1	Leaves persistent, blade margins entire		(2)
+	Leaves deciduous; leaf blade margins serrulate (serrate), or entire or undulate		(9)
2 (1)	Leaf blade surfaces eglandular-hairy (hairs branched) and/or stipitate-glandular-hairy abaxially, hairs usually quickly deciduous to becoming matted and obscure, but not lepidote		(3)
+	Leaf blade surfaces with glandular-peltate scales (lepidote) abaxially, scales sometimes obscured by various eglandular hairs		(5)
3 (2)	Pedicels glabrous; petioles glabrous.		6 Rhododendron macrophyllum
+	Pedicels multicellular, stipitate-glandular- or eglandular-hairy; petioles multicellular-hairy (hairs ± branched), often glabrescent		(4)
4 (3)	Calyx lobes 2-6 mm; pedicels and ovaries stipitate-glandular-hairy; leaf blades: apex acuminate to acute, length/width ratio (2.4-)2.6-5.5(-8), abaxial surface smooth to slightly roughened.		7 Rhododendron maximum
+	Calyx lobes 0.5-1.7 mm; pedicels and ovaries multicellular eglandular-hairy (i.e., hirsute); leaf blades: apex rounded/mucronate to obtuse or acute, length/width ratio (1.3-)1.5-3(-3.5); abaxial surface minutely and obscurely papillose.		8 Rhododendron catawbiense
5 (2)	Corollas white or pale to dark pink to rose or purple, petals connate 3/4+ their lengths; capsules basipetally dehiscent; leaf scales broad-rimmed		(6)
+	Corollas white to cream, petals slightly connate basally; capsules acropetally dehiscent; leaf scales not broadly rimmed		(7)
6 (5)	Shrubs to 3(-5) m; leaf blades (1-)5-8(-13) × (1-)2-3.5(-5.5) cm; corollas 15-37 mm, tube 8-22 mm, upper lobe usually green-spotted; stamens 10; capsules 6-14 mm.		4 Rhododendron minus
+	Shrubs 0.5(-0.7) m; leaf blades 0.4-2(-2.5) × 0.2-0.7(-0.9) cm; corollas (6.5-) 7.5-14(-15) mm, tube 1.5-6.5 mm, upper lobe not spotted; stamens 5-10; capsules 4-7 mm.		5 Rhododendron lapponicum
7 (5)	Twigs unicellular-hairy, papillate, and with flattened, glandular scales; petioles with glandular-peltate scales without broad rim and few ferruginous, long-crisped hairs; abaxial leaf surfaces sparsely to densely papillate, with few ferruginous, long-crisped hairs intermixed with conspicuous glandular- peltate scales.		1 Rhododendron columbianum
+	Twigs and petioles with ± dense covering of ferruginous, multicellular, long-crisped hairs; abaxial leaf surfaces (and especially midvein) with usually dense, ferruginous, multicellular, crisped hairs, these often intermixed with white, short, straight to curved hairs, glandular-peltate scales thus ± inconspicuous		(8)
8 (7)	Pedicels with at least a few ferruginous, elongated, eglandular hairs, covered with ferruginous, long-crisped, unicellular and/or peltate scales, sometimes also with long-stalked, multicellular, glandular hairs; stamens 10; adaxial leaf surfaces with veins deeply impressed, abaxial with ferruginous, crisped hairs (usually more abundant on midrib, which is usually visible), sometimes forming dense, ± uniform mat; leaf blades ± linear (often much longer than wide); fruits borne on sharply recurved pedicels.		2 Rhododendron tomentosum
+	Pedicels with unicellular and/or glandular-peltate scales and sometimes ferruginous, long-crisped, multicellular hairs; stamens usually 5-10; adaxial leaf surfaces with veins usually not impressed, abaxial with ferruginous, crisped hairs forming usually dense, ± even covering, (usually concealing midvein); leaf blades ovate-lanceolate, sometimes narrowly elliptic to linear; fruits borne on broadly recurved pedicels.		3 Rhododendron groenlandicum
9 (1)	Inflorescences lateral, 1-2-flowered; flowers pendulous, ± radially symmetric. ......		9 Rhododendron albiflorum
+	Inflorescences terminal, 3-24-flowered; flowers erect to horizontal, bilaterally symmetric		(10)
10 (9)	Corollas glabrous (or sparsely stipitate-glandular-hairy) on outside, tube to 1/4 as long as lobes or absent; stamens 5-7 or 10; seeds with testa tightly appressed		(11)
+	Corollas often unicellular-hairy on outside, tube usually equaling or much longer than lobes; stamens 5; seeds with testa usually ± loose		(12)
11 (10)	Corolla tubes absent due to deep division between 2 lower lobes and between lateral and lower lobes; upper corolla lobes 3-8 mm; stamens 10; filaments unicellular-hairy proximally; capsules moderately multicellular, stipitate-glandular- and eglandular-hairy; seed tails flattened; leaf margins usually revolute, sometimes plane, apex acute to rounded.		10 Rhododendron canadense
+	Corolla tubes 3-8 mm; upper corolla lobes 10-18 mm; stamens (5-)7; filaments glabrous; capsules very sparsely to moderately multicellular, stipitate-glandular-hairy; seed tails ± stellate-globular; leaf margins plane, apex acuminate.		11 Rhododendron vaseyi
12 (10)	Flowers opening after leaves (i.e., essentially all leaves unfolded, and vegetative bud scales absent)		(13)
+	Flowers opening before or with (or sometimes right after in R. occidentale) leaves (i.e., at least some leaves still folded or vegetative bud scales still present)		(17)
13 (12)	Twigs usually glabrous; abaxial surface of leaves glabrous		(14)
