USA: Northern West Virginia, south through the mountains of Virginia, North Carolina, Tennessee and Georgia. Generally restricted to the Ridge and Valley Province and Southern Appalachians, with occasional populations in the Cumberland Mountains and in the eastern edge of the Cumberland Plateau in Ohio and Kentucky. Not known west of die Tennessee River Valley in Tennessee. Extending into the Upper Piedmont in North and South Carolina and as far soutii as Meriwether Co., Georgia. Collections from Tarrytown, New York that are R. calendulaceum have been suggested as native in the past (Barnhart, 1895). However, all of the collections are nearly a century old and this is well north of the range of the species. Most likely these were cultivated plants that had persisted.
Regularity: Regularly occurring
Occurrence in North America
Flame azalea is a native, deciduous, erect, much-branched shrub that
grows to 10 feet (3 m) in height. Its morphology and phenology are
highly variable . The fruit is a capsule [8,20,29]. Rhododendrons
(Rhododendron spp.) have a diffuse shallow root system . Flame
azalea is not rhizomatous .
Shrub or small tree to 10m tall, usually non-rhizomatous; young twigs reddish brown, sparsely to densely covered with unicellular hairs and multicellular eglandular hairs. Vegetative bud scales glabrous abaxially; margin unicellular-ciliate. Leaf blade membranaceous, ovate or obovate to elliptic, (4.5-)5.6-7.7(-9.1) x (1.3-)1.8-2.6(-3.3)cm; base acute to oblique; apex acute to obtuse, often mucronate; adaxial surface sparsely covered with multicellular eglandular and unicellular hairs or with only multicellular eglandular hairs, or glabrous; the midvein densely covered with unicellular hairs; abaxial surface sparsely to densely covered with multicellular eglandular hairs or glabrous; the midvein densely covered with unicellular and multicellular eglandular hairs,rarely densely covered with unicellular hairs and multicellular eglandular hairs, or with only unicellular hairs; margin entire, ciliate with multicellular eglandular hairs; petiole 0.3-0.6(-0.7)cm long, sparsely to densely covered with unicellular hairs and multicellular eglandular hairs. Flower bud scales chestnut brown; abaxial surface glabrous, rarely very sparsely covered with unicellular hairs; margin glandular. Flowers appearing before or as the leaves expand; inflorescence a shortened raceme of 5 to 9 flowers. Pedicels (0.7-)0.8-L2(-1.4)cm long, sparsely to densely covered with unicellular hairs and densely covered with multicellular gland-tipped hairs, or with multicellular eglandular hairs, or both. Sepals (0.1-)0.2-0.3cm long, often varying in length on the same flower; margins glandular-fimbriate, frequently setose; abaxial surface sparsely to densely covered with unicellular hairs and multicellular gland-tipped hairs, rarely with unicellular hairs and multicellular eglandular hairs or with only unicellular hairs. Corolla orange to flame-coloured, fragrance acrid, the tube longer than the limb and abruptly expanding into it; upper corollalobe (1.4-)1.6-2.2(-2.3) x (1.2-)1.5-2.2(-2.5)cm; lateral lobes 1.8-2.6(-3.0) x (0.9-)1.0-1.4(-l .5)cm; corolla tube (1.6-) 1.8-2.2(-2.4)cm long, (0.2-)0.3-0.4(-0.5)cm wide at base; outer surface of corolla sparsely to densely covered with unicellular hairs and densely covered with multicellular gland-tipped hairs which often continue up the corolla lobes; inner surface of corolla sparsely to densely covered with unicellular hairs. Stamens (5.2-)5.7-6.7(-7.2)cm long, with dense terete or flattened unicellular hairs on proximal (1.9-)2.3-3. l(-3.2)cm of filament, exserted (3.4-)3.7-4.8(-5.4)cm beyond throat of corolla. Style (5.5-)5.9~6.8(-7.2)cm long, exserted (3.8-)4.2-5.3(-6.0)cm beyond throat of corolla, with dense unicellular hairs on proximal (0.0-) 0.3-1.3(-1.7)cm; stigma 0.1-0.2(-0.3)cm wide. Ovary 0.3-0.4(-0.5)cm long, (0.1-)0.2-0.3cm wide at the base, densely covered with multicellular eglandular hairs and dense unicellular hairs, rarely with multicellular gland-tipped hairs and unicellular hairs, or with all three types of hairs. Capsules (1.5-)1.8-2.6(-2.9) x (0.5-)0.6-0.8(-0.9)cm, ovate, sparsely covered with unicellular hairs and sparsely to moderately covered with either multicellular eglandular hairs or less often eglandular hairs. Seeds pale to dark chestnut brown, ovate or elliptic to fusiform, (1.7-)2.4-3.9 (-4.2) x (0.6-)0.9-1.5(-1.9)mm, body (0.8-)l.l-1.7(-2.1) x (0.2-)0.4-0.7(-0.8)mm, the testa expanded and dorsiventrally flattened, surrounding the body, the cells elongate, with transverse end-walls.
