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Rhododendron thomsonii Hook. f.

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Credits

Article from Bean's Trees and Shrubs Hardy in the British Isles

Recommended citation
'Rhododendron thomsonii' from the website Trees and Shrubs Online (treesandshrubsonline.org/articles/rhododendron/rhododendron-thomsonii/). Accessed 2024-12-04.

Genus

Other taxa in genus

Glossary

calyx
(pl. calyces) Outer whorl of the perianth. Composed of several sepals.
corolla
The inner whorl of the perianth. Composed of free or united petals often showy.
ovary
Lowest part of the carpel containing the ovules; later developing into the fruit.
apex
(pl. apices) Tip. apical At the apex.
glabrous
Lacking hairs smooth. glabrescent Becoming hairless.
glaucous
Grey-blue often from superficial layer of wax (bloom).
hybrid
Plant originating from the cross-fertilisation of genetically distinct individuals (e.g. two species or two subspecies).
style
Generally an elongated structure arising from the ovary bearing the stigma at its tip.
subspecies
(subsp.) Taxonomic rank for a group of organisms showing the principal characters of a species but with significant definable morphological differentiation. A subspecies occurs in populations that can occupy a distinct geographical range or habitat.

Credits

Article from Bean's Trees and Shrubs Hardy in the British Isles

Recommended citation
'Rhododendron thomsonii' from the website Trees and Shrubs Online (treesandshrubsonline.org/articles/rhododendron/rhododendron-thomsonii/). Accessed 2024-12-04.

Rhododendron thomsonii

Rhododendron thomsonii CHM 3057, collected in Bhutan, has attractively dark red flowers. Ray Wood, Castle Howard, UK. April 2020. Image John Grimshaw.

Shrub or small tree, 0.6–3.5(–6) m; bark smooth, reddish, peeling; young shoots glabrous or sparsely glandular. Leaves 3–7.5(–11) × 2–5.5(–7.5) cm, orbicular to obovate or elliptic, base rounded to cordate, entirely glabrous (occasionally with a few hairs below), lower epidermis, strongly glaucous-papillate, with some red-stalked glands; petioles glabrous or sparsely glandular. Flowers 3–10, in a lax truss; calyx 2–20 mm, irregular to cupular, often coloured; corolla fleshy, deep crimson, campanulate, with nectar pouches, 35–50 mm; ovary glabrous or glandular, style glabrous. Flowering April-May. Royal Horticultural Society (1997)

Distribution  Bhutan China S Tibet India Sikkim, Arunachal Pradesh

RHS Hardiness Rating H5

Conservation status Least concern (LC)

Taxonomic note R. thomsonii hybridizes in the wild with R. campylocarpum (see under R. × candelabrum). Royal Horticultural Society (1997)

An evergreen, glabrous shrub up to 14 ft high and more in diameter in the Cornish gardens. Leaves roundish oval, 2 to 4 in. long, two-thirds as wide, round at the apex except for a short, abrupt tip, and rounded or slightly heart-shaped at the base, dark green above, blue-white or glaucous green below; stalk about 3⁄4 in. long. Flowers borne in March or April, six to ten in a loose terminal cluster; pedicels up to 1 in. long. Calyx 1⁄2 to 3⁄4 in. deep, rarely shorter, cup-shaped, often tinged with red. Corolla bell-shaped, fleshy, rich blood-red, 2 to 3 in. across, five-lobed. Stamens ten, glabrous. Ovary and style glabrous. Bot. Mag., t. 4997. (s. and ss. Thomsonii)

Native of the Himalaya as far west as Nepal; introduced by J. D. Hooker in 1850 from Sikkim, where it grows at 10,000 to 13,000 ft. It first flowered in 1857 with Messrs Methven of Edinburgh, who had taken a scion from one of their seedlings and grafted it on R. ponticum.

R. thomsonii is hardy at Kew, but needs a sheltered position, and even then its flowers and young growths are very liable to injury by late frost. In more favoured gardens it is magnificent, and really deserves a place even in a garden with the climate of Kew, since it makes a handsome specimen and the flowers are so splendid that a display one year in three is ample return for the room it occupies. It received an Award of Garden Merit in 1925.

