A useful vegetative feature often mentioned for this genus is the presence of a pair of glands (sometimes 1 or 3) near the summit of the petiole or toward the base of the blade. These are obscure or absent in some species, and of course they will not in themselves separate Prunus from some quite different genera, such as Salix, in which certain species may have similar glands. In flower or fruit, however, Prunus is an easily recognized genus, with deep perigynous cup and the fruit a drupe with a single large stone or pit enclosing the seed. Many species flower before the leaves are fully grown, and the key is designed for use with or without flowers.
A considerable number of species are cultivated for flowers or fruit besides those recorded here as escaped to roadsides or elsewhere. These may persist in old yards. There are also numerous horticultural forms of the cultivated species, including double-flowered ones for the ornamentals, and they may also hybridize.
1. Inflorescence an elongate raceme of 12–20 or more flowers, terminating a new leafy branchlet of the current year.
2. Leaves with blades ovate or elliptic to lanceolate, mostly broadest at or below the middle, ± glossy above, the teeth ± incurved, giving the margin a crenulate aspect; calyx lobes entire or nearly so (at most 5 glandular teeth), persistent with the floral tube beneath the maturing fruit.
2. Leaves with blades elliptic to obovate, mostly broadest at or above the middle, dull above, the teeth narrowly acuminate and ascending to spreading, giving the margin a sharply and finely serrate aspect; calyx lobes with numerous irregular gland-tipped teeth, deciduous with the floral tube, leaving only a smooth disc beneath the maturing fruit.
1. Inflorescence a corymb or umbel of fewer than 12 flowers (or flowers solitary), sessile or on short lateral shoots (leafy or not).
3. Flowers and fruit sessile or nearly so, mostly solitary (or 2); ovary and fruit densely pubescent; flowers pink to white.
4. Year old twigs pubescent, leaves densely pubescent beneath, even at maturity; fruit less than 1.5 cm in diameter, red.
4. Year old twigs glabrous, leaves essentially glabrous beneath even when young, at most a few hairs along the midvein; fruit much larger, yellow or orange.
5. Leaves ovate; petioles of larger leaves ca. 14–30 mm long, about half as long as the blade; flowers white (pink in bud).
5. Leaves lanceolate to narrowly elliptic or even oblanceolate, petioles of larger leaves ca. 5–11 mm, much shorter than the blades; flowers pink.
3. Flowers and fruit on distinct slender pedicels, often not solitary; ovary and fruit glabrous; flowers in most species white.
6. Calyx lobes glabrous throughout (at most, glandular-margined); fruit (cherry) ± spherical, not grooved, not glaucous, the pit rounded rather than 2-edged.
7. Plant a low spreading shrub (native), with mostly decumbent or ascending elongate branches; leaf blades oblanceolate (to obovate-elliptic), the teeth obscure or absent on lower third or half; calyx lobes with irregularly glandular-toothed margins; petals less than 7.5 (–9) mm long.
7. Plant an erect small to large tree or tall bushy shrub; leaf blades ovate to obovate, regularly toothed to the base; calyx lobes without glands (or a few glandular teeth in the large-flowered P. cerasus); petals various.
8. Petals 4–7.5 mm long; fruit less than 1 cm in diameter.
9. Inflorescence a few-flowered corymbose raceme (i.e., with a definite central axis); petals glabrous; leaf blades round-ovate, less than 1.5 times as long as wide; fruit nearly black.
9. Inflorescence usually umbellate (occasionally ± corymbose); petals hairy on the outside at the base; leaf blades usually at least twice as long as wide; fruit bright red.
8. Petals ca. 9–15 mm long; fruit ca. 1.5–2.5 cm in diameter.
10. Calyx with entire lobes, constricted below them; bud scales at base of umbel not leaf-like, the inner ones divergent or reflexed; leaves retaining some pubescence, especially along the midrib, beneath, the blades ca. 7–15 cm long at maturity and the petioles with conspicuous glands near the summit; fruit sweet.
10. Calyx with glandular-toothed lobes, not constricted below them; bud scales often with leaf-like tips, the inner ones erect; leaves becoming glabrous beneath, the blades mostly 4–8 cm long at maturity with glands toward the base (rather than on the petiole); fruit sour.
6. Calyx lobes pubescent, at least sparsely at the base above (rarely glabrous in P. nigra); fruit (plum) with a longitudinal shallow furrow or groove, usually glaucous, the pit 2-edged, often somewhat flattened.
11. Teeth of leaf sharp (almost bristle-tipped), glandless; calyx lobes without marginal glands (rarely a few teeth at end in P. americana).
12. Petals 4–6 mm long; calyx lobes usually slightly pubescent on the lower side; leaf blades mostly ovate in outline, acute or barely acuminate; fruit less than 10 (–12) mm in diameter when dry (said to be ca. 15 mm when fresh), the pit ca. 5–8 mm broad; dry jack pine plains and sandy oak savannas (rarely mesic sites).
12. Petals (at least the largest) (6.5–) 7–11 mm long; calyx lobes usually glabrous on the lower (outer) side; leaf blades mostly oblong-elliptic to obovate in general outline but abruptly acuminate to a conspicuous prolonged tip; fruit ca. 10–25 mm in diameter when dry (larger fresh), the pit ca. 10–15 mm broad; usually edges of mesic forests, thickets, and stream borders.
11. Teeth of leaf gland-tipped (evident on young leaves unfolding as the flowers open), the rounded tip with a callous scar if gland is shed; calyx lobes with glandular margins (except in P. angustifolia).
13. Margins of calyx lobes pubescent but not glandular; petals ca. 5–7 mm long, white; young twigs shiny reddish brown; leaves small, the blades mostly less than 5 cm long.
13. Margins of calyx lobes glandular; petals ca. (4–) 5–12 mm long, white or pale pink; twigs and leaves various.
14. Flowers in clusters of 2–4; leaves with blades acuminate at tip; petioles with glands at summit.
15. Petals 8–12 mm long, often pale pink, especially with age; floral tube and sometimes even young leaves red tinged; larger mature leaves usually less than twice as long as wide, apex abruptly acuminate.
15. Petals (4–) 5–9 mm long; white; floral tube and young leaves greenish; larger mature leaves more than twice as long as wide, apex long-acuminate.
14. Flowers solitary or in pairs; leaves with obtuse to rounded blades (except the dwarf shrub P. glandulosa); petioles usually without glands.
16. Plants dwarf (less than ca 0.8 m tall) colonial, suckering shrubs; flowers "double," with numerous petals; leaves tapering to a ± acute tip.
16. Plants taller, greater than 1 m tall before flowering, often developing a clear trunk, even if suckering; flowers with 5 petals; leaves with obtuse to rounded blades.
17. Twigs not spiny, often glabrous; petals ca. 8–12 mm long; fruit over 2.5 cm in diameter, pendent at maturity.
17. Twigs spiny, the young ones finely pubescent; petals ca. 5–8 mm long; fruit usually less than 1 (–1.5) cm in diameter, erect at maturity.