Rhododendron viscosum Torr.

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Article from Bean's Trees and Shrubs Hardy in the British Isles

Recommended citation
'Rhododendron viscosum' from the website Trees and Shrubs Online (treesandshrubsonline.org/articles/rhododendron/rhododendron-viscosum/). Accessed 2024-05-28.



  • Azalea viscosa L.
  • Rhododendron oblongifolium (Small) Millais
  • Rhododendron serrulatum (Small) Millais

Other taxa in genus


(pl. calyces) Outer whorl of the perianth. Composed of several sepals.
The inner whorl of the perianth. Composed of free or united petals often showy.
Lowest part of the carpel containing the ovules; later developing into the fruit.
The inner whorl of the perianth. Composed of free or united petals often showy.
Protruding; pushed out.
Lacking hairs smooth. glabrescent Becoming hairless.
Bearing glands.
midveinCentral and principal vein in a leaf.
Generally an elongated structure arising from the ovary bearing the stigma at its tip.


Article from Bean's Trees and Shrubs Hardy in the British Isles

Recommended citation
'Rhododendron viscosum' from the website Trees and Shrubs Online (treesandshrubsonline.org/articles/rhododendron/rhododendron-viscosum/). Accessed 2024-05-28.

Deciduous shrub or small tree, to 6m; young twigs usually eglandular-hairy, occasionally with gland-tipped hairs. Leaves (3–)4–6(–8) × 1.3–2.3(–3.1) cm ovate or obovate to elliptic, lower surface glabrous, sometimes glaucous, occasionally with eglandular and/or gland-tipped hairs. Flower bud scales with outer surface sparsely to densely covered with unicellular hairs or glabrous, margin unicellular-ciliate, occasionally glandular below. Flowers with a sweet fragrance, appearing after the leaves have expanded, 3–14, in a shortened raceme; calyx 1–2(–5) mm; corolla white, occasionally with a pink or purplish tinge, rarely completely pink, funnelform, tube gradually expanding into limb, outer surface usually covered with unicellular and gland-tipped hairs, 20–57 mm. Capsule covered with eglandular or gland-tipped hairs. Flowering May-July. Royal Horticultural Society (1997)

Distribution  United States E & S

Habitat s.l.-1,500 m

RHS Hardiness Rating H6

Awards AM 1921 (F.G. Strover, South Norwood, London) as Azalea viscosa glauca; flowers white. AGM 1993.

Conservation status Least concern (LC)

Taxonomic note This is a variable and widespread species. Royal Horticultural Society (1997)

A deciduous shrub of bushy habit eventually 6 to 8 ft high, with twiggy branches, hairy when young. Leaves thinly arranged along the shoot or in a tuft of five or six at the end, obovate, 1 to 2 in. long, tapering to a short stalk at the base; dark green and glabrous above, paler and bristly along the midrib beneath; margins bristly. Flowers produced during June and July at the end of the previous year’s shoots, six to twelve together in a cluster. Calyx small, and like the slender flower-stalk, glandular-hairy. 34 in. long, curved, hairy. Corolla white or pink, 1 to 114 in. long, the lower half is a narrow tube often more highly coloured, the upper half five expanded oblong lobes 34 in. long. The whole corolla, but especially the tubular part, is covered with sticky hairs. Stamens exserted. Ovary clad with usually gland-tipped bristles; style longer than the stamens, downy in the lower part. (s. Azalea ss. Luteum)

Native of eastern N. America; introduced in 1734, and still one of the most delightful of garden shrubs because of its late blossoming and its exquisitely fragrant flowers. It is the reputed parent, or one of the parents of a great number of garden azaleas. Loddiges in their catalogue for 1836 gave a list of one hundred and seven varieties, which, according to Loudon, were hybrids or varieties of R. viscosum. The identity of many of these old varieties is lost, but some are still to be obtained under their old names. Relatively few, however, show any viscosum ‘blood’, but rather that of R. periclymenoides and R. calendulaceum. The viscosum group at the present time is, as a matter of fact, a rather limited one, but is well distinguished by the lateness in flowering, strong fragrance, and the viscous blossoms.

R serrulatum (Small) Millais

Azalea serrulata Small

Closely related to R. viscosum, which it replaces from mid-Georgia south to Florida and west to south-eastern Louisiana. According to Rehder its distinguishing characters are the red-brown branchlets, the leaves of firmer texture, finely but distinctly serrated at the margin, and the longer corolla-tube (about twice as long as the lobes, against one-and-a-half times as long in R. viscosum). It is remarkable for flowering very late in its natural habitat, but is of no importance for British gardens.

var. glaucum (Ait.) Torr.

Azalea viscosa var. glauca Ait

The swamp honey­suckle is variable in the wild, more especially in the colour of the flowers and leaves. This is its most distinct variety, with pure white, fragrant flowers, and leaves blue-white on the lower, or sometimes on both, surfaces. A very charming shrub, flowering late like the type.Other variants are described by Rehder in A Monograph of Azaleas, pp. 160–5. On the variability of R. viscosum Dr Henry Skinner writes: ‘From the dwarf, twiggy and semi-evergreen bushes of the marshes of South Carolina to the tall, grey-leaved and large-flowered shrubs of the pond margins of Cape Cod, the Swamp azalea is much more changeable than its sister of the Gulf coast [R. serrulatum]… . It seems certain that not a little of the trouble is due to R. viscosum and arborescens having met on occasion in the northern states… . In some of these northern swamps genes have been so freely exchanged between these two species that nomenclatural assignment of present populations becomes virtually impossible’ (R.C.Y.B. 1957, p. 28).