Like Rubus and Amelanchier, Crataegus is a genus easy to recognize, but with an entrenched reputation for difficulty due to hybridization, polyploidy, and apomixis (see Phipps, 2005). Nevertheless, we hope the preponderance of good specimens should be readily referable to species in this treatment. Crataegus is much in need of a modern collecting program in Michigan, many of our specimens being old.
Ideally, in Crataegus, plants should be marked so that flowering and fruiting specimens can be made from the same individual. The keys presented here work best with both flowers and fruit (or experience with certain species). However, old fruit may be found on or beneath a flowering plant, and thus (if there is no chance of mixture with fruit from another plant) having both flower and fruit characters can be met in part at a single time. As with other larger genera, the more one becomes used to the variability of the genus and to the precise meanings of terms applied to the parts, the easier the recognition of species becomes.
The number of stamens can be determined in many cases by a careful study of the apex of the fresh fruit, where remnants of all the filaments may be present. Likewise, the margins of the calyx lobes can often be examined on fruit that is not too deteriorated. The number of styles and nutlets is usually the same, although in some cases the former could be more, due to failure of nutlets to develop. Anther color is safely determined only from fresh, unopened anthers. Inflorescences and leaves at flowering time tend to be more pubescent than the same structures at fruiting time; a few scattered hairs, therefore, in a late-season collection are more significant than a similar amount of pubescence in the spring.
A “lobed” leaf in Crataegus has lobes relatively small (or shallow) compared to most other genera (e.g., Quercus). The statements in the keys are based largely on the twigs, the portions ordinarily collected and studied. The field student will note that the trunks often bear larger, compound thorns. The leaves of the short shoots that bear flowers (here termed floral leaves), often differ in shape from the leaves of more vigorous vegetative shoots, which are often more deeply lobed, but may also be more variable.
Nutlets in Crataegus have very thick, hard walls and are basically triangular in cross-section, with two ± flat sides by which they fit together inside the fruit, and a rounded, often ± gently grooved “back.” In some species, the flat sides (lateral surfaces) of the nutlets have conspicuous pits. Nutlets need to have the pulp carefully scraped down to the hard walls to see these pits.
In cases of uncertainty, try an unknown plant under both leads of a couplet in the key. Soon it will be found that under one of them no species has the particular combination of stamen number, anther color, pubescence of leaves and inflorescences, etc., demonstrated by the specimen. These combinations are distinctive.
About 16 species, mostly easily recognizable, constitute the vast majority of all specimens thus far collected in Michigan, and are known from a dozen to many collections each. Crataegus succulenta and C. macrosperma are apparently common throughout Michigan. Others that are fairly common in the Upper Peninsula and northernmost Lower Peninsula are C. chrysocarpa, C. irrasa, and C. douglasii; of these, C. chrysocarpa ranges occasionally farther south.
The commonest additional species in the Lower Peninsula are C. pruinosa, C. crus-galli, C. punctata, C. calpodendron, C. coccinea, and C. mollis. Others fairly common are C. brainerdii, C. holmesiana, C. intricata, C. jesupii, C. dodgei, and C. margaretta. The remaining species are known from very few collections each, and some have not been recently seen.
We have listed synonyms only in so far as needed for correlation with the Michigan Flora treatment. For more comprehensive discussion and the placement of all the numerous microspecies described from Michigan, see Michigan Flora. The innumerable varieties that have been made under some species have also generally not been mentioned except when they impact the position of a plant in keys or are recognized as species in some other works. Phipps, O’Kennon, & Lance (2003) provide more information on almost all Michigan species, often with illustrations.
1. Leaves with some of the primary lateral veins running to (or toward, forking just before) the sinuses as well as to the points of the lobes; blades ± deltoid in general outline or small and deeply lobed; thorns under 5 cm long; stamens ca. 20.
2. Blades of leaves 3-lobed in general outline, ± deltoid, the apex sharply acute, the sinuses not more than halfway to the midrib; calyx deciduous in fruit, the ends of the (3–) 5 nutlets clearly exposed, almost slightly exserted; anthers yellow; thorns up to 4.5 (–5.5) cm long.
2. Blades of leaves mostly rather deeply 3–7-lobed, the apex ± rounded and toothed, the sinuses in many leaves extending at least halfway to the midrib; calyx persistent on fruit, the ends of the 1–2 nutlets ± hidden in depression; anthers pink; thorns (not sharp-tipped spur-branches) up to 2.5 cm long, usually much shorter.
