Rhododendron maximum, Rosebay Rhododendron – 5 (3)

By Kevin Kavanagh

Joyce Kilmer Memorial Forest, tucked away inside the Nantahala National Forest of North Carolina’s Appalachian Mountains, is one of those North American pilgrimages that all botanists should make at least once in their lives. As a graduate student in forest ecology, I had read about it in the scientific literature, one of the best examples of an old-growth ‘cove’ forest on the continent: massive tulip trees (Liriodendron tulipifera) more than 500 years old, eastern hemlocks (Tsuga canadensis) rivalling their west-coast rainforest cousins in size and age, and, along the streams, dense understorey stands of rosebay rhododendrons (Rhododendron maximum) forming a dark canopy overhead.

To call it a canopy is not embellishment. I set up my tent under – not beside – a tangled grove of rosebay rhododendrons that rose close to seven metres (about 20 feet) high, their large elliptic leaves forming a delicate pattern against the bright yellows of overstorey tulip trees in peak autumn coloration. Along mountain streams, the trails wove in among expansive rhododendron thickets that in places rose more than 10 metres (30 feet) overhead on gnarled stems (trunks!), the largest of which were close to 30 centimetres (one foot) in diameter.

The species range extends from New York State and New England southward to northern Georgia and Alabama, principally in the mountain systems that comprise the Appalachian chain. Although hardy in eastern Canadian gardens, Rhododendron maximum does not penetrate naturally into Canada. While there are historical accounts from Ontario, Quebec, New Brunswick and Nova Scotia, none of them, to my knowledge, have been authenticated.

In its native range, rosebay rhododendron (also known as great-laurel or great rhododendron) thrives along cool, moist streams in the shade of hemlocks and a rich myriad of hardwoods. More rarely, it will mix with other rhododendron species on moist, exposed, high elevation sites where the resultant vegetation community is known as a ‘heath bald’.

The smooth leaves (among the largest of eastern rhododendron species at 20-25 centimetres or eight inches long) are arranged in a whorl at the end of each branch. As a broadleaf evergreen, its leaves remain year round. Among rhododendron species its leaves are the first to droop and curl up tightly when the temperature drops below 0C (32F), a strategy to prevent moisture loss in the leaf tissue. As the temperature rises above freezing again, the leaves unfurl and look striking in the winter garden.

Another attractive feature of this species is its superior cold hardiness. Many authorities rate it hardy to -40C/F and it has easily survived a -30C (-22F) temperature in my Toronto garden with no visible foliar damage. Its preferred sheltered, streamside habitat suggests that high humidity is beneficial to this species so placing it out of windy and exposed locations is important. It is very shade-tolerant and can survive and even flower (lightly) under the canopies of many forest species. It looks its best in partial sun along the edge of a woodland garden, although exposures to strong sun at mid-day in summer and strong late-winter sun are best avoided, especially in northern climates where the ground can freeze for six or more weeks. Exposure to late winter sun will significantly warm the leaves, causing moisture loss, and when the roots are frozen, the plant is robbed of its ability to replenish moisture to its leaves. This explains why rhododendrons in northern gardens are more frequently killed by winter desiccation than the absolute low temperatures they experience. In cold climates, artificial shading in winter from southern exposures can make a big difference in rhododendron survival.

The flowers are subtle ranging from faint pink (almost white) to, more rarely, a deep full pink. The colour is always darker in the buds just prior to opening. While most rhododendrons flower in spring or early summer in advance of stem and leaf development, the flowers of rosebay rhododendrons open in mid- to late July, with the result that they are somewhat hidden among the newly expanded vegetative growth.

Rhododendron maximum is a member of the diverse heath family, Ericaceae, and has similar cultural requirements to others in this group. Acidic soil rich in leafy organic matter (especially decayed oak and pine needles) is best. A layer of leaf litter in winter is important to minimize the number of freeze/thaw cycles that can damage the fine shallow root mass. Rhododendrons hate having the soil worked around their base as this damages the fine rootlets. In the wild, rosebay rhododendrons prefer the slopes adjacent to mountain streams, but they do not appreciate getting soggy wet feet. Good drainage is important. That said, if planted under or among other trees and shrubs (especially maples), R. maximum will need supplemental watering during dry spells to successfully compete with the established and often aggressive root systems of the larger trees. Transplanting in spring is optimal. If the plant is container-grown, loosen up the soil ball slightly and soak it for a couple of hours before planting to ensure that the plant is fully hydrated.

Although at home in a woodland setting, rosebay rhododendron can also grace the shrub-border, adding texture year-round. And, despite its potential to achieve substantial sizes in the wild, it rarely exceeds two to three metres (six to 10 feet) in cultivation. This may make it the ideal choice for the gardener who has been too intimidated to try a broad-leaved evergreen rhododendron, especially in northern growing zones. The challenge is finding R. maximum in nurseries. It has never found favour with landscaping enthusiasts because it lacks the flower profusion of its cousins such as R. catawbiense. R. maximum can most reliably be found in specialty nurseries featuring native plants. Growing this species from seed is practically an art and should be attempted only if you have patience and are willing to research its germination and seedling requirements.

Whether or not you choose to grow rosebay rhododendron, a trip to a natural site to see its midsummer bloom or its evergreen foliage in the depth of winter is highly recommended. It will buoy the spirits of any plant enthusiast.

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Kevin Kavanagh is the Director of the Canadian Biodiversity at Risk Program at World Wildlife Fund Canada. An avid gardener, Kevin has naturalized his backyard in Toronto and is designing a new garden at his cottage in the heart of Carolinian Canada. Rhododendrons and native plants, especially trees and shrubs of the southeastern United States, fill much of the space in both gardens.