+	Twigs multicellular-hairy (glandular or eglandular) and/or unicellular-hairy; abaxial leaf surface glabrous or sparsely or densely eglandular-hairy		(15)
14 (13)	Corollas red to orange-red or orange; capsules eglandular-hairy and sparsely unicellular-hairy; seed testa expanded, dorsiventrally flattened.		21 Rhododendron prunifolium
+	Corollas white or sometimes light pink (filaments and style dark pink to red); capsules multicellular, stipitate-glandular-hairy, sometimes also sparsely unicellular-hairy; seed testa not dorsiventrally flattened.		22 Rhododendron arborescens
15 (13)	Corollas red, upper lobe with blotch or darker-colored spot, tube abruptly expanding into lobes; floral bud-scale margins glandular proximally (usually ciliate distally), abaxial surface glabrous.		23 Rhododendron cumberlandense
+	Corollas white (or sometimes pink), upper lobe with or without yellow or orange blotch, tube gradually expanding into lobes; floral bud-scale margins usually ciliate, glandular along proximal 2/3 or with glands and unicellular hairs mixed proximally, abaxial surface glabrous or densely unicellular-hairy		(16)
16 (15)	Corollas densely glandular-hairy on outer surface (i.e., tube densely scattered, conspicuous, stipitate-glandular-hairy, hairs extending in line up corolla lobes), upper lobe without blotch; flowers strongly scented at night; styles white or greenish to pinkish.		24 Rhododendron viscosum
+	Corollas weakly glandular-hairy on outer surface, upper lobe with yellow to orange blotch; flowers strongly scented during mid day; styles white.		25 Rhododendron eastmanii
17 (12)	Upper corolla lobes with contrasting blotch often appearing as darker-colored area		(18)
+	Upper corolla lobes without contrasting or darker-colored blotch		(22)
18 (17)	Corolla lobes spreading nearly as broadly as tube is long, tube usually abruptly expanding into lobes, corolla yellow to orange, red-orange, or red		(19)
+	Corolla lobes shorter than or equaling tube, tube gradually expanding into lobes, corolla white, sometimes pink tinged, or yellow to orange		(20)
19 (18)	Floral bud scales with glandular margins, abaxial surface glabrous; corollas scattered, multicellular, stipitate-glandular-hairy on outside.		12 Rhododendron calendulaceum
+	Floral bud scales with ciliate margins, abaxial surface glabrous or densely unicellular-hairy; corolla tubes multicellular eglandular-hairy abaxially, sometimes very weakly glandular.		13 Rhododendron flammeum
20 (18)	Corollas yellow to orange (tube often orange-red or red), upper lobe with indistinct, darker yellow, orange, or red blotch; floral bud-scale margins glandular.		14 Rhododendron austrinum
+	Corollas white, sometimes pink tinged, upper lobe with contrasting yellow blotch; floral bud-scale margins unicellular, sometimes with unicellular hairs mixed with glands, or glandular		(21)
21 (20)	Capsules moderately to densely unicellular-hairy and multicellular eglandular-hairy; floral bud scales glabrous or glabrate abaxially, margins unicellular-ciliate.		15 Rhododendron alabamense
+	Capsules sparsely unicellular-hairy (unicellular hairs rarely absent) and multicellular eglandular- and stipitate-glandular-hairy, or without eglandular hairs; floral bud scales sparsely to densely unicellular-hairy abaxially, margins unicellular-ciliate or with glands and cilia mixed, or with only glands.		16 Rhododendron occidentale
22 (17)	Corolla lobes multicellular stipitate-glandular-hairy (hairs forming lines that continue along outer surface), corolla white, usually flushed pink or rose; plants to 0.5-1 m, strongly rhizomatous		17 Rhododendron atlanticum
+	Corolla lobes scattered, multicellular stipitate-glandular- or eglandular-hairy on outer surface (hairs not forming distinct lines), corolla pink to white; plants to 3-6 m, not strongly rhizomatous		(23)
23 (22)	Corollas pink, scattered, multicellular eglandular-hairy on outer surface; floral bud scales glabrous abaxially, margins moderately unicellular-ciliate; leaf blades glabrous or nearly so.		18 Rhododendron periclymenoides (in part)
+	Corollas pink or white with pink tube, multicellular stipitate-glandular-hairy; floral bud scales glabrous or sparsely to densely unicellular-hairy abaxially; leaf blades sparsely to densely unicellular-hairy		(24)
24 (23)	Floral bud scales glabrous abaxially; pedicels (and sepal margins) usually eglandular, often not unicellular-hairy or only sparsely unicellular-hairy; leaf blades glabrous or only sparsely unicellular-hairy..		18 Rhododendron periclymenoides (in part)
+	Floral bud scales sparsely to densely unicellular-hairy abaxially; pedicels (and sepal margins) eglandular or glandular, (sparsely) moderately to densely unicellular-hairy; leaf blades sparsely to densely unicellular-hairy abaxially		(25)
25 (24)	Pedicels usually eglandular, 4-17 mm; leaf blade margins inconspicuously ciliate (cilia appressed to margins); capsules densely unicellular-hairy.		19 Rhododendron canescens
+	Pedicels usually glandular, 5-26 mm; leaf blade margins conspicuously ciliate (cilia ascending away from margins); capsules sparsely unicellular-hairy		20 Rhododendron prinophyllum