Rhododendron viscosum(L.) Torr.

swamp azalea

WFO wfo-0001047632 Accepted WFO 2026-06 8 photographs CC0 / CC BY

Plate 1 figs. a–h · 8 separate observations

Rhododendron viscosum, photographed by Matt Pelikan
fig. a Matt Pelikan, CC BY 4.0 / 2022-06-10 / obs. 204960525
Rhododendron viscosum, photographed by Jeff Clark
fig. b Jeff Clark, CC BY 4.0 / obs. 188847087
Rhododendron viscosum, photographed by Jake McCumber
fig. c Jake McCumber, CC BY 4.0 / obs. 162727239
Rhododendron viscosum, photographed by leahmv
fig. d leahmv, CC BY 4.0 / obs. 171089395
Rhododendron viscosum, photographed by Mike Gooley
fig. e Mike Gooley, CC0 1.0 / obs. 141859650
Rhododendron viscosum, photographed by Mark Apgar
fig. f Mark Apgar, CC BY 4.0 / obs. 137711382
Rhododendron viscosum, photographed by Eli Talhouk
fig. g Eli Talhouk, CC BY 4.0 / obs. 137288436
Rhododendron viscosum, photographed by Lucian Vazquez
fig. h Lucian Vazquez, CC BY 4.0 / obs. 137070178

Every figure is a research-grade observation under CC0, CC BY or CC BY-SA, rehosted with the photographer’s name, the licence and the observation it came from. Photographs under a NonCommercial licence are excluded from this site and are never stored, which costs us a great many pictures and is not negotiable.

Native range 23 botanical countries

Regions where Rhododendron viscosum is native: Alabama, Arkansas, Connecticut, Delaware, Florida, Georgia, Louisiana, Maine, Maryland, Massachusetts, Mississippi, New Hampshire, New Jersey, New York, North Carolina, Oklahoma, Pennsylvania, Rhode I., South Carolina, Tennessee, Texas, Vermont, Virginia AlabamaArkansasConnecticutFloridaGeorgiaLouisianaMaineMarylandMassachusettsMississippiNew HampshireNew JerseyNew YorkNorth CarolinaOklahomaPennsylvaniaSouth CarolinaTennesseeTexasVermontVirginia DelawareRhode I.
Native distribution of Rhododendron viscosum, after Kew’s World Checklist of Vascular Plants. Introduced, extinct and doubtful records are excluded, so this is where the plant is from, not everywhere it now grows. Regions too small to draw at this scale are marked with a dot.
RegionTDWG codeContinent
Alabama ALA NORTHERN AMERICA
Arkansas ARK
Connecticut CNT
Delaware DEL
Florida FLA
Georgia GEO
Louisiana LOU
Maine MAI
Maryland MRY
Massachusetts MAS
Mississippi MSI
New Hampshire NWH
New Jersey NWJ
New York NWY
North Carolina NCA
Oklahoma OKL
Pennsylvania PEN
Rhode I. RHO
South Carolina SCA
Tennessee TEN
Texas TEX
Vermont VER
Virginia VRG

Region boundaries approximated from Natural Earth (public domain) and mapped to TDWG World Geographical Scheme for Recording Plant Distributions (WGSRPD) level-3 botanical countries (Brummitt 2001). Indicative, not the official WGSRPD geometry.

Flowering 352 in flower of 424 examined

Peak flowering in Jul. Each bar is the share of Rhododendron viscosum observations in which someone actually recorded the reproductive state and found the plant in flower, not the raw number of flowering records. That distinction matters: people observe plants far more in spring than in winter, so a bare count of flowering records partly measures when people go outside. Dividing by the number examined removes that. 352 of 424 examined observations were in flower, every one of them research grade. The whisker on each bar is a 95% Wilson interval. 4 months have fewer than 5 examined observations, so no proportion is drawn for them. This is still a global aggregate and not a forecast for your garden: the same species flowers on different dates in different hemispheres. Where a species has fewer than 30 flowering records we do not draw this chart at all. Computed from 10.15468/dl.cgje2x.

Where it actually grows measured, from 1,818 observations

Condition 5th percentile Median 95th percentile
Coldest month, mean daily low -7.9 °C -3.0 °C 5.6 °C
Warmest month, mean daily high 21.3 °C 27.9 °C 33.0 °C
Annual rainfall 1,131 mm 1,244 mm 1,495 mm
Rainfall in the driest quarter 243 mm 279 mm 329 mm

It is found where winters bring hard frost. This is not care advice and it is not a forecast. It is a measurement: we sampled the climate at every one of the 1,818 research-grade observations of Rhododendron viscosum that carry a coordinate, and this is the range those places actually span. The 5th and 95th percentiles are used rather than the minimum and maximum, because a single cultivated specimen in a heated conservatory should not widen a tropical plant's range to the Arctic.

This is not a hardiness zone. A USDA zone is the average annual extreme minimum temperature. The figure above is the mean daily minimum of the coldest month, which is a different quantity and is typically far warmer. Reading one as the other would place a plant several zones too warm, so we do not publish a hardiness zone, because we do not have one. Climate from CHELSA V2.1 (Karger et al. 2017); occurrences from 10.15468/dl.cgje2x.

Also published as 32 synonyms

A synonym is not an error. It is a record of botanists disagreeing, in print, about where this plant belongs. Each of these was somebody’s considered answer.

  • Anthodendron viscosum (L.) Rchb.
  • Azalea glauca Lam.
  • Azalea hispida Pursh
  • Azalea nitida Pursh
  • Azalea oblongifolia Small
  • Azalea scabra Lodd. ex A.Gray
  • Azalea serrulata Small
  • Azalea tomentosa Dum.Cours
  • Azalea virens (Michx.) Dum.Cours.
  • Azalea viscida Christm.
  • Azalea viscosa L.
  • Azalea viscosa var. aemulans (Rehder) Ashe
  • Azalea viscosa var. hispida (Pursh) Hook.
  • Azalea viscosa var. montana (Rehder) Ashe
  • Azalea viscosa var. nitida (Pursh) A.Gray
  • Azalea viscosa var. virens Michx.
  • Rhododendron coryi Shinners
  • Rhododendron glaucum (Lam.) Sweet
  • Rhododendron glaucum G.Don
  • Rhododendron hispidum (Pursh) Torr.
  • Rhododendron nitidum (Pursh) Torr.
  • Rhododendron oblongifolium (Small) Millais
  • Rhododendron ornatum Sweet
  • Rhododendron serrulatum (Small) Millais

and 8 more.

Sourcesevery claim on this page

  1. World Flora Online Plant List. accepted name, authority, classification. CC0. Retrieved 2026-07-12.
  2. iNaturalist. photographs and flowering annotations, CC0 / CC BY / CC BY-SA only. per photograph. Retrieved 2026-06-27.
  3. Wikidata. common name (P1843), joined on the World Flora Online identifier (P7715). CC0. Retrieved 2026-07-13.
  4. Kew, World Checklist of Vascular Plants (WCVP v16). native distribution by TDWG level-3 botanical country, and life form. CC BY 3.0. Retrieved 2026-06-04.

We publish what we can source and we say so when we cannot. This page has no care advice and no toxicity claim, because we do not yet have those from a source we can cite.