The green links below add additional plants to the comparison table. Blue links lead to other Web sites.
enable glossary links
Rhododendron macrophyllum

big leaf rhododendron, California rhododendron, California rose bay, coast rhododendron, Pacific rhododendron, western rhododendron

smooth azalea, sweet azalea

Habit Shrubs or trees, to 5 m, sometimes rhizomatous. Shrubs or trees, to 6 m, usually not rhizomatous.
Stems

bark smooth to vertically furrowed, shredding;

twigs with basally branched, crisped/matted, eglandular hairs, very quickly glabrate.

bark smooth to vertically furrowed, shredding;

twigs glabrous or, very rarely, sparsely, widely scattered, unicellular and multicellular eglandular-hairy.

Leaves

persistent;

petiole glabrous;

blade elliptic to slightly ovate or obovate, (6–)8.5–14(–20) × 2.5–5.5(–7.5) cm, thick, coriaceous, margins entire, plane to revolute, glabrous, apex acute to obtuse or slightly acuminate, surfaces scattered eglandular-hairy (hairs branched basally, crisped, very quickly deciduous), abaxial surface ± smooth.

deciduous;

petiole glabrous or multicellular eglandular-hairy;

blade ovate to obovate, 3–8(–10.5) × 1.3–3.5 cm, thin, chartaceous, margins entire, plane, ciliate, eglandular-hairy, apex acute to obtuse, often mucronate, abaxial surface glabrous, (sometimes glaucous), adaxial surface glabrous, (lustrous).

Inflorescences

10–20-flowered;

bracts similar to bud scales.

3–7-flowered;

bracts similar to bud scales.

Pedicels

30–60 mm, glabrous.

6–21 mm, glabrous or stipitate-glandular-hairy, sometimes also unicellular-hairy.

Flowers

opening after development of leaves (of flowering shoots), erect to horizontal, fragrant;

calyx lobes 1–1.5 mm, glabrous, except margins eglandular- and stipitate-glandular-hairy;

corolla white to pink or rose-purple, with yellowish green spots on upper lobe, broadly campanulate, 24–48 mm, outer surface glabrous, petals connate, lobes 14–30 mm, tube gradually expanding into lobes, 10–23 mm;

stamens 10, included, ± unequal, 16–37 mm.

opening after leaves, erect to horizontal, very fragrant;

calyx lobes 0.8–6(–9) mm, surfaces and margins scattered, stipitate-glandular- and/or, sometimes, eglandular-hairy;

corolla white or, sometimes, light pink (contrasting with dark pink to red style and filaments), without blotch on upper lobe, funnelform, 30–55 mm, stipitate-glandular-hairy (hairs continuing in lines up lobes), otherwise glabrous or sparsely unicellular-hairy on outer surface, petals connate, lobes 10–24 mm, tube ± gradually expanded into lobes, 20–37 mm (equaling or much longer than lobes);

stamens 5, much exserted, ± unequal, 44–82 mm.

Capsules

borne on erect pedicels, 13–25 × 4–7 mm, eglandular-hairy (hairs ferruginous, branched or unbranched) and, often, stipitate-glandular-hairy.

borne on erect pedicels, 8–17 × 4.5–8 mm, ± densely multicellular stipitate-glandular-hairy, sometimes also sparsely unicellular-hairy.

Seeds

without distinct tails, flattened portion of testa well developed at each end;

testa expanded, dorsiventrally flattened, loose.

without distinct tails;

testa not dorsiventrally flattened, usually ± loose.

Floral

bud scales multicellular eglandular-hairy (hairs branched basally), and unicellular-hairy (hairs short to elongate) abaxially, margins eglandular-hairy (hairs branched).

bud scales usually glabrous abaxially, margins unicellular-ciliate.

2n

= 26.

= 26.

Rhododendron macrophyllum

Rhododendron arborescens

Phenology Flowering spring–summer. Flowering late spring–summer.
Habitat Forest and forest margins, thickets Stream banks, rocky streamsides, heath balds, swampy woods or bogs
Elevation 50-1600 m [160-5200 ft] 90-1500 m [300-4900 ft]
Distribution
from FNA
CA; OR; WA; BC
[WildflowerSearch map]
[BONAP county map]
from FNA
AL; GA; KY; MD; NC; PA; SC; TN; VA; WV
[WildflowerSearch map]
[BONAP county map]
Discussion

Rhododendron macrophyllum, R. maximum, and R. catawbiense represent subg. Hymenanthes (Blume) K. Koch in North America; the subgenus is represented by hundreds of species in temperate eastern Asia and is characterized by its branched, eglandular hairs (D. F. Chamberlain 1982). These showy plants are frequently used as ornamentals.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Rhododendron arborescens is most closely related to R. viscosum, as evidenced by their glabrous floral bud scales and flowers that appear after the leaves have expanded (K. A. Kron 1993). It can be distinguished by its glabrous branchlets, red style and filaments (which contrast with the white corollas), and distinctive seeds that lack loose, expanded testae. These two species occasionally hybridize; hybrids with R. cumberlandense also are known.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Source FNA vol. 8, p. 462. Treatment authors: Walter S. Judd, Kathleen A. Kron. FNA vol. 8, p. 470. Treatment authors: Walter S. Judd, Kathleen A. Kron.
Parent taxa Ericaceae > subfam. Ericoideae > Rhododendron Ericaceae > subfam. Ericoideae > Rhododendron
Sibling taxa
R. alabamense, R. albiflorum, R. arborescens, R. atlanticum, R. austrinum, R. calendulaceum, R. canadense, R. canescens, R. catawbiense, R. columbianum, R. cumberlandense, R. eastmanii, R. flammeum, R. groenlandicum, R. lapponicum, R. maximum, R. minus, R. occidentale, R. periclymenoides, R. prinophyllum, R. prunifolium, R. tomentosum, R. vaseyi, R. viscosum
R. alabamense, R. albiflorum, R. atlanticum, R. austrinum, R. calendulaceum, R. canadense, R. canescens, R. catawbiense, R. columbianum, R. cumberlandense, R. eastmanii, R. flammeum, R. groenlandicum, R. lapponicum, R. macrophyllum, R. maximum, R. minus, R. occidentale, R. periclymenoides, R. prinophyllum, R. prunifolium, R. tomentosum, R. vaseyi, R. viscosum
Synonyms R. californicum Azalea arborescens
Name authority D. Don ex G. Don: Gen. Hist. 3: 843. 1834 , (Pursh) Torrey: Fl. N. Middle United States, 425. 1824 ,
Web links