Two of the stalwart shrubs used in Pittsburgh landscapes are rhododendron and azalea. Both in the Rhododendron genus, they are prized for their evergreen foliage, which in mid-spring provides a backdrop for their beautiful flowers. However, the beautiful colors and forms of their blossoms seem to end too quickly. The good news is that there are azaleas that extend the bloom period by flowering from late spring to early summer.
These later-blooming shrubs consist of a small group that are either Pennsylvania natives or re-blooming azaleas, and together they have the potential to extend the blooming season for three seasons of the year -- spring, summer and fall! The optimal soil pH range for growing rhododendrons is 5.0 to 6.0. Contact a local nursery or Penn State Extension for a soil test kit.
Native azaleas are underutilized in the typical home landscape. Natives add the benefit of attracting pollinators to the garden while contributing fragrance and fall color. To maximize the performance of these plants, they should be grown in acidic, organically rich, moist, well-drained soil and exposed to several hours of sun daily. The following is a selection of Pennsylvania natives and cultivars. All are deciduous and start to bloom in late spring or early summer:
• Sweet azalea (Rhododendron arborescens) -- This is a large, loosely branched shrub that grows 8-12 feet tall with an equal spread. Fragrant white or pink-tinged flowers with red stamens form in the summer. The glossy, bright green leaves change to deep red and purple in the fall.
• Flame azalea (R. calendulaceum) -- grows 4-10 feet tall and wide. In early summer, it has showy funnel-shaped flowers in yellow, orange or red. The medium green foliage changes to yellow and red before the leaves drop. 'Wahsega' is a cultivar with deep red flowers.
• Swamp azalea (R. viscosum) -- has a loose, open habit, reaching 5-6 feet tall and wide. In midsummer, clusters of white clove-scented flowers appear. Fall color is orange to maroon. This species tolerates wet soils. Three cultivars are worth seeking out:
'Betty Cummins' has deep pink flowers. 'Pink Mist' has pink-tinged white flowers. 'Delaware Blue' has lightly scented flowers with a pink shell, white overtones and pink throats. The deep green foliage turns an outstanding yellow in the fall.
These are the result of the quest by plant breeders to develop shrubs with extended blooming periods. These azaleas are evergreen, available in a variety of colors and bloom in the spring, summer and fall. The cultural requirements are the same as for any azalea -- acidic, organically rich, moist, well-drained soil and exposure to several hours of sun daily. The current groups of re-blooming azaleas are known as Encore and Bloom-A-Thon.
• Encore azaleas were introduced by a breeder in Alabama and have been in the marketplace for more than 15 years. They are available in 29 colors and sizes and are recommended for hardiness zones 7 to 10. Although cold-hardiness trials have suggested that some varieties are suitable for USDA plant hardiness zones 6a and 6b, which include most of Western Pennsylvania, local growers say their performance here is still questionable.
• Bloom-A-Thon azaleas were developed by Bob and Lisa Head in Seneca, S.C., and introduced in 2012. They are available in four colors -- red, double pink, lavender and white -- their hardiness rating is zones 6 to 9. No reports were found to determine the hardiness of these azaleas in our area.
Adventurous gardeners may want to experiment with re-blooming azaleas to determine whether they are truly hardy in zones 6a and 6b. I plan to trial several this year, starting in late spring. If our efforts are successful, these beautiful shrubs will add another dimension to local gardens.
Steve Piskor is a Penn State master gardener and Pennsylvania certified horticulturist. Columns by master gardeners sometimes appear in place of the Garden Q&A by Sandy Feather, a Penn State Extension educator.