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Rhododendron macrophyllum

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

Lapland rhododendron, Lapland rose-bay

Habit Shrubs or trees, to 5 m, sometimes rhizomatous. Shrubs, to 0.5(–0.7) m, rhizomatous.
Stems

bark smooth to vertically furrowed, shredding;

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

prostrate to erect;

bark ± smooth to vertically furrowed, shredding;

twigs with straw-colored to ferruginous, multicellular, broad-rimmed, peltate scales 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.

persistent;

petiole with broad-rimmed, glandular-peltate scales;

blade oblong-elliptic to elliptic, ovate, or obovate, 0.4–2(–2.5) × 0.2–0.7(–0.9) cm, coriaceous, margins entire, plane to revolute, with broad-rimmed scales, apex obtuse to rounded or mucronate, surfaces with conspicuous, straw-colored or golden to ferruginous, broad-rimmed, glandular-peltate scales and unicellular-hairy abaxially.

Inflorescences

10–20-flowered;

bracts similar to bud scales.

fasciculate racemes, 3–6-flowered;

bracts similar to bud scales.

Pedicels

30–60 mm, glabrous.

2.5–14 mm, with ferruginous scales and often 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 before development of leaves, erect to horizontal, fragrant;

calyx lobes 0.5–2 mm, with ferruginous scales and unicellular-hairy, margins ciliate, unicellular and multicellular eglandular-hairy;

corolla rose to purple, sometimes white, without blotch, broadly funnelform, (6.5–)7.5–14(–15) mm, glabrous on outer surface, petals connate (for 3/4+ their lengths), lobes 4.5–8.5 mm, tube gradually expanding into lobes, 1.5–6.5 mm;

stamens 5–10, slightly exserted, ± unequal, 7–13 mm;

filaments glabrous or proximally unicellular-hairy.

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, 4–7 × 2–3 mm, with ferruginous, peltate scales and unicellular-hairy.

Seeds

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

testa expanded, dorsiventrally flattened, loose.

without 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 ferruginous-lepidote, sometimes also unicellular-hairy abaxially, margins ± ciliate, unicellular and multicellular, elongate eglandular-hairy.

2n

= 26.

= 26, 52.

Rhododendron macrophyllum

Rhododendron lapponicum

Phenology Flowering spring–summer. Flowering spring–summer.
Habitat Forest and forest margins, thickets Arctic and alpine tundra, rocky barrens and heaths or thickets, raised beach ridges, sandy stream banks
Elevation 50-1600 m [160-5200 ft] 0-1900 m [0-6200 ft]
Distribution
from FNA
CA; OR; WA; BC
[WildflowerSearch map]
[BONAP county map]
from FNA
AK; ME; NH; NY; WI; AB; BC; MB; NF; NS; NT; NU; ON; QC; YT; Greenland; Europe (Russia, Scandinavia); Asia (Siberia)
[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 lapponicum is known to hybridize with R. tomentosum in Greenland, producing R. ×vanhoeffenii Abromeit.

(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. 462. 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. cumberlandense, R. eastmanii, R. flammeum, R. groenlandicum, R. macrophyllum, R. maximum, R. minus, R. occidentale, R. periclymenoides, R. prinophyllum, R. prunifolium, R. tomentosum, R. vaseyi, R. viscosum
Synonyms R. californicum Azalea lapponica
Name authority D. Don ex G. Don: Gen. Hist. 3: 843. 1834 , (Linnaeus) Wahlenberg: Fl. Lapp., 104. 1812 ,
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