Shell pink. Apple blossom. Cotton candy. Pale rose-pink. However you describe the colors of the large, open-faced, white to pink flowers of royal azalea [pdf] (Rhododendron schlippenbachii), now is the time to see them in glorious bloom at the Arboretum. We have several accessions of this large deciduous shrub, including impressive mass plantings on Bussey Hill (465-70*MASS-A) and along Valley Road north of Beech Path (1861-80-MASS).
Royal azalea is native to Korea [pdf] and parts of northeastern China, the Russian Far East, and northern Japan. It’s winter hardy to USDA zone 5 (average annual minimum temperatures -10 to -20°F [-23.4 to -28.8°C]) and, as with most azaleas, prefers acidic, moist yet well drained, humus-rich soils. In its native haunts royal azalea grows in open deciduous forests, and in cultivation it performs well in part-day sun or dappled shade. This multi-season ornamental is also noted for its outstanding red-orange-yellow fall foliage color (p.22) [pdf], which develops best with at least some direct sun exposure.
Rhododendron is the genus name for plants commonly known as azaleas and rhododendrons. Azaleas were once placed in a separate genus (Azalea, conveniently), but have now long been lumped into Rhododendron. While we gardeners tend to be fond of dividing and defining plants by their common name, Rhododendron is a great example of why we shouldn’t. There are a few “rules” for dividing azaleas from rhododendrons, the most prominent of which are that rhododendrons are usually evergreen and their flowers have 10 stamens, while azaleas are usually deciduous and have 5 stamens. But there are plenty of exceptions within Rhododendron species, including Korean rhododendron (Rhododendron mucronulatum), which is deciduous, and this post’s feature, royal azalea, which is deciduous but its flowers have 10 stamens.
No matter the common names, please enjoy the spring and early summer floral display of the Arboretum’s many Rhododendron accessions. For more on our broadleaf evergreen rhododendrons, see these articles on Rhododendron Dell: part 1 [pdf], part 2 [pdf].