborbicular fruit, represent C. macrosperma in the strict sense, but intermediates are quite common.
Crataegus pastorum is similar to the typical form. Crataegus demissa is a scarce, poorly understood form with the most proportionately wide, broad-based, but small leaves. Forms with particularly large leaves include C. acutiloba and C. matura. These usually have the most numerous lateral veins and lobes, which are particularly acuminate, and are most common in Quebec, New England, and the Canadian Maritime provinces. Crataegus acutiloba is distinguished by rather oblong and large leaves (to 8 cm) with cuneate to broadly cuneate bases, acute and usually reflexed lobes, and narrow, oblong-pyriform fruits. Crataegus matura has the largest leaves in the species (to 9 cm) with broad-cuneate to subtruncate bases, particularly deeply incised lobes, but with suborbiculate to broadly oblong fruits. A puzzling observation is that the two large-leaved forms may have shallowly concave pyrene sides. Crataegus pentandra has long and quite narrow leaves, with a cuneate to rounded, rather than truncate, base, usually rather shallow lobes as well as narrow fruits.
The recently investigated Minnesota populations of Crataegus macrosperma barely differ from the C. apiomorpha form of C. fluviatilis except in lacking inflorescence pubescence; they are further discussed under the latter. Crataegus cyanophylla from Illinois is probably conspecific with C. macrosperma. The identity of C. tenuifolia Britton, usually placed here, requires investigation (see comments under C. holmesiana). Crataegus tenella Ashe, from southeastern Pennsylvania and Delaware, represents intergradation with the southeastern Virginia populations of C. iracunda. Forms with nearly glabrous young adaxial leaf surfaces and slender blackish thorns found not uncommonly in the southeast may represent introgression with C. pruinosa or a species in ser. Populneae.