Spring Brings Rhododendron Blooms

Visit the Arboretum and Botanical Garden to enjoy the beauty and diversity of seasonal flowers.

By Connor Ryan, Rhododendron Collections Manager

In the Research Department at Holden Forests & Gardens we love rhododendrons. The diversity in flower, leaves and form. The vast number of species (1000+!). Their ability to quite freely hybridize. Their fascinating evolutionary history and resulting physiological traits and ecological niches. Their human history — from attractive garden plants to life-saving medicines and everything in between.

The history and diversity of the genus (which includes both rhododendrons and azaleas) provide countless avenues for ornamental display, scientific research and other scholarship, and plant conservation. In anticipation of the spring season, I want to share some of the most notable and charismatic members of our collection. With about 750 unique types of rhododendrons in the collection, there is plenty to see on the arboretum grounds.

While it is possible to see a rhododendron in bloom at HF&G from late March into September, spring is the time to see the collection at its peak. The season starts with the lepidote, scaly-leaved rhododendron species and hybrids. First up are the closely related species in the Rhododendron dauricum complex, which dot the rhododendron gardens with pink to purple flowers as early as late

March. From this complex is Rhododendron mucronulatum ‘Cornell Pink,’ which, though often mistakenly called an azalea, is one of the few deciduous rhododendrons. Its floating, pink flowers on bare stems signal the start of rhododendron season. Soon to follow are the ‘PJM’ rhododendrons, which my wife and I jokingly call “Purple J. Meatball,” a nod to the round, meaty shape most landscapers shear them into. The PJMs are derived from a simple but fortuitous cross between the northern Asian Rhododendron dauricum and the Carolinian Rhododendron carolinianum. These and other early bloomers can be found in both arboretum rhododendron gardens, plus the Cleveland Botanical Garden and David G. Leach Research Station.

Another favorite early bloomer in our gardens is the royal azalea (Rhododendron schlippenbachii). Royal azalea is perhaps the finest of all the deciduous azaleas, with large, lightly fragrant pink flowers on naked stems in late April to early May. Many of our royal azaleas stem from plant collecting trips HF&G staff took to Korea in the 1980s. A similar but later bloomer, the pinkshell azalea (Rhododendron vaseyi) is also a favorite. In the wild it is found only in the mountains of Western North Carolina. Compared to the royal azalea it shares flower color and exceptional fall color, but its leaves and flowers are smaller. Horticulturist MaryAnn Thesing recently planted this species originally collected on Whiteside Mountain in North Carolina along a ravine in the Helen S. Layer Rhododendron Garden. You might also catch the white-flowered form (‘White Find’) established on the western edge of the Helen S. Layer Garden or David Leach’s dark pink selection (‘Spring Spangle’) in the Leach Station gardens.

As rhododendron season jogs along, we pick up much of the rest of the pack in May. It is a slow build through April, but by mid-May many of the evergreen and deciduous azaleas have hit their stride. Deciduous Ghent azaleas Warren Corning sourced from Holland come ablaze just west of Logsdon Pond at the arboretum. These are among the oldest rhododendrons in our collection, dating to the 1950s. Drifts of evergreen azaleas can be seen through the Eliot and Linda Paine Rhododendron Discovery Garden and the Layer Rhododendron Garden. Many of these were developed locally, either by Anthony Shammarello in South Euclid or Girard Nurseries in Geneva. My current favorites from this group are ‘Girard’s Unsurpassable’ and ‘Elsie Lee.’ At the botanical garden, the Japanese garden could just as well be called the Azalea Garden, where the evergreen azaleas predominate among other broadleaf evergreens and conifers. There is also a substantial planting of evergreen azaleas and large leaf rhododendrons in the Waterfall Garden.

On the south end of the Rhododendron Discovery Garden, the Florida flame azalea (Rhododendron austrinum) will explode into orangey, fragrant, honeysuckle-esque flowers in mid to late May. We were pleasantly surprised to find this Gulf Coast native species in full bloom in 2015 following temperatures below -20°F that winter. This is further evidence of southeastern US azalea species having migrated to glacial refugia in past ice ages. These species may have slowly moved to warmer places but retained cold hardiness traits from their ancestors. Consequently, most if not all the southern azaleas grow well at HF&G.

Just as the azaleas get their flower on, many of the large leaf rhododendrons join the party. The arboretum and Leach Station gardens showcase an immense collection of large leaf rhododendrons visitors seldom see in home landscapes. Shade of pink, purple, red, white and even orange and yellow abound — products of several decades of hybridizing both in Northeast Ohio and further afield. Our pride and joy ‘ Maud Corning’ will perfume the Layer Rhododendron Garden with its sweet, spicy scent often hitting you before you see the plant itself. At the botanical garden you can see several of the standard purple and pink-flowered rhododendrons commonly available at local nurseries. These are the triedand-true performers for our area collectively called Ironclads. Perhaps the largest rhododendron at the arboretum is also one of those Ironclads, ‘ Catawbiense Boursault.’ You can find it in the large bed to the north of Beech Knoll.

By the end of May, you will likely have seen mesh bags or hanging flags attached to various azaleas and rhododendrons. This is a sign of science at work — internal and external researchers regularly use the Rhododendron collection to further our knowledge about plant biology and horticulture. Just as spring is a hectic time for the horticulture staff, researchers are running mad through the gardens to ensure they visit their research plants in bloom.

While many of the azaleas and rhododendrons will carry into June, this month is when the rosebay rhododendron (Rhododendron maximum) and its hybrids take command. Rosebay rhododendron is Ohio’s only native large leaf rhododendron, occurring most notably at Rhododendron Cove State Nature Preserve southeast of Columbus. It blooms late for a large leaf rhododendron, and that trait is passed on to its hybrids. At the Leach Station and on Beech Knoll, you might spot the curious “Red Max,” a naturally occurring red-flowered variation on Rhododendron maximum known originally from a few plants on Mt. Mitchell in North Carolina. At the botanical garden, Rhododendron maximum and its hybrids are also planted