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Species In Our Midst
Rhododendron maximum

by John Perkins

Rhododendron maximum
Picture by S & J Perkins
Rhododendron maximum in Salem, NH

Rhododendron maximum

Rhododendron maximum, The Rosebay Rhododendron, is the only elepidote (having no scales) species native to New England. It grows in woods, often in dense shade, from sea level to 6000 feet in eastern North America from Georgia to Quebec and Nova Scotia. Growing along streams and on moist, rocky wooded slopes, R. maximum tolerates the low pH associated with damp, shady almost bog-like conditions. Rhododendron State Park in USDA Zone 4, Fitzwilliam, New Hampshire contains a large stand of this species.

In the central and southern regions, R. maximum and R. catawbiense, both members of the Section Ponticum, Subsection Pontica, form dense thickets which in pioneer days caused travel across the Appalachian and Allegheny mountains to be very difficult.

The epithet 'maximum' refers to the large size of a mature specimen which although normally 15 feet, may become tree-like reaching 40 feet. The plant habit is spreading, broader than tall. The handsome leaves are large, 4 to 12 inches long, glossy dark green, and thinly indumented. The branchlets, petioles, and flower parts are glandular. These glands are quite apparent on the new growth causing it to feel sticky to the touch.

The inflorescence (truss) of 15-30 funnel-shaped flowers is presented in a upright candelabroid umbel. The 5-lobed flower is 1 inch long, white to pale pink, occasionally deep pink to rose red in color with yellow-green spotting. The calyx, although small, is much larger in R. maximum than in R. catawbiense or other Pontica allies. Bloom time is late-season with peak being mid-July in northern locations. Hardiness is rated at -15° to -20° F (-25° to -29° C).

Despite being native to United States few named cultivars are available. R. maximum var. leachii is compact having smaller leaves with waved margins. 'Whirligig', a form of this species with twisted leaves, is offered by RareFind Nursery. 'Shortoff' is an almost dwarf, compact form (4 feet by 7 feet in 20 years) which I highly recommend and is offered by Broken Arrow Nursery. 'Fowler' is another compact form collected in North Carolina. More dwarf forms of this species are rumored by southern rhododendron collectors but as of yet I have never seen any nursery offering such a plant for sale. An ivory form has been used in breeding for hardy yellows.

The "Red" R. maximum, discovered in the Mount Mitchell region of North Carolina, is among the more legendary rhododendrons of North America. Numerous articles have been written on this plant. Biologist are intrigued by its chimera nature and horticulturists long for a good red-flowered large-leafed hardy parent. However, the unstable nature of this plant makes the flower color very changeable not only from location to location and year to year but differing from branch to branch and flower to flower on the same plant.

'Mt. Mitchell' (Gable), 'Mount Mitchell' (Leach), 'Ruddy Red Max' and 'Red Maximum' (Skinner) are named cultivars which most certainly have the "Red" R. maximum in their background. Polly Hill Arboretum offers New Englanders the opportunity to see two different forms of the red maximum.

Being difficult to propagate by cuttings, this species is seldom available from the Chapter's Van Veen order or the Plants-4-Members sale. However, R. maximum is one of the easier species to grow from seed and clones can be grafted. Mature plants multiply naturally by layering and can be coaxed into layering easily with a "well dropped brick". The dense thickets of R. maximum in the wild is a result of this amazing propagation trait.

Natural hybrids between R. maximum and R. catawbiense occur in the wild where their ranges overlap. The well known 'Russell Harmon' may be an example of such an occurrence. Moreover, R. maximum has been successfully crossed with all of the other members of the subsection Pontica and many members of the section Ponticum.

Although many wonderful hybrids have been made using R. maximum, few have become commercially viable due to any of the following reasons: the difficulty of cutting propagation, the intolerance of full sun, failure to bloom at a young age, and late bloom after peak "garden center" sales. 'Albert Close', 'Harold Amateis ', ' Independence Day ', 'Judy Spillane', 'Lady Eleanor Cathcart', ' Maxecat ', ' Midsummer ', 'Mountain Marriage', ' Paul Vossberg ', 'Red River', 'Summer Rose', 'Summer Snow', and 'Summer Summit' can be found in nurseries specializing in rhododendrons.

The confusing names 'Maximum Roseum' and 'Maximum Album' are both most likely R. ponticum hybrids that probably do not contain any R. maximum.

Many hybridizers from Pennsylvania including Gable, Pride, Delp, Fetterhoff, and Blough used R. maximum in their breeding program. Paul James who is now evaluating a large collection of Delp hybrids considers the late blooming maximum hybrids to be among Delp's best. Rhododendron Hybrids by Salley and Greer is an excellent source for learning about the named plants that resulted from these breeding programs. Plants-4-Members often offers unnamed and lesser known maximum hybrids for sale.

Those looking for a compact large flowered rhododendron for full sun or wind swept areas will be quite disappointed by R. maximum. However, although its flowers are small and lack fragrance, R. maximum's extreme hardiness, late bloom time, dark green foliage, and ability to flower in dense shade makes it a handsome addition to the background of a large New England woodland shade garden.

John Perkins, Salem, NH

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