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Key to Prunus, Key B: CHERRIES, subgenera Cerasus and Lithocerasus

Rosaceae

Prunus

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1
Plants shrubs, to 1.5 (-3) m tall; [subgenus Lithocerasus; section Microcerasus].
  2
Inflorescences of 1-2 flowers; drupes reddish, 10-15 mm long; [exotic, persistent from cultivation]
 ⇒ Prunus glandulosa
  2
Inflorescences of 2-5 flowers; [native].
    3
Youngest twigs minutely and densely puberulent (use 10× magnification); leaf blades ca 2.6× as long as wide; plants erect, stones mostly orbicular, (5-) 6-8 (-9) mm long, 5-6 mm wide; leaf apices usually obtuse, sometimes acute or rounded; [plants of acid sandy or rocky upland barrens, savannas, or woodlands]
 ⇒ Prunus susquehanae
    3
Youngest twigs usually glabrous, sometimes sparsely puberulent; leaf blades ca. 3.3-3.7× as long as wide; plants decumbent or erect; stones fusiform, ovoid, or subglobose, 6-9.5 mm long, 4.5-5 mm wide (P. pumila var. depressa) or 6-7 mm wide (P. pumila var. pumila); leaf apices short-acuminate, acute, or obtuse; [usually in open, sandy or gravelly habitats].
      4
Stems prostrate to decumbent; leaf blades oblanceolate, averaging 3.7× as long as wide; stones ovoid to fusiform, 4.5-5 mm wide; [s. PA northward through New England]
 ⇒ Prunus pumila var. depressa
      4
Stems usually erect-ascending, sometimes decumbent or sprawling; leaf blades elliptic, obovate, or oblanceolate, lengths ca. 2.9-3.3× as long as wide; stones subglobose to ovoid, 5-7 mm wide; [endemic to sandy coastal areas of the Great Lakes; known in our area only from Presque Isle, PA]
 ⇒ Prunus pumila var. pumila
1
Plants trees, well over 3 m tall when mature; [subgenus Cerasus].
  5
Leaf serrations single to double, the tips of the serrations acuminate to attenuate; leaf blades strongly acuminate to attenuate-caudate.
    6
Leaf serrations attenuate; tree with upright form; leaf apices attenuate-caudate; twigs, petioles, and leaf blades glabrous; [subgenus Cerasus, section Sargentiella]
 ⇒ Prunus speciosa
    6
Leaf serrations acuminate; tree with weeping or upright (spreading) form; twigs, petioles, and leaf blades hairy (the blades at least along the veins on the lower surface); [subgenus Cerasus, section Microcalymma]
      7
Petioles 10-20 mm long.
        8
Leaf blade base obtuse to rounded; leaf blades < 1.7× as long as wide
 ⇒ Prunus sargentii
        8
Leaf blade base cuneate; leaf blades > 1.8× as long as wide
 ⇒ Prunus yedoensis
  5
Leaf serrations small and obscure or well-developed but rounded to acute; leaf blades acute to acuminate.
    9
Petals 4-7.5 mm long; fruit < 1 cm in diameter; [subgenus Cerasus, section Phyllomahaleb].
      10
Inflorescence with a central axis, thus nearly or actually racemose; fruit blackish; leaves 1-1.5× as long as wide; [exotic tree]
 ⇒ Prunus mahaleb
      10
Inflorescence umbellate to corymbose, the central axis absent or poorly developed; fruit red; leaves 2-5× as long as wide; [native tree]
 ⇒ Prunus pensylvanica
    9
Petals 9-15 mm long; fruit 1.3-2.5 cm in diameter; [subgenus Cerasus, section Eucerasus].
      11
Leaves 7-15 cm long, persistently hairy beneath, at least along the midrib and veins; pair of petiolar glands on the petiole near the blade; fruit sweet when ripe
 ⇒ Prunus avium
      11
Leaves 4-8 cm long, glabrous beneath once fully-expanded; pair of petiolar glands on the base of the leaf blade; fruit sour when ripe
 ⇒ Prunus cerasus