Key Plant Community Associations
Flame azalea occurs in mixed deciduous forests [11,19,26]. It occurs in
the well developed shrub layer of oak (Quercus spp.) forests of southern
and western exposures, and with more mesic site species in ravines.
Flame azalea is an important understory shrub in forests formerly
codominated by American chestnut (Castanea dentata) [1,26].
Flame azalea occurs with other ericaceous shrubs including rosebay
rhododendron (Rhododendron maximum), mountain-laurel (Kalmia latifolia),
highbush cranberry (Vaccinium corymbosum), and huckleberry (Gaylussacia
spp.) . It occurs at low coverage in the grassy bald vegetation type
and the grassy bald edge ecotone .
sites. It occurs on submesic to subxeric sites at lower elevations and
on submesic sites at elevations above 5,000 feet (1,500 m) .
Adequate humidity and soil moisture are required. Rhododendrons grow
best on acidic soils from pH 4.5 to 5.5 .
This species is known to occur in the following ecosystem types (as named by the U.S. Forest Service in their Forest and Range Ecosystem [FRES] Type classification):
FRES13 Loblolly - shortleaf pine
FRES15 Oak - hickory
FRES18 Maple - beech - birch
Habitat: Cover Types
This species is known to occur in association with the following cover types (as classified by the Society of American Foresters):
44 Chestnut oak
52 White oak - black oak - northern red oak
53 White oak
55 Northern red oak
Habitat: Plant Associations
This species is known to occur in association with the following plant community types (as classified by Küchler 1964):
K104 Appalachian oak forest
K109 Transition between K104 and K106
K111 Oak - hickory - pine forest
Found in open, dry sites on southern and western exposures of hills and mountain-sides.
Broad-scale Impacts of Plant Response to Fire
fire in the southern Appalachians of North Carolina provides information
on prescribed fire
and postfire response of plant community species,
including flame azalea,
that was not available when this species review
was originally written.
Plant Response to Fire
Flame azalea probably sprouts from the root crown when top-killed.
Tall shrub, adventitious-bud root crown
Flame azalea is probably fire resistant because of its ability to sprout
from the root crown. Fire may open up maturing forest canopies and
rejuvenate declining flame azalea. Flame azalea occurs in oak woods
that periodically experience fire .
Rhododendron fruits split along the sides soon after ripening and
release many small seeds which are disseminated short distances by
wind. Moist mineral soil or a short moss seedbed is required for
seedling establishment [9,16].
Rhododendrons sprout from the root crown when top-killed .
Propagation techniques from cuttings are described [5,23]. Day-night
temperatures and durations for maximizing flame azalea seedling growth
are reported .
Growth Form (according to Raunkiær Life-form classification)
Life History and Behavior
Flame azalea flowers in late April and May at lower elevations and in
June and early July at higher elevations . Flowers appear before or
with the leaves and last several weeks. Fruit matures July through
Molecular Biology and Genetics
Barcode data: Rhododendron calendulaceum
Statistics of barcoding coverage: Rhododendron calendulaceum
Public Records: 1
Specimens with Barcodes: 1
Species With Barcodes: 1
National NatureServe Conservation Status
Rounded National Status Rank: N5 - Secure
NatureServe Conservation Status
Rounded Global Status Rank: G5 - Secure
Although historically reported in these states, no extant populations
are known [3,30]. Flame azalea is listed as endangered in Ohio .
It is secure throughout the rest of its range.
Relevance to Humans and Ecosystems
It is a deciduous shrub, 120–450 cm tall. The leaves are 3–7 cm long, slightly dull green above and villous below. The flowers are 4–5 cm long, usually bright orange, but can vary from pastel orange to dark reddish-orange.
Rhododendron calendulaceum by Ellis Rowan, 1901
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Names and Taxonomy
calendulaceum (Michx.) Torr. (Ericaceae) [8,20]. There are no currently
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