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Re-Blooming Azaleas

Extended flowering period a big plus!

By: Steve Piskor
Penn State Master Gardener &
PA Certified Horticulturist ©2013


Two of the stalwart shrubs used in Pittsburgh landscapes are rhododendron and azalea. Many 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 azaleas consist of a small group that are either Pennsylvania natives or re-blooming azaleas and together they have the potential of extending the blooming season for three seasons of the year – spring, summer and fall!


Native Azaleas

Native azaleas are underutilized in the typical home landscape. Native plants add the benefit of attracting pollinators to the garden. Many contribute fragrance and fall color to the garden. 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: 

▪ Rhododendron arborescens (sweet azalea) – This azalea 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.

▪ Rhododendron calendulaceum (flame azalea) – Grows 4 –10 feet tall and wide.  In early summer, showy, funnel-shaped flowers, in yellow, orange or red are formed.  The medium-green foliage changes to yellow and red before the leaves drop.

▪ Rhododendron c. ‘Wahsega’ (‘Wahsega’ flame azalea) – This cultivar is similar in size and habit to the species. The flowers of the azalea have a deep red coloration.

▪ Rhododendron viscosum (swamp azalea) – This native azalea has a loose, open habit, reaching 5 – 6 feet tall and wide.  In mid-summer, clusters of white, clove-scented flowers are formed.  Fall color is orange to maroon.  This species tolerates wet soils.

▪ Rhododendron v. ‘Betty Cummins’ (‘Betty Cummins’ swamp azalea) – This cultivar is similar in habit to the species, except with deep pink flowers.

▪ Rhododendron v. ‘Delaware Blue’ (‘Delaware Blue’ swamp azalea) – Another cultivar that grows to 5 feet tall and wide and blooms in summer.  The flowers are lightly scented and have a  pink shell, white overtones and pink throats.  The deep-green foliage turns to an outstanding yellow in the fall.

▪ Rhododendron v. ‘Pink Mist’ (‘Pink Mist’ swamp azalea) – This cultivar is also similar in habit to the species.  The flowers form in summer and are white, with a pink-tinged coloration.


Re-blooming Azaleas

The quest by plant breeders to develop shrubs with extended blooming periods has resulted in the introduction of re-blooming azaleas.  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 – grow in 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 over 15 years. They are available in 29 varieties of colors and sizes.  They are recommended for hardiness zones 7 to 10; however, cold-hardiness trials suggest that some of the azalea varieties are suitable for zones 6b (-5 to 0 degrees F) and 6a (-10 to -5 degrees F).  According to the 2012 USDA Plant Hardiness Zone Map, all of Allegheny County is zone 6a or 6b.  Therefore, the trial-tested 6a and 6b azaleas should grow in our area.  However, initial reports indicate that the performance of the 6a/6b azaleas is questionable for Allegheny County.  This may be due in part to the fact that they are grown in Alabama!

Bloom-A-Thon® azaleas were developed by Bob and Lisa Head in Seneca, South Carolina.  The azaleas were introduced in 2012 and are available in four colors – red, double pink, lavender, white.  Their hardiness rating is zones 6 to 9.  No reports were found to determine the hardiness of these azaleas in our area.

Despite the caveat of the southern heritage of these azaleas, adventurous gardeners may want to experiment with re-blooming azaleas to determine whether they are truly hardy in zones 6a and 6b.  For my part, I plan to trial several of the re-blooming azaleas, starting late spring.  If all of our efforts are successful, these beautiful shrubs will add another dimension to the garden.


Azalea Factoids

Azaleas are in the rhododendron genus.

▪ All azaleas are rhododendrons; not all rhododendrons are azaleas.

▪ Rhododendrons are mainly evergreen; many azaleas are deciduous.

▪ Rhododendrons tend to have bell-shaped flowers; azalea flowers are funnel-shaped.

▪ Rhododendrons have 10 or more stamens per flower; azaleas usually have 5 stamens per flower. [Note:  A stamen is the male part of the flower that juts out from the flower’s center]

▪ 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.


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