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Species In Our Midst
Rhododendron hirsutum

by Sally Perkins

Rhododendron hirsutum var. 'Flore Pleno'
Picture by S & J Perkins
Rhododendron hirsutum var. 'Flore Pleno' in Salem, NH

Rhododendron hirsutum Linnaeus 1753

Rhododendron hirsutum is a lepidote (with scales) in subgenus Rhododendron, subsection Rhododendron. This is a distinct subsection of species that has been isolated in the central European high mountain ranges.

R. hirsutum is found in the Central and Austrian Alps, the Italian Dolomites, and the mountains of Slovenia growing on or adjacent to limestone at elevations from 1,200 to 6,000 feet (350-1800 m). It grows in woodlands, screes, among dwarf pine and moorland.

R. hirsutum is a compact to broadly upright shrub to 3 feet (1 m). The scaly branchlets usually have bristly hairs or very short hairs. The evergreen leaves are obovate to oblanceolate, 1/3 to 1 inch (0.8 to 2.6 cm) long and are held 2 years. The upper leaf surface is a pale shining green with few scales and the lower surface is laxly scaly with large dark brown scales 2-4 times their diameter apart. The leaf margin is crenulate (small scalloped edge) with long bristly hairs projecting outward.

The inflorescence is terminal with 4-12 flowered clusters that have a short rhachis or flowering stem. The corolla is tubular with 5 spreading hairy-margined lobes, rose-pink, scarlet or crimson in color. There is a rare white form. The 10 stamens are uneven in length and approximately the length of the corolla tube with soft hairs at the base of the filaments. The style is 1 1/2 to 2 1/2 times the length of the ovary. The calyx is persistent on the capsule, 5-lobed, 0.08 to 0.2 inch (2-5 mm) long with long hairs. Bloom time is late season, blooming at the end of June for me in Salem, New Hampshire.

The species is historically significant for being the first cultivated rhododendron of record in the western world, in a 1656 plant list of botanist John Tradescant's garden. Linnaeus described it in 1753 with the species epithet meaning hairy. It, like R. ferrugineum, is commonly called the "Alpine Rose" emphasizing the need for scientific name usage.

R. hirsutum receives attention for its ability to live and thrive on limestone but it can grow in acid conditions in cultivation. Experts consider it a good rock garden plant (Davidian), a slow but easy garden plant (Leach) or somewhat difficult plant to grow (Cox). I have grown it and feel it is the latter and at best the first. In my limited experience I think it is difficult to transplant once established. I have a few "seedlings" 4 years old and about 3 inches (7 cm) tall.

When I first saw R. hirsutum growing on Monte Baldo in Italy although not being in bloom, I was struck by its strikingly bristly hairy leaves and compact habit. It tucked itself among exposed limestone outcroppings while the more robust R. ferrugineum, which was in bloom on the same mountain, was found in the moist swales. Had I known what to look for, I might have found the natural hybrid between RR. ferrugineum and hirsutum 'Intermedium', which has, as its name implies, intermediate traits between the two species.

R. hirsutum is rated at -15°F (-26°C) by Greer with late to very late season bloom and 1 foot (30 cm) in height in 10 years. R. hirsutum 'Flore Pleno' is a double-flowered form of garden origin (? hybrid) which performs well in my garden, giving an extended bloom but without the bristly hairiness. The form lanciniatum has deeply incised leaves that sound interesting but I have never seen it.

The hybrid 'Myrtifolium', (minus x hirsutum), is hardy to -15°F (-26°C) with mid-June bloom of rose tubular flowers but its lovely new foliage hides them. Otherwise the species is little used in commercial hybridizing.

For the new student of rhododendron species, interested in the species-look with easier success, I would recommend the hybrid 'Myrtifolium' or R. hirsutum 'Flore Pleno' but if you want to get the distinct bristly hairs you will have to rise to the challenge and try the species in a well drained location maybe with a piece of chalk tucked into the soil.

Sally Perkins, Salem, NH.

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