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

early azalea, election-pink, hoary azalea, roseshell azalea

Habit Shrubs, to 2.5 m, rhizomatous. Shrubs, to 3(–5) m, usually not rhizomatous.
Stems

bark ± smooth to furrowed;

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

bark smooth to vertically furrowed, shredding;

twigs scattered, multicellular eglandular-hairy (hairs unbranched), otherwise densely to sparsely unicellular-hairy.

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.

deciduous;

petiole usually multicellular eglandular-hairy and unicellular-hairy;

blade ovate to obovate, 3–9 × 1.2–3.7 cm, thin, membranous, margins entire, plane, conspicuously ciliate, eglandular-hairy (hairs ascending away from margins), apex acute to obtuse, often mucronate, abaxial surface sparsely to densely unicellular-hairy, sometimes also eglandular-hairy, adaxial surface usually sparsely unicellular-hairy, sometimes glabrous, often also scattered eglandular-hairy.

Inflorescences

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

bracts similar to bud scales.

4–13-flowered;

bracts similar to bud scales.

Pedicels

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

5–26 mm, usually stipitate-glandular-hairy, or, sometimes, also eglandular-hairy, otherwise sparsely to moderately unicellular-hairy.

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.

opening before or with leaves, erect to horizontal, fragrant (spicy-scented);

calyx lobes 0.5–4 mm, surfaces and margins scattered stipitate-glandular- and/or eglandular-hairy, otherwise sparsely to densely unicellular-hairy;

corolla usually bright pink, without blotch on upper lobe, funnelform, 25–45 mm, scattered, multicellular stipitate-glandular-hairy (hairs not forming distinct lines), otherwise sparsely to moderately unicellular-hairy on outer surface, petals connate, lobes 10–23 mm, tube gradually expanded into lobes, 11–27 mm (equaling or longer than lobes);

stamens 5, much exserted, ± unequal, 32–53 mm.

Capsules

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

borne on erect pedicel, 10–28 × 3–7 mm, stipitate-glandular-hairy, otherwise sparsely unicellular-hairy.

Seeds

with distinct tails;

testa closely appressed.

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

testa expanded, dorsiventrally flattened, ± loose.

Floral

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

bud scales very sparsely to densely unicellular-hairy abaxially, especially near midvein, margins unicellular-ciliate.

2n

= 26.

= 26.

Rhododendron albiflorum

Rhododendron prinophyllum

Phenology Flowering late spring–summer. Flowering spring.
Habitat Coniferous forests, alpine thickets, stream banks, seeps on rock outcrops Acidic thickets or bogs, swampy to dry, rocky woods, bluffs, ravines, or along streams
Elevation 800-3500 m [2600-11500 ft] 100-1500 m [300-4900 ft]
Distribution
from FNA
CO; ID; MT; OR; WA; AB; BC
[WildflowerSearch map]
[BONAP county map]
from FNA
AL; AR; CT; IL; KY; MA; MD; MO; NC; NH; NJ; NY; OH; OK; PA; RI; TN; TX; VA; VT; WV
[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 prinophyllum is unusual in its strongly disjunct distribution. Plants in the western portion of the range often have longer corolla tubes and are most similar to R. canescens and R. periclymenoides, and can be distinguished from both by their broader, more gradually expanded corolla tubes and usually consistently glandular sepal margins, pedicels, and ovaries. Hybrids are known with R. calendulaceum and R. periclymenoides. The name R. roseum (Loiseleur) Rehder, which has been used for this species, is illegitimate because it was superfluous when published (K. A. Kron 1989).

(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. 469. 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. groenlandicum, R. lapponicum, R. macrophyllum, R. maximum, R. minus, R. occidentale, R. periclymenoides, R. prunifolium, R. tomentosum, R. vaseyi, R. viscosum
Synonyms Azaleastrum albiflorum, R. albiflorum var. warrenii Azalea prinophylla
Name authority Hooker: Fl. Bor.-Amer. 2: 43, plate 133. 1834 , (Small) Millais: Rhododendrons, 229. 1917 ,
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