1	Leaves persistent, alternate, blades distinctly coriaceous; petals 0 (rarely 4 in R. pilosa)		(2)
+	Leaves deciduous, alternate, subopposite, or opposite, sometimes fascicled on short shoots, blades herbaceous to subcoriaceous; petals 0 or 4		(6)
2 (1)	Leaf blade secondary veins with basal pair diverging much more acutely than distal pairs; inflorescences cymes or flowers solitary; sepals 5; drupes dark red, becoming black, stones 3.		9 Rhamnus alaternus
+	Leaf blade secondary veins all diverging at nearly same angle; inflorescences fascicles or flowers solitary; sepals 4(–5); drupes red, stones 2		(3)
3 (2)	Leaf blade margins entire or coarsely serrate, blunt-serrate, or spinulose-serrate		(4)
+	Leaf blade margins spinulose to spinose-dentate		(5)
4 (3)	Shrubs, 1.2–2(–4) m, usually armed with thorns; leaf blades 0.7–2 cm, elliptic to obovate or ovate; California mainland.		1 Rhamnus crocea
+	Shrubs or trees, 2.5–6(–10) m, unarmed; leaf blades (1.5–)2–5(–6) cm, broadly elliptic to oblong or ovate-oblong; California Channel Islands.		2 Rhamnus pirifolia
5 (3)	Branchlets stiff, glabrous or densely and softly hirtellous; both leaf blade surfaces glabrous.		3 Rhamnus ilicifolia
+	Branchlets flexible, sparsely to densely softly hirtellous; both leaf blade surfaces sparsely to densely softly hirtellous.		4 Rhamnus pilosa
6 (1)	Leaves usually alternate, sometimes subopposite, blade secondary veins 4–11 pairs; shrubs or small trees 0.5–3(–4) m, unarmed		(7)
+	Leaves usually opposite to subopposite, sometimes alternate or fascicled on short shoots, blade secondary veins 2–7(–8) pairs; shrubs or trees (1–)2–10 m, armed with thorns		(11)
7 (6)	Sepals 5; petals 0; drupe stones 3; shrubs 0.5–1(–1.5) m; wetlands and seeps.		8 Rhamnus alnifolia
+	Sepals 4; petals 4; drupe stones 2 (3 in R. arguta); shrubs or small trees 0.5–3(–4) m; drier habitats		(8)
8 (7)	Petioles 10–28 mm; pedicels 10–24(–30) mm; drupe stones 3; leaf blades usually broadly ovate, ovate, oblong-ovate, or ovate-cordate, sometimes ovate-orbiculate, margins spinulose-serrate.		14 Rhamnus arguta (in part)
+	Petioles 3–8 mm; pedicels (0–)0.5–4 mm; drupe stones 2; leaf blades lanceolate, elliptic-lanceolate, oblong-lanceolate, elliptic, or elliptic-oblong, margins crenulate, crenulate-serrate, or serrulate		(9)
9 (8)	Branchlets 6+ cm; leaf blade apices acute to acuminate; c, e United States.		5 Rhamnus lanceolata
+	Branchlets 2–5 cm; leaf blade apices acute to obtuse or rounded; w United States		(10)
10 (9)	Bud scales thin, glossy, pale golden to yellowish; leaf blades usually lanceolate, sometimes elliptic-lanceolate or oblong-lanceolate, 3–8 cm, both surfaces glabrous.		6 Rhamnus smithii
+	Bud scales coriaceous, dull, dark red to reddish brown; leaf blades usually elliptic to elliptic-lanceolate or elliptic-oblong, 1.5–5(–5.5) cm, both surfaces minutely hirtellous.		7 Rhamnus serrata
11 (6)	Leaf blade bases rounded to truncate or cordate; drupe stones 3–4		(12)
+	Leaf blade bases cuneate or acute; drupe stones 2		(13)
12 (11)	Leaf blade margins crenate-serrate, secondary veins 2–4 pairs; pedicels 2–4 mm; drupes 5–6(–8) mm.		10 Rhamnus cathartica
+	Leaf blade margins spinulose-serrate, secondary veins (3–)4–5 pairs; pedicels 10–24(–30) mm; drupes 7–10 mm.		14 Rhamnus arguta (in part)
13 (11)	Leaf blades obovate to elliptic-oblanceolate or broadly oblanceolate, adaxial surfaces usually minutely hirtellous at least along main veins on distal 1/3, rarely glabrous.		13 Rhamnus japonica
+	Leaf blades usually elliptic to oblong, oblong-obovate, obovate-elliptic, or oblong-lanceolate, sometimes ovate to obovate, rarely oblanceolate-elliptic, adaxial surfaces glabrous or sparsely pilose on veins		(14)
14 (13)	Leaf blades green abaxially, abaxial surface with straight, colorless hairs on veins, secondary veins 3–5(–6) pairs; pedicels 10–20 mm.		11 Rhamnus davurica
+	Leaf blades yellowish green abaxially, abaxial surface with wavy or curled, yellow hairs along veins or at least in vein axils, secondary veins (4–)5–7(–8) pairs; pedicels 5–10 mm.		12 Rhamnus utilis