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Species In Our Midst
Rhododendron viscosum

by Ian Donovan


Rhododendron viscosum
Picture by S & J Perkins
Rhododendron viscosum in Salem, NH


Rhododendron viscosum

If I were asked to choose, I would say that the deciduous azaleas in Subgenus Pentanthera, Section Pentanthera, are my favorite rhododendrons. Our native Rhododendron viscosum has been placed in Subsection Pentanthera along with all the other North American deciduous azaleas. It was the first American azalea to be grown in England. Bishop Henry Compton of London grew it in c.1680 from seed sent by Reverend John Banister, 1650-1692, an English missionary-naturalist who was accidentally shot to death by a game hunter while he himself was hunting plants.

The specific epithet, viscosum, is from the Latin and means "gluey". The folk name, Swamp Honeysuckle, refers to the sticky substance on the outside of the long, slender corolla tube. It comes from minute hairs with a knob, or gland, on the end that secretes the sticky material, hence the term glandular. This is an important diagnostic characteristic.

The other common name, Swamp Azalea, derives from the plant's favored habitat on the edges of creeks, swamps and ponds; bayheads; low flatlands; moist thickets; shrub balds; and acid bogs where moisture is regularly present. The roots, of course, are not submerged in the water because they must have oxygen. Rhododendron viscosum is a widely distributed and variable species that can be found from Vermont and Maine to peninsular Florida, west to Texas and north to Arkansas from sea level to 1500m (5,000 feet).

In her 1993 revision of Subsection Pentanthera, Dr. Katherine A. Kron confirmed the 1981 combining of previously described regional species R. serrulatum, R. coryi, R. oblongifolium, and R. viscosum into the one species, R. viscosum. Peter and Kenneth Cox in their Encyclopedia of Rhododendron Species (1997) have retained these geographical variants as distinct "Groups" so as to recognize that there are different plant forms over its range, depending upon where the plant originates. Greer rates its hardiness as -15°F (-32°C).

Rhododendron viscosum flowers primarily from May to August but can flower as early as March or as late as December in the South. The 3 to 14 flowers in a short raceme, which usually appear after the leaves have expanded, may occasionally be tinged pink or purple. The stamens and style are not colored.

The plant may grow to a multistem shrub of 6m (20 feet), but is usually about 5 to 8 feet in height. It is sometimes stoloniferous, and the branchlets are pubescent (covered in short hairs) and bristly. The deciduous leaves are obovate to oblanceolate, 1.5 to 6.0+ cm long and 0.5 to 3.0 cm broad.

Just down Valley Street from my home by the old millpond between Pembroke and Duxbury, R. viscosum blooms in mid-June with its small white flowers emitting a pleasantly sweet, spicy fragrance. These small white flowers by themselves are not showy, but the shrub in full bloom makes a great impact in the garden where more white is always welcome.

Ian Donovan, Pembroke, MA


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