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

bog Labrador-tea, common Labrador tea, Labrador-tea, rusty Labrador-tea

Habit Shrubs, to 2.5 m, rhizomatous. Shrubs, 0.2–1.5 m, rhizomatous.
Stems

bark ± smooth to furrowed;

twigs multicellular eglandular-hairy (hairs unbranched) and unicellular-hairy.

erect and/or prostrate;

bark smooth, sometimes peeling or shredding with age;

twigs unicellular-hairy and with flattened, glandular scales, scales often obscured by dense, ferruginous, long-crisped, unbranched, multicellular hairs.

Leaves

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.

persistent, (fragrant when crushed);

petiole with unicellular and/or peltate scales and sometimes ferruginous, long-crisped, multicellular hairs;

blade ovate-lanceolate, sometimes narrowly elliptic to linear, 2–5 × 1.5–2.5 cm, coriaceous, margins entire, weakly to strongly revolute, glabrous, apex acute, abaxial surface with sparse to dense glandular-peltate scales without broad rim, scales often obscured by dense (to sparse), ± even covering of ferruginous, long-crisped, unbranched, multicellular, eglandular hairs usually concealing midvein, adaxial surface ± rugose with scattered, lepidote scales and sometimes also unicellular-hairy along sometimes impressed midrib.

Inflorescences

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

bracts similar to bud scales.

slightly rounded, 10–35-flowered;

bracts densely lepidote, sometimes with long-crisped hairs abaxially, margins ciliate, hairs long-crisped.

Pedicels

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

12–25 mm, with unicellular and/or glandular-peltate scales, sometimes multicellular-hairy (hairs ferruginous, long-crisped).

Flowers

± 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.

radially symmetric, opening after leaves (of flowering shoots), ± erect, not fragrant;

calyx lobes ca. 1–1.5 mm, outer surface densely to sparsely unicellular-hairy (hairs tan) and multicellular stipitate-glandular-hairy (hairs red) on margins;

corolla white to cream, without blotch, ± rotate, 2–8 mm, inner surface densely unicellular-hairy, petals appearing distinct or only slightly connate basally, lobes 5–7 mm;

stamens (5–)8(–10), exserted, ± equal, 3.8–9.5(–11) mm;

filaments glabrous or proximally unicellular-hairy.

Capsules

borne on erect pedicels, 6–8 × 5–6 mm, stipitate-glandular-, eglandular-, and unicellular-hairy.

borne on broadly recurved pedicels, 3–5.5 × 4–6 mm (slightly longer than wide), with sparse, lepidote scales, sometimes also long-crisped-hairy, acropetally dehiscent.

Seeds

with distinct tails;

testa closely appressed.

somewhat elongated beyond narrow ends;

testa closely appressed.

Floral

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

bud scales with lepidote scales, unicellular-hairy abaxially, margins unicellular-hairy.

2n

= 26.

= 26.

Rhododendron albiflorum

Rhododendron groenlandicum

Phenology Flowering late spring–summer. Flowering spring-mid summer.
Habitat Coniferous forests, alpine thickets, stream banks, seeps on rock outcrops Bogs, spruce forests, muskeg, swamps, stream margins, talus slopes, tundra
Elevation 800-3500 m [2600-11500 ft] 0-2000 m [0-6600 ft]
Distribution
from FNA
CO; ID; MT; OR; WA; AB; BC
[WildflowerSearch map]
[BONAP county map]
from FNA
AK; CT; ID; MA; ME; MI; MN; NH; NY; OR; PA; VT; WA; WI; AB; BC; MB; NB; NF; NS; NT; NU; ON; PE; QC; SK; YT; Greenland
[WildflowerSearch map]
[BONAP county map]
Discussion

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.)

Rhododendron groenlandicum, R. columbianum, and R. tomentosum customarily have been placed in the genus Ledum. Ledum is here considered to be a subsection of Rhododendron subg. Rhododendron (as subsect. Ledum), a placement supported by the presence in these species of comparable complex, multicellular, glandular, peltate scales and phylogenetic analyses of morphological and molecular data. The glandular scales of species of subsect. Ledum lack the radiating, broad-rimmed fringe-cells found in some members of subg. Rhododendron (and characteristic of R. minus and R. lapponicum) but are essentially identical to those of species of subsect. Edgeworthia, e.g., R. pendulum (see K. A. Kron and W. S. Judd 1990). More than 500 species of subg. Rhododendron occur in tropical and temperate eastern Asia (J. Cullen 1980; D. F. Chamberlain et al. 1996).

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

Source FNA vol. 8, p. 464. Treatment authors: Walter S. Judd, Kathleen A. Kron. FNA vol. 8, p. 460. Treatment authors: Walter S. Judd, Kathleen A. Kron.
Parent taxa Ericaceae > subfam. Ericoideae > Rhododendron Ericaceae > subfam. Ericoideae > Rhododendron
Sibling taxa
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
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. lapponicum, R. macrophyllum, R. maximum, R. minus, R. occidentale, R. periclymenoides, R. prinophyllum, R. prunifolium, R. tomentosum, R. vaseyi, R. viscosum
Synonyms Azaleastrum albiflorum, R. albiflorum var. warrenii Ledum groenlandicum
Name authority Hooker: Fl. Bor.-Amer. 2: 43, plate 133. 1834 , (Oeder) Kron & Judd: Syst. Bot. 15: 67. 1990 ,
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