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Species In Our Midst
Rhododendron tomentosum

by Joe Bruso

Rhododendron tomentosum 'Milky Way'
Picture by RSF
Rhododendron tomentosum 'Milky Way' in Federal Way, WA

Rhododendron tomentosum

R. tomentosum is one of 5 species in subgenus Rhododendron (lepidotes or scaly-leaved rhododendrons), section Rhododendron, subsection Ledum. Recent placement by taxonomists within genus Rhododendron represents a big demotion for a plant which, along with its 4 related species (groenlandicum, hypoleucum, tolmachevii and neoglandulosum) formerly comprised its own genus (Ledum). Because of this recent re-classification, older Rhododendron references do not describe it. Greer's Guidebook to Available Rhododendrons doesn't mention it. Cox's The Encyclopedia of Rhododendron Species gives a succinct description of R. tomentosum and its relatives. Better coverage can be found in some horticultural references under its previous name, Ledum palustre. R. tomentosum's closest relative outside subsection Ledum is thought to be R. micranthum. Our geographically closest representative in this subsection is R. groenlandicum, which is very common at higher elevations in New Hampshire's White Mountains.

R. tomentosum forms a bushy, erect to spreading evergreen, usually rounded shrub, up to 3 feet high and wide. Commonly called "Marsh Tea" or "Wild Rosemary" it grows well in peat bogs, preferring moist soil in full shade. It is hardy to Zone 2, ranging from northern North America to northern Europe and Asia.

Two subspecies have been described, based primarily on growth habit and range:

  • ssp. tomentosum is upright and bushy.
  • ssp. subarcticum is prostrate or decumbent, with a more northerly range than ssp. tomentosum.

Five petaled Flowers, 1/2 inch across, in white, cream or rose appear from spring until early summer. They are held in rounded, terminal "corymbs", 2 inches across.

The leaves are narrowly oblong to linear, 1/2" to 2" long, dark green above, with rufous-lanate indumentum (rusty colored hair) beneath, and recurved margins. New shoots are a lso rusty-haired. The leaves are aromatic and leathery and are reported to have narcotic qualities. They have been used in Norway and Germany to make beer more intoxicating.

Relatively few hybrids have been created with R. tomentosum, and its close relatives. It appears to be difficult to cross Ledum species with rhododendrons outside their subsection. Some of the reported crosses have had their authenticity questioned. For example: 'Arctic Tern' is possibly a cross between a "Ledum" species and another rhododendron. 'Brilliant' is a purported cross between R. tomentosum and 'Elizabeth', an elepidote hybrid. Some question whether R. tomentosum was involved in this cross.

Natural hybrids between subsection Ledum and R. lapponicum have been reported. Controlled crosses have been made between R. tomentosum and R. lapponicum, R. neoglandulosum, and R. hippophaeoides. In all cases, R. tomentosum was the seed parent, the reciprocal cross failing.

The Rhododendron Species Foundation sells a selection of R. tomentosum called 'Milky Way', which is often illustrated in an advertisement in the Journal of the American Rhododendron Society.

Joe Bruso, Hopkinton, MA

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