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Rhododendron macrophyllum

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

Cumberland rhododendron, diploid flame or Cumberland azalea

Habit Shrubs or trees, to 5 m, sometimes rhizomatous. Shrubs, to 2 m, usually 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 very sparsely to conspicuously multicellular eglandular-hairy (hairs unbranched), otherwise glabrous or densely unicellular-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 usually multicellular eglandular-hairy and usually unicellular-hairy;

blade ovate to obovate, 3.1–8.1 × 1.3–3.5 cm, thin, membranous to chartaceous, margins entire, plane, unicellular-ciliate, apex acute to obtuse, often mucronate, abaxial surface glabrous or very sparsely eglandular-hairy, (usually glaucous), adaxial surface glabrous or sparsely scattered eglandular-hairy.

Inflorescences

10–20-flowered;

bracts similar to bud scales.

3–7-flowered;

bracts similar to bud scales.

Pedicels

30–60 mm, glabrous.

4–9 mm, eglandular-hairy, rarely stipitate-glandular-hairy (then sepals eglandular), otherwise sparsely to densely 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 have expanded, erect to horizontal, acrid-scented;

calyx lobes 1–3 mm, scattered, long-stipitate, usually eglandular-hairy, and unicellular-hairy, margins extremely long stipitate-eglandular-hairy (rarely long stipitate-glandular-hairy, then pedicels eglandular);

corolla red, with indistinct blotch or darker-colored region on upper lobe, funnelform, 27–45 mm, scattered stipitate-glandular-hairy, otherwise ± densely unicellular-hairy on outer surface, petals connate, lobes 14–30 mm, tube abruptly expanding into lobes, 14–24 mm (equaling or much longer than lobes);

stamens 5, much exserted, ± unequal, 42–62 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, 12–28 × 5–8 mm, sparsely to densely long stipitate-eglandular-hairy and sparsely to moderately unicellular-hairy.

Seeds

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

testa expanded, dorsiventrally flattened, loose.

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

testa expanded, dorsiventrally flattened, ± 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 glabrous abaxially, margins usually ciliate distally, glandular proximally.

2n

= 26.

= 26.

Rhododendron macrophyllum

Rhododendron cumberlandense

Phenology Flowering spring–summer. Flowering summer.
Habitat Forest and forest margins, thickets Ridge tops, mixed deciduous forests
Elevation 50-1600 m [160-5200 ft] 300-1800 m [1000-5900 ft]
Distribution
from FNA
CA; OR; WA; BC
[WildflowerSearch map]
[BONAP county map]
from FNA
AL; GA; KY; NC; SC; TN; VA
[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 cumberlandense has a geographic distribution that is concentrated in the Cumberland Plateau and Mountains region with some populations in the southern Blue Ridge. It is most closely related to R. calendulaceum and can be distinguished from the latter by its flowering well after the leaves have expanded, eglandular pedicel and sepal margins, and usually abaxially conspicuously glaucous leaves. Hybrids are known with R. arborescens and R. viscosum. Rhododendron bakeri (Lemmon) Skinner very likely represents a hybrid between R. flammeum and R. canescens, although it has often been synonymized with R. cumberlandense (see K. A. Kron 1993).

(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. 471. 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. arborescens, R. atlanticum, R. austrinum, R. calendulaceum, R. canadense, R. canescens, R. catawbiense, R. columbianum, 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 cumberlandensis
Name authority D. Don ex G. Don: Gen. Hist. 3: 843. 1834 , E. L. Braun: Rhodora 43: 33. 1941 ,
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