3. Styles and nutlets usually 2; leaves less deeply divided (most sinuses scarcely halfway to the midrib).
3. Style and nutlet 1; leaves deeply divided (most sinuses extending more than halfway to the midrib).
1. Leaves with the primary lateral veins running only to (or toward) the points of the lobes (if any); blades, thorns, and stamens various; native.
4. Nutlets with deep to shallow pits or depressions on their lateral surfaces; flowering in June or latter half of May.
5. Mature fruit purplish black, glaucous; thorns mostly 1.5–2.5 cm long; inflorescence glabrous or very sparsely villous; stamens 10 or fewer; nutlets 3–4 (–5), rounded at the ends.
5. Mature fruit red or orange; thorns mostly 2.5–9.5 cm long; inflorescence densely villous to glabrous (only in C. succulenta); stamens ca. 10 or 20; nutlets 2–3, or if more, then acute at the ends.
6. Leaves acute to rather broadly rounded or obtuse in outline at the apex, at least the midrib beneath often pubescent; nutlets 2–3, with a definite pit occupying most of each half of the ventral face, smoothly rounded or nearly so dorsally; thorns mostly 2.5–9.5 cm long; inflorescences often ± villous.
7. Mature leaf blades thin, the veins (except sometimes for midrib) scarcely if at all impressed above, strigose above and usually pubescent beneath; inflorescences, branchlets of current year, and petioles all usually villous or lightly tomentose; thorns ca. 2.5–5 cm long, often sparse or even absent; stamens ca. 20; late flowering (usually early June).
7. Mature leaf blades ± leathery, thickened at margins, the veins usually deeply impressed above, glabrous to pubescent on both surfaces; inflorescences, new branchlets, and petioles sparsely villous to glabrous (if inflorescence somewhat villous, at least the young branchlets nearly always glabrous, the veins deeply impressed, and/or the stamens ca. 10); thorns ca. 2.5–9.5 cm long, usually numerous; stamens ca. 20 or 10; mid-season flowering (mid- to late May in southern Michigan).
6. Leaves ± narrowly acute or short-acuminate in outline at the apex, completely glabrous beneath (the rare exceptions hybrids?); nutlets 2–5, with rather shallow and irregular depressions on the ventral face, usually very strongly ridged and grooved dorsally; thorns mostly 3.5–4.5 (–5.5) cm long; inflorescences mostly glabrous.
8. Calyx tube of mature fruit 4–5 mm broad, 0.5–1 mm long, elevating the prominent calyx lobes.
8. Calyx tube of fruit at most 3–4 mm broad, the lobes sessile or nearly so, not conspicuously elevated.
4. Nutlets plane, not pitted, laterally; flowering in April, May, or early June.
9. Blades of at least the floral leaves (in many species also the vegetative leaves) ± acute to broadly or (more commonly) narrowly tapered or cuneate at the base.
10. Bracteoles of the inflorescence numerous, with numerous conspicuous stalked glands on the margins (best seen when young); petioles glandular; stamens ca. 10; inflorescences and leaves essentially glabrous (or a few hairs on the veins above); inflorescence few-flowered (rarely more than 8-flowered), ± simple or umbelliform.
10. Bracteoles of inflorescence usually few and/or the glands sparse or absent; petioles eglandular or the inflorescences villous; stamens and leaves various; inflorescences usually many-flowered.
11. Blades (especially of floral leaves) mostly obovate to oblong-elliptic, broadest above or rarely at the middle, unlobed or very obscurely lobed near the apex, mostly 1.5–3 or more times as long as broad, usually thick or even stiff and leathery (except sometimes in shade leaves).
12. Leaves glossy above, the veins not (or only slightly) impressed; petioles mostly less than 1 cm long; styles and nutlets 1–3; inflorescence usually glabrous, stamens ca. 10 or 20.
12. Leaves dull above, the veins rather conspicuously impressed; petioles mostly 1–2 cm long; styles and nutlets 3–5; inflorescence usually ± villous; stamens ca. 20.
11. Blades (at least of floral leaves) mostly elliptic to ovate, broadest at or below the middle, often ± lobed, usually 1–1.5 times as long as broad, often thin.
13. Inflorescences and lower surfaces of leaves glabrous or nearly so; leaves strigose with short appressed hairs above, at least when young; nutlets often slightly pitted ventrally, especially before full maturity; stamens usually 20 with pink anthers (occasionally 5–15); vegetative leaves usually ovate or broadly elliptic.