Crossed with other species, R. thomsonii has given rise to some of the finest and best known hybrid grexes, notably Luscombei (with R. fortunei), Shilsonii (with R. barbatum), and Cornish Cross (with R. griffithianum), ‘Ascot Brilliant’ is an old hybrid between R. thomsonii and a hardy hybrid, raised by Standish over a century ago; more recent commercial hybrids with R. thomsonii as one parent are ‘Sir John Ramsden’ and ‘J. G. Millais’.

From the Supplement (Vol. V)

subsp. lopsangianum (Cowan) Chamberlain R. lopsangianum Cowan – The distinctive characters of this subspecies are given on pages 786–7, under R. lopsangianum.

The two varieties of R. thomsonii mentioned are considered to be natural hybrids between R. thomsonii and R. campylocarpum – R. × candelabrum Hook.f. – a cross occurring occasionally where the two species are in contact.

Rhododendron thomsonii

Rhododendron thomsonii collected by Joseph Hooker in Sikkim, c. 1850, as grown at the Yorkshire Arboretum in descent from an original plant at the Sunningdale Nurseries (March 2019). Image John Grimshaw.

Rhododendron thomsonii

The slightly waxy external surface of the flowers of Rhododendron thomsonii CHM 3057 (Ray Wood, Castle Howard, April 2022). Image John Grimshaw.

Rhododendron thomsonii

Rhododendron thomsonii subsp. thomsonii. Dawyck Botanic Garden, UK, 07 May 2008. Image John Anderson.

R lopsangianum Cowan

Allied to R. thomsonii, but not so tall-growing, and with smaller flowers and leaves. The leaves are markedly papillose beneath and the calyx is small, not forming a distinct cup. Described in 1937 from specimens collected by Ludlow and Sherriff in the Tibetan Himalaya and introduced by them. It is named after the then Dalai Lama of Tibet.

R viscidifolium Davidian

In describing this species in 1966, Davidian remarks that it agrees with R. thomsonii var. pallidum in some respects, differing in the copper-red flowers, the small calyx, and the tomentose or glandular ovary. The leaves are glandular and sticky beneath, whence the specific epithet. It was discovered by Ludlow, Sherriff, and Taylor in the Tibetan Himalaya on the Lo La and was introduced by them in 1938. It is in cultivation at Glenarn in Dunbartonshire.

subsp. lopsangianum (Cowan) D.F.Chamb.

Synonyms
R. lopsangianum Cowan

Leaves 3–4.5cm long; calyx 24mm; shrubs 0.6–1.8m.

Distribution China (S Tibet).

Habitat 2,500–4,300m.

Subsp. lopsangianum is in some respects intermediate between subsp. thomsonii and R. sherriffii; some plants in cultivation have a few scattered hairs on the lower leaf surface.

Rhododendron thomsonii subsp. lopsangianum

Rhododendron thomsonii subsp. lopsangianum LS&T 6561. Valley Gardens, Windsor Great Park, UK. 01 April 2017. Image John Anderson.

Rhododendron thomsonii subsp. lopsangianum

Rhododendron thomsonii subsp. lopsangianum LS&T 6561. Valley Gardens, Windsor Great Park, UK. 01 April 2017. Image John Anderson.

subsp. thomsonii

Leaves 5–11cm long, calyx (6–)10–18, shrubs 1.3–6m.

Distribution Nepal, N India (Sikkim, Arunachal Pradesh), Bhutan.

Habitat 3,000–4,000m.

Awards AM 1973 (Crown Estate Commissioners, Windsor); flowers red in throat, darkening at rim.

var. candelabrum (Hook, f.) C. B. CI.

Synonyms
R. candelabrum Hook, f

Flowers paler than in the typical variety. Calyx smaller. Ovary glandular. Described from Sikkim.

var. pallidum Cowan

Flowers rose-pink. Calyx large. Ovary glandular. Described from specimens collected by Ludlow and Sherriff in the Tibetan Himalaya.

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