Rhododendron calendulaceum

(Michaux) Torrey

Fl. N. Middle United States, 425. 1824 ,.

Common names: Flame azalea
Endemic
Basionyms: Azalea calendulacea Michaux Fl. Bor.-Amer. 1: 151. 1803
Treatment appears in FNA Volume 8. Treatment on page 465. Mentioned on page 458, 466, 469, 470, 471.

Shrubs or trees, to 4 (–10) m, usually not rhizomatous. Stems: bark smooth to vertically furrowed, shredding; twigs sparsely to conspicuously multicellular eglandular-hairy (hairs unbranched), otherwise sparsely to moderately unicellular-hairy. Leaves deciduous; petiole multicellular eglandular-hairy and unicellular-hairy; blade ovate to obovate, 4.5–9.1 × 1.3–3.3 cm, thin, membranous to chartaceous, margins entire, plane, ciliate, eglandular-hairy, apex acute to obtuse, often mucronate, abaxial surface glabrous or densely eglandular-hairy, often also very sparsely to moderately unicellular-hairy, adaxial surface glabrous or sparsely scattered eglandular-hairy, often also sparsely unicellular-hairy. Floral bud-scales usually glabrous abaxially, margins glandular. Inflorescences 5–9-flowered; bracts similar to bud-scales. Pedicels 7–14 mm, stipitate-glandular-hairy and/or eglandular-hairy, otherwise sparsely to densely unicellular-hairy. Flowers opening before or as leaves expand, erect to horizontal, acrid-scented; calyx lobes 1–3 mm, scattered usually stipitate-glandular-hairy and unicellular-hairy, margins long stipitate-glandular-hairy and/or eglandular-hairy; corolla orange to red-orange, with contrasting, darker-colored, orange to yellow blotch on upper lobe, funnelform, 30–54 mm, densely multicellular stipitate-glandular-hairy, otherwise sparsely to densely unicellular-hairy on outer surface, petals connate, lobes 14–30 mm, (spreading nearly as broadly as tube long), tube abruptly expanding into lobes, 16–28 mm (longer than lobes); stamens 5, much exserted, ± unequal, 52–72 mm. Capsules borne on erect pedicels, 15–29 × 5–9 mm, sparsely to moderately long stipitate-glandular-hairy and/or eglandular-hairy, and sparsely unicellular-hairy. Seeds without distinct tails, flattened portion of testa well developed at each end; testa expanded, dorsiventrally flattened, ± loose. 2n = 52.


Phenology: Flowering spring–summer.
Habitat: Mixed deciduous forests
Elevation: 200-1000 m

Distribution

V8 910-distribution-map.gif

Ga., Ky., Md., N.C., Ohio, Pa., S.C., Tenn., Va., W.Va.

Discussion

Rhododendron calendulaceum, frequently confused with the diploid R. cumberlandense, is an allotetraploid, and may have been derived from a hybrid between an ancestor of R. cumberlandense and one of R. prinophyllum (K. A. Kron 1993). It is distinguished from R. cumberlandense by its flowers opening before or with (versus after) the expansion of the leaves, glandular or eglandular (versus usually eglandular) pedicels, glandular or eglandular (versus usually eglandular) sepal margins (R. cumberlandense never with both pedicels and sepal margins glandular), and nonglaucous (versus often glaucous) abaxial leaf surface. Rhododendron calendulaceum is one of the showiest shrubs of the southern Appalachians; it is known to hybridize with R. prinophyllum and R. canescens.

Rhododendron calendulaceum and the remaining species (through 25. R. eastmanii) are the North American representatives of sect. Pentanthera G. Don, a group characterized by corollas that are typically unicellular-hairy on the outside and with a narrow tube, flowers with five, declinate, and usually much-exserted stamens, and seeds with ± loose testae (K. A. Kron 1993; Kron and M. Creel 1999). All species are used as ornamentals because of their colorful flowers(F. C. Galle 1985).

Selected References

None.

Lower Taxa

None.

unicellular-hairy +, eglandular-hairy +  and glabrous +
unicellular-hairy +, eglandular-hairy +, scattered +  and glabrous +
Mucronate (?) +, Acute (?) +  and Obtuse (?) +
Walter S. Judd +  and Kathleen A. Kron +
(Michaux) Torrey +
smooth +  and vertically furrowed +
not flaky +
Azalea calendulacea +
4.5 cm45 mm <br />0.045 m <br /> (9.1 cm91 mm <br />0.091 m <br />) +
ovate;obovate +
membranous;chartaceous +
thin;1.3cm;3.3cm +
orange +  and yellow +
scale-like +
unicellular-hairy +  and stipitate-glandular-hairy +
0.1 cm1 mm <br />0.001 m <br /> (0.3 cm3 mm <br />0.003 m <br />) +
1.5 cm15 mm <br />0.015 m <br /> (2.9 cm29 mm <br />0.029 m <br />) +
unicellular-hairy +, eglandular-hairy +, stipitate-glandular-hairy +  and long +
0.5 cm5 mm <br />0.005 m <br /> (0.9 cm9 mm <br />0.009 m <br />) +
Flame azalea +
multicellular +
orange +  and red-orange +
unicellular-hairy +  and stipitate-glandular-hairy +
rotate to crateriform campanulate cylindric globose or urceolate +
3 cm30 mm <br />0.03 m <br /> (5.4 cm54 mm <br />0.054 m <br />) +
Ga. +, Ky. +, Md. +, N.C. +, Ohio +, Pa. +, S.C. +, Tenn. +, Va. +  and W.Va. +
200-1000 m +
undifferentiated +
fusiform +
acrid-scented +
erect +  and horizontal +
usually ovoid;cylindric +
Mixed deciduous forests +
multicellular +
terminal +  and axillary +
deciduous +
14mm +  and 30mm +
eglandular-hairy +, stipitate-glandular-hairy +  and long +
parietal +, axile +  and placentation +
tenuinucellate +  and unitegmic +
unicellular-hairy +, eglandular-hairy +  and stipitate-glandular-hairy +
0.7 cm7 mm <br />0.007 m <br /> (1.4 cm14 mm <br />0.014 m <br />) +
scale-like +
reduced +
not sticky +
multicellular +
unicellular-hairy +  and eglandular-hairy +
Flowering spring–summer. +
4-5-carpellate +
Fl. N. Middle United States, +
distinct +
tailed +, flattened-ellipsoidal +  and fusiform +
unequal +
procumbent +  and prostrate +
creeping +  and sprawling +
peltate +  and capitate +
straight +
Undefined tribe Empetraceae +
distinct +
Rhododendron calendulaceum +
Rhododendron +
species +
flattened +
expanded +
expanding +
1.6 cm16 mm <br />0.016 m <br /> (2.8 cm28 mm <br />0.028 m <br />) +
multicellular +
unicellular-hairy +  and eglandular-hairy +
evergreen +, deciduous +  and perennial +
tree +  and shrub +
heterotrophic +, autotrophic +  and mycotrophic +