Rhododendrons are inextricably linked to the history of our landscape. Horticulturalists had endeavored to hybridize these colorful flowering shrubs for northern and western gardens for well-over a hundred years, and they have proved worth the effort because of their evergreen variety and beauty in the landscape. Starting in the mid-1880s, Arboretum staff methodically evaluated these cultivars and published information on their hardiness. In 1917, E. H. Wilson selected the best of these trial plants and affectionately named them ‘ironclads’—these hybrids can be enjoyed in the landscape today as peak blooming times have arrived (May/June).
Rhododendron Dell, which extends along both sides of Bussey Brook at the base of Hemlock Hill, showcases historical and modern cultivars from British and Northeast American hybridizers. This landscape, which explodes with color during the early summer, is most easily accessible at South Street Gate. Now is the time to visit!
For this blog post, we interviewed Sue Pfeiffer, Arboretum Horticulturist who cares for our Rhododendrons year-round.
When did you start working with the Arboretum rhododendrons?
I started managing the rhododendron collection in 2012. At that time, the plants were doing well, and had reached considerable size. They were phenomenal examples of how well rhododendrons grow in the northeast, and how profusely they flower. The plants had grown together, flowering solely on the upper canopy that had established. Due to their size, however, the profusion of blooms were on the upper branches, making it difficult to fully appreciate the variety, size, and color of the show they displayed in June. From a horticultural collections perspective, pruning began to promote new growth from the base that would allow blooming on these lower branches for visitors to enjoy.
What horticultural challenges do you face when caring for Rhododendrons?
The initial challenge of reducing the height of these established plants was not knowing how the plants would respond to pruning as many had never been pruned, or if they had, it had been a very long time. Careful selection and a 3-4 year pruning plan was established to ensure that new growth would push out from the base.
Another major challenge with the collection lies with preservation of those rhododendron planted along the brook. With increased water flow due to upstream paving, erosion of the bank is taking its toll on the plants. We are at risk of losing important historical cultivars as they slowly creep and fall over into the brook.
What are some peak times for blooming?
Peak bloom typically ranges from the end of May to mid-June, but with such a rich variety of cultivars and species, you are guaranteed to find rhododendron in bloom anytime during the months of May and June. If you include azaleas, the window of bloom time stretches from mid-March to July!
Rhododendron Dell seems popular, have you heard people talk about it?
“Rhody Dell,” as we like to call it, is a very special place. From a collections viewpoint, it represents a historical narrative of rhododendron breeding and species introduction in New England. Locally, it is a beautiful woodland oasis to wander among the dark leaves and bright flowers of the rhododendron, listen to flow of the brook, and to take a breath among the giant hemlocks.
Where else have you seen rhododendron in other places/countries? Were you impressed?
For me, the most impressive rhododendrons were at the Lost Gardens of Heligan in Cornwall, England. The 400-year old estate was lost for decades before being rediscovered and restored in the 1990s. The ancient rhododendrons on the estate, some being some of the first brought back from the Himalayas in the mid-1800s, feel more like small forests when strolling through the plants. You can walk underneath the rhododendron, as opposed to around the collection. The climate in Cornwall is just slightly better than in New England, which allows the rhododendrons to attain spectacular size.
Do you have a favorite cultivar or hybrid? What ironclads have withstood to present day?
While it is difficult to name a favorite cultivar or hybrid especially since there are thousands of cultivars to choose from, if I had to choose from our collection, it would have to be Rhododendron ‘Duke of York’. This cultivar, whose form is more similar to a small tree in our collection, has its compact growth and deep pink flowers and has proven to be the most reliable bloomer. I have recently been charmed by Rhododendron ‘Cadis’, introduced by Joseph Gable (Stewartstown, PA). ‘Cadis’ starts off with deep pink buds which contrast with its dark green leaves, but opens to a soft pink, fragrant bloom. I am also fond of the original ironclads that were developed and have stood the test of time, namely ‘Album Elegans’, ‘Album Grandiflorum’, ‘Roseum Elegans’, ‘Purpureum Elegans’, and ‘Purpureum Grandiflorum’.
Are there Integrated Pest Management (IPM) concerns for rhododendrons?
The main health concerns in the collection is dieback due to the fungus Botryosphaeria, which causes dieback of branches from late winter to early summer. A second health issue is the presence of another fungal disease spread by leafhoppers called bud blast, which infects and kills the buds prior to opening.
Sargent, Charles Sprague [Rhododendrons.] Bulletin of Popular Information no. 57 (June 5, 1914). [pdf]
Wyman, Donald. “Flower Colors of Hardy Hybrid Rhododendrons.” Arnoldia 9:7-8 (1949). [pdf]
Wyman, Donald. “Seventy-five years of growing rhododendrons in the Arnold Arboretum.” Arnoldia 29:6 (1969). [pdf]
Brooks, Richard. “A Fresh Look at a Traditional Favorite: Rhododendrons.” Arnoldia 60:1 (2000). [pdf]
Madsen, Karen. “In Pursuit of Ironclads.” Arnoldia 60:1 (2000). [pdf]
Port, Kyle. “Rediscovering Rhododendron Dell, Part 1.” Arnoldia 70:4 (2013). [pdf]
Port, Kyle. “Rediscovering Rhododendron Dell, Part 2.” Arnoldia 71:1 (2013). [pdf]