Rhododendron tomentosum Harmaja

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'Rhododendron tomentosum' from the website Trees and Shrubs Online (treesandshrubsonline.org/articles/rhododendron/rhododendron-tomentosum/). Accessed 2022-12-02.

Genus

Other taxa in genus

Glossary

calyx
(pl. calyces) Outer whorl of the perianth. Composed of several sepals.
crisped
Curled or crumpled.
included
(botanical) Contained within another part or organ.
linear
Strap-shaped.
midrib
midveinCentral and principal vein in a leaf.
revolute
Rolled downwards at margin.
tomentum
Dense layer of soft hairs. tomentose With tomentum.
variety
(var.) Taxonomic rank (varietas) grouping variants of a species with relatively minor differentiation in a few characters but occurring as recognisable populations. Often loosely used for rare minor variants more usefully ranked as forms.

Credits

New article for Trees and Shrubs Online.

Recommended citation
'Rhododendron tomentosum' from the website Trees and Shrubs Online (treesandshrubsonline.org/articles/rhododendron/rhododendron-tomentosum/). Accessed 2022-12-02.

Small, erect or decumbent shrub, 0.3–1.2 m; young shoots ferruginous-lanate, glandular. Leaves 0.6–5 × 0.1–0.5(–1.2) cm, linear to narrowly elliptic-oblong, margin strongly revolute, upper surface dark green, dull, lower surface densely ferrugineous-lanate, epidermis with or without short setulose hairs, sometimes also with reddish glands, scales rimless, golden. Flowers many, in a loose terminal umbellate corymb; calyx minute; corolla white, rotate, 4–8 mm; stamens 7–10; ovary glandular, style glabrous. Flowering June-July. Royal Horticultural Society (1997)

Distribution  Holarctic

Habitat s.l.-2,000 m

RHS Hardiness Rating H6

Conservation status Least concern (LC)

Taxonomic note Formerly in the genus Ledum, now considered to form part of Rhododendron, described by Bean as Ledum palustre. For other species formerly treated as Ledum see Rhododendron columbianum (Bean’s Ledum glandulosum) and Rhododendron groenlandicum (Bean’s Ledum groenlandicum).

Entry amended to relect the transfer of Ledum to Rhododendron, June 2022, Bean text unchanged except for name changes. JMG)

A dwarf evergreen shrub of thin habit, 1 to 4 ft high; young shoots clothed with rust-coloured wool. Leaves linear, 1⁄2 to 11⁄4 in. long, 1⁄12 to 1⁄4 in. wide, the margins much recurved and thus reducing their width, dark dull green above, covered with rust-coloured wool beneath. Flowers white, 1⁄2 in. across, produced during April and May in terminal clusters; calyx minutely toothed; stamens more numerous than in R. groenlandicum, usually seven to eleven; seed-vessel egg-shaped.

R. tomentosum (including its varieties) is distributed throughout the arctic and subarctic regions of the northern hemisphere; in Europe it extends southward into central Germany, Austria, and the Carpathians, and may be truly native in one locality in Scotland; it also ranges southward in the mountains of western N. America and in N.E. Asia. According to Aiton it was introduced to Britain in 1762, but as a garden shrub it is inferior to R. groenlandicum and much less common. The differences between them are pointed out under that species.

f. decumbens (Ait.) O. Fedtsch.

Synonyms
L. palustre var. decumbens Ait.
L. p. subsp. decumbens (Ait.) Hulten
L. decumbens (Ait.) Lodd

A decumbent shrub to 8 in. high with linear revolute leaves less than {1/8} in. wide and usually less than {3/4} in long. It was originally described from a plant said to have been introduced from Hudson Bay but is of fairly wide distribution within the range of the species.

f. dilatatum (Wahlenb.) O. Fedtsch.

Synonyms
L. palustre var. dilatatum Wahlenb

This has broader, oval-oblong leaves but is otherwise like the typical state of the species. It was described from material from Lapland, but similar plants occur here and there throughout the range of the species. Some plants from Japan and other parts of N.E. Asia, and also from western North America, have been referred to var. dilatatum but are better placed in the complex described below.

var. hypoleucum Bean

This well-marked variant was described in the first edition of this work (Vol. 2 (1914), p. 11) from a plant at Kew raised from Japanese seeds received from Prof. Sargent of the Arnold Arboretum on 4 January 1893. It has short, relatively broad leaves, with only the margins revolute, so that nearly all the undersurface is visible, and this is strikingly white from a dense covering of crisped white hairs, except on the yellowish midrib, which bears a few dark or rusty hairs, some of which may occur elsewhere on the surface. If some closely matching Japanese specimens are taken into account, the dimensions of leaf-blade in this variety are {5/8} to 1{5/8} in. long, {1/6} to {1/2} in. wide. In 1916 the famous Russian botanist V. L. Komarov described L. hypoleucum (as a new species) from specimens collected on the coast of the Russian Far East and on Sakhalin. This differs from L. palustre var. hypoleucum Bean only in its larger leaves (up to 2{3/4} in. long, {11/16} in. wide, flde Komarov). Other specimens from N.E. Asia agree in size of leaf with either var. hypoleucum Bean or L. hypoleucum Komar., but have the crisped white tomentum overlaid with a more or less dense covering of rusty wool, such as is characteristic of typical L. palustre. It was Japanese material of this nature that Nakai named L. palustre var. diversipilosum. Finally, specimens from Sakhalin and the Amur region with the rusty wool almost completely hiding the white hairs, or the latter absent, have been described as L. macrophyllum Tolmatchev.Some of the material discussed above has been erroneously included in L. palustre var. dilatatum. The Alaskan plants named L. pacificum by Small have also been referred to this variety but seem better placed under L. groenlandicum.


subsp. subarcticum (Harmaja) G.Wallace

Leaves 0.6-2 x 0.1-0.3cm, lower epidermis with few or no setulose hairs.

Distribution Arctic regions of Europe, America and Russia, also Japan (Hokkaido) and Korea.

Taxonomic note (Ledum minus hort., L. palustre L. var. decumbens Aiton, R. subarcticum Harmaja)


subsp. tomentosum

Leaves 1.2-5 x 0.2-0.5(-1.2)cm, lower epidermis covered with short setulose hairs.

Distribution N & C Europe, Russia (European part, extending to S Siberia).

Habitat s.l.-2,000m.

Taxonomic note (Ledum palustre L.)