Crataegus [unranked] Tenuifoliae Sargent, Silva 13: 34. 1902; Crataegus sect. Tenuifoliae (Sargent) Eggleston

Shrubs or trees, 20–70(–80) dm, main trunk dominant. Stems: trunk bark buff to gray-brown, fibrous, checked into longitudinal plates, freshly exposed bark orange-brown; ˂branches spreading; twigs ± straight˃, new growth usually glabrous, sometimes sparsely pubescent, 1-year old deep reddish brown, older gray; thorns on twigs usually numerous, usually recurved, 1-year old shiny, dark brown to black, stout to slender, 2.5–6 cm. Leaves: petiole ˂1–3 cm˃, length 30–60% blade, glabrous, sessile-glandular or eglandular; blade ˂pale green abaxially, mid dark green adaxially˃, elliptic to ovate or ovate-deltate, 3–6(–8) cm, thin to chartaceous, base cuneate to rounded or truncate to subcordate, lobes 0 or 3–6 per side, sinuses shallow to deep, lobe apex usually acute, margins serrate, venation craspedodromous, veins (3 or)4–7 per side, apex usually acute, sometimes subacute or acuminate, abaxial surface glabrous, veins pilose young in some, adaxial appressed-scabrous-pubescent young, glabrescent. Inflorescences (4 or)5–10(–20)-flowered, convex panicles; branches glabrous or villous; bracteoles caducous, ˂absent or few to several˃, narrow, membranous, margins short-stipitate-glandular. Flowers (10–)13–17(–26) mm diam.; hypanthium usually glabrous, sometimes villous at base; sepals narrowly triangular, much shorter than petals, margins subentire or glandular-serrate, rarely nearly eglandular; stamens 5–10 or 20, anthers pink to red or purple; styles 3–5. Pomes red, rarely with orange or orange-red spots, ellipsoid to suborbicular, sometimes broadly pyriform, 8–15(–20) mm diam., glabrous; flesh mellow; sepals erect-patent to recurved or erose, ˂non-accrescent˃; pyrenes (2 or)3–5.

Species 7 (7 in the flora): North America.

Members of ser. Tenuifoliae range through nearly the whole of the eastern United States (except Florida) and also in adjacent Canada. Crataegus wootonianais a disjunct species in the mountains of New Mexico.

Relatively small and thin leaves, entire or weakly serrate sepals, and pink to reddish anthers characterize most ser. Tenuifoliae. Bracteole frequency in this series appears to correlate positively with inflorescence hairiness; the species flower relatively early among sympatric congeners and are often strikingly tinged with anthocyanins at leaf expansion.

Members of ser. Tenuifoliae are most similar to those of ser. Coccineae (the latter usually being larger in all parts and usually more hairy), to ser. Rotundifoliae via Crataegus flabellata, and to some members of ser. Populneaeae, this last series a grouping of taxa intermediate between ser. Pruinosae and ser. Tenuifoliae.

The interserial hybrids Crataegus ×fretalis and C. ×lucorum key out in the eighth and fourth couplets, respectively.