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Rhododendron macrophyllum

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

Cascade azalea, white rhododendron, white-flower azalea, white-flower rhododendron

Habit Shrubs or trees, to 5 m, sometimes rhizomatous. Shrubs, to 2.5 m, rhizomatous.
Stems

bark smooth to vertically furrowed, shredding;

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

bark ± smooth to furrowed;

twigs multicellular eglandular-hairy (hairs unbranched) and 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 multicellular eglandular- and stipitate-glandular-hairy and unicellular-hairy;

blade narrowly elliptic or ovate to obovate, 2–9 × 0.8–3 cm, thin, membranous to chartaceous, margins minutely serrate, plane, ciliate when young, eglandular- and stipitate-glandular-hairy, apex acute to ± rounded, surfaces scattered eglandular-hairy, ± glabrescent.

Inflorescences

10–20-flowered;

bracts similar to bud scales.

lateral (axillary, i.e., above leaf scars, spaced along shoots of previous year), fasciculate, 1–2-flowered;

bracts similar to bud scales.

Pedicels

30–60 mm, glabrous.

to 9–15 mm, eglandular- and stipitate-glandular-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.

± radially symmetric, opening soon after (and borne below) expanded leaves, pendulous, very fragrant (similar to vanilla and jasmine);

calyx lobes 5–17 mm, eglandular- and stipitate-glandular-hairy, margins glandular-hairy;

corolla white, rarely marked with yellow, bowl-shaped, 9–22 mm, minutely unicellular-hairy or glabrous on outer surface, petals connate, lobes 6–15 mm, tube expanding into lobes, 3–9 mm;

stamens 9(–12), included, ± unequal, 5.5–14 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, 6–8 × 5–6 mm, stipitate-glandular-, eglandular-, and unicellular-hairy.

Seeds

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

testa expanded, dorsiventrally flattened, loose.

with distinct tails;

testa closely appressed.

Floral

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

bud scales stipitate-glandular- and eglandular-hairy abaxially, margins stipitate-glandular-hairy.

2n

= 26.

= 26.

Rhododendron macrophyllum

Rhododendron albiflorum

Phenology Flowering spring–summer. Flowering late spring–summer.
Habitat Forest and forest margins, thickets Coniferous forests, alpine thickets, stream banks, seeps on rock outcrops
Elevation 50-1600 m [160-5200 ft] 800-3500 m [2600-11500 ft]
Distribution
from FNA
CA; OR; WA; BC
[WildflowerSearch map]
[BONAP county map]
from FNA
CO; ID; MT; OR; WA; AB; BC
[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 albiflorum is especially distinctive due to its axillary, white, somewhat pendulous, and nearly actinomorphic flowers, and it is placed in the monotypic subg. Candidastrum (Sleumer) Philipson & Philipson (W. R. Philipson and M. N. Philipson 1986). It is occasionally used as an ornamental. The disjunct population in Colorado has somewhat smaller calyx lobes and corollas and shorter stamens; it is sometimes recognized as var. warrenii (M. A. Lane et al. 1993). This variety is not recognized here because of the extent of morphological overlap between that population and those of the Pacific Northwest.

(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. 464. 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. 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. macrophyllum, R. maximum, R. minus, R. occidentale, R. periclymenoides, R. prinophyllum, R. prunifolium, R. tomentosum, R. vaseyi, R. viscosum
Synonyms R. californicum Azaleastrum albiflorum, R. albiflorum var. warrenii
Name authority D. Don ex G. Don: Gen. Hist. 3: 843. 1834 , Hooker: Fl. Bor.-Amer. 2: 43, plate 133. 1834 ,
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