Plant of the Week

Rhododendron arborescens range map.
Rhododendron arborescens range map. USDA PLANTS Database.

Smooth azalea.
Smooth azalea (Rhododendrom arborescens). Photo: Thomas G. Barnes @ USDA-NRCS PLANTS Database.

Smooth azalea.
Smooth azalea (Rhododendron arborescens). Photo by Fred Huber.

Smooth azalea (Rhododendron arborescens)

By Fred Huber

Smooth azalea is named for its fragrant flowers that are white to pale pink in color with five red stamens that extend well beyond the petals (known as exerted stamens) are quite showy. In the fall, the foliage turns deep red to purple in color. Smooth azalea can be fast growing and can become tall, reaching 18 feet in height. The flowers bloom as early as April or as late as September.

The smooth azalea grows in the eastern part of the United States, but is chiefly Appalachian. It is a drought sensitive species that can be found along rocky riversides, wooded stream banks, swamps, high elevation forests, and shrub balds. It prefers moist, acidic soil and requires full sun to light shade. The leaves are shiny green on top and waxy below with three to seven flowers. Sweet azalea plants at higher altitudes have smaller leaves and are shorter.

Smooth azalea is most closely related to swamp azalea (R. viscosum), as evidenced by their glabrous floral bud scales and flowers that appear after the leaves have expanded. It can be distinguished by its glabrous branchlets, red style and filaments (which contrast with the white corollas), and distinctive seeds that lack loose, expanded testae. These two species occasionally hybridize; hybrids with Cumberland azalea (R. cumberlandense) also are known.

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Yellow Marsh Marigold (Caltha palustris).
Yellow Marsh Marigold (Caltha palustris)