Vascular Plants of North Carolina
Account for Swamp Azalea - Rhododendron viscosum   (L.) Torrey
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Section 6 » Order Ericales » Family Ericaceae
Author(L.) Torrey
DistributionThough it has a more-or-less statewide distribution, it is mainly found in the Coastal Plain and the Mountains, and less so in the western Piedmont. It is quite spotty in occurrence in the eastern Piedmont and northwestern Coastal Plain. The map below probably contains some Coastal Plain counties that actually refer to the recently elevated Rhododendron serrulatum.

The range of the species is centered near NC, though it occurs essentially only in Atlantic and Gulf coast states, plus AR and OK. It ranges north to ME and southwest to eastern TX, but is absent in the Great Lakes states.
AbundanceFairly common in most of the Coastal Plain, but far outnumbered in the Sandhills region by the recently split R. serrulatum. It is infrequent to fairly common in the Mountains, but is rare to locally uncommon in most of the Piedmont and northwestern Coastal Plain. It is possibly absent in a few counties in the northern part of these latter two provinces. The abundance in the Coastal Plain is not completely known now, since many records previously included as R. viscosum now refer to R. serrulatum.
HabitatThis species occurs mainly in wetlands, such as montane bogs, stream banks, pond margins, wet thickets, and swamp margins. It also grows on heath balds to a lesser extent. Where it grows along stream margins, it is mostly those that are not rocky, which is the main habitat of the similar-looking R. arborescens. The recently split R. serrulatum is the primary "clammy azalea" of Sandhills streamhead pocosins, seepages, and other strongly acidic wetlands, including wet pine savannas.
PhenologyBlooms from June to July, after the leaves have fully emerged; fruits from July to October.
IdentificationThis is a deciduous shrub that grows mainly to 3-6 feet tall, but averages about 5 feet tall. It has somewhat darker green and shinier leaves than does R. periclymenoides, though R. arborescens can look quite similar. Swamp Azalea, however, has the floral tubes being very glandular (sticky), and the white flower is extremely fragrant, more so than other species in the genus in NC. The recently split R. serrulatum is quite similar; however, this latter species has the corolla tube glabrous inside, and the corolla tube is at least twice the length of the lobes of the flowers. R. serrulatum can also grow to 20 feet tall, and the winter buds have 15-20 scales (as opposed to 8-12 scales in R. viscosum). The "new" R. viscosum has the corolla tube pubescent inside, and the corolla tube is less than twice as long as the lobes of the corolla.
Taxonomic CommentsRAB (1968) and some other references assigned two varieties for it -- var. viscosum and var. serrulatum. Many references did not agree, and simply did not include varieties for R. viscosum. However, Weakley (2018) considers the two taxa as full species; thus, according to that reference and this website, there are no other taxa included within the revised R. viscosum.

Other Common Name(s)Clammy Azalea is another frequently used name.
State RankS5 [S4S5]
Global RankG5
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