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13. Inflorescences and lower surfaces of leaves villous to glabrous; leaves glabrate or with short appressed hairs above when young; nutlets plane ventrally; stamens various but anthers usually yellow; vegetative leaves often broad, ± rotund.
14. Stamens ca. 10 or fewer.
15. Blades of floral leaves with lobes rather sharp and spreading, usually developed below the middle; inflorescences glabrous to densely villous; styles and nutlets mostly 3–5; calyx lobes ± conspicuously glandular-serrate; vegetative leaves with the base about as acute as, or more acute than, the apex; leaves glabrous to sparsely villous beneath, the teeth rather conspicuously gland-tipped.
15. Blades of floral leaves with lobes shallow or nearly absent, mostly developed only beyond the middle; inflorescences glabrous (loosely villous in one uncommon variety); styles and nutlets mostly 2–3; calyx lobes essentially entire (glands few and small or none); vegetative leaves with the apex often more acute than the base; leaves glabrous beneath, with teeth obscurely, if at all, gland-tipped.
14. Stamens ca. 15–20.
16. Inflorescence and leaves on both surfaces ± villous; Upper Peninsula.
16. Inflorescences at most slightly villous; leaves completely glabrous beneath (and usually quite or nearly so above); southern half of Lower Peninsula.
17. Leaf serrations sharp; apex and lobes acute; styles 4–5; anthers pink.
17. Leaf serrations rather crenate; apex and lobes of blades ± rounded in outline; styles 2–4; anthers yellow.
9. Blades of both floral and vegetative leaves mostly broadly rounded, truncate, or subcordate at the base.
18. Inflorescence, calyx, and leaves (at least along main veins) beneath ± densely villous-tomentose; fruit short-villous at least at the ends; stamens in common species 20, the anthers white or yellow (rarely pink).
19. Stamens ca. 20, white or yellow; fruit nearly spherical (occasionally oblong-obovoid); Lower Peninsula.
19. Stamens ca. 10, pink; fruit pear-shaped or obovoid; rare in the Upper Peninsula.
18. Inflorescence, calyx, and leaves glabrous or pubescent; fruit glabrous; stamens ca. 10 or 20 but anthers in most species pink to purple.
20. Stamens ca. 15–20; anthers pink to purple or white to yellowish; young leaves glabrous to pubescent above.
21. Leaves glabrous or nearly so on both sides when young; inflorescences glabrous; calyx lobes entire or weakly and sparsely serrate; ripe fruit with rather thin dry flesh.
21. Leaves strigose above, at least when young; inflorescences glabrous to villous; calyx lobes usually glandular-serrate; ripe fruit mellow or succulent.
22. Flowers 2–2.5 cm wide.
22. Flowers less than 2 cm wide.
23. Inflorescences ± villous at flowering time.
23. Inflorescences completely glabrous.
20. Stamens 10 or fewer; anthers pink to purple; young leaves strigose above.
24. Inflorescences villous or glabrous; leaves often with a few hairs beneath, at least along veins or in axils; calyx lobes rather prominently glandular-serrate; ripe fruit with mellow or succulent flesh.
25. Stamens 15–20; inflorescence and leaves (at least when young) villous.
25. Stamens ca. 10 or fewer; inflorescence and leaves glabrous or pubescent.
26. Fruit at most slightly longer than thick; floral leaves ovate, broadest at the base.
26. Fruit distinctly obovoid or oblong, noticeably longer than thick; floral leaves mostly oblong-ovate to elliptic, often slightly narrowed at the base.
24. Inflorescences glabrous (± villous in C. fluviatilis); leaves completely glabrous beneath; calyx lobes usually entire or weakly serrate; ripe fruit various.
27. Fruit bright red, soft and succulent when ripe; leaves thin at maturity, the tips of the lobes generally spreading or reflexed (note out-curving of tips of lower main lateral veins).
28. Inflorescences completely glabrous at flowering time; widespread.
28. Inflorescences ± villous; rare in southeast Michigan.
27. Fruit bright or dull red or greenish with thin dry firm or mealy flesh when ripe; leaves firm, the tips of the lobes usually not spreading.
29. Vegetative leaves ± lobed, usually ± shallowly lobed, rather than deeply incised.
29. Vegetative leaves sharply lobed, often deeply incised (sometimes halfway to middle).