Rhododendron irroratum Franch.

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

Recommended citation
'Rhododendron irroratum' from the website Trees and Shrubs Online (treesandshrubsonline.org/articles/rhododendron/rhododendron-irroratum/). Accessed 2020-01-17.

Genus

Infraspecifics

Other species 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.
glabrous
Lacking hairs smooth. glabrescent Becoming hairless.
glandular
Bearing glands.
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.

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Credits

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

Recommended citation
'Rhododendron irroratum' from the website Trees and Shrubs Online (treesandshrubsonline.org/articles/rhododendron/rhododendron-irroratum/). Accessed 2020-01-17.

Shrub or small tree, 1.5-9 m. Leaves coriaceous, 7-14 x 2-3.7 cm, oblanceolate to elliptic, apex acuminate, lower surface glabrous though with persistent red punctate hair bases overlying the veins. Flowers 12-17, in a dense truss, white or cream to deep pink (in cultivation), with at least a few greenish or more commonly purple flecks, campanulate or tubular-campanulate, with nectar pouches, 35-50 mm; ovary and style stalked-glandular. Flowering March-May. Royal Horticultural Society (1997)

Distribution  China SW Malaysia

RHS Hardiness Rating H5

Conservation status Least concern (LC)

An evergreen shrub of rather stiff habit up to 12 ft high; occasionally a tree twice as high in the wild; young shoots minutely downy and glandular. Leaves of firm and rather leathery texture, narrowly elliptical, sharply pointed, tapered at the base; 2 to 512 in. long, 1 to 134 in. wide; ultimately glabrous on both surfaces; veins in thirteen to sixteen pairs; stalk 13 to 35 in. long. Flowers in a terminal hemispherical truss, 4 in. wide, of about fifteen blooms opening in April. Calyx with five shallow triangular teeth, covered like the flower-stalk with minute glands; the latter is 12 to 1 in. long. Corolla bell-shaped, with the five rounded lobes notched and wavy at the margin; 112 to 2 in. long and wide; the colouring varies, and although the ground is white, sometimes pure, sometimes yellowish, sometimes suffused with rose, it is always or nearly always more or less spotted, sometimes conspicuously so. Stamens ten, white or yellowish, about 1 in. long, minutely downy towards the base; anthers brown. Ovary densely glandular, as is also the style the whole of its length. Bot. Mag., t. 7361. (s. and ss. Irroratum)

Native of N.W. Yunnan at altitudes of 8,000 to 10,000 ft. It was discovered by Père Delavay near Lankiung in 1886 and introduced by him to France, thence to Kew, where it first flowered in 1893. However, the species is now mainly represented in cultivation by plants from the seeds collected by Forrest and by Dr Rock in the region from the Tali range northward to the Chungtien plateau and eastward beyond the Yangtse to Yungpeh. It seems to be a common species on the Sungkwei pass N.E. of Lankiung, in scrub and thickets.

R. irroratum is variable in the colour of its flowers. The form with white or blush-coloured flowers is represented in gardens by the lovely Rock 59220 and also by his 59212, 59620, and 59581, all represented in the Species Collection, Windsor Great Park. A form with pale creamy-yellow flowers was introduced by Forrest from the Sungkwei pass under his number F.5851. According to his field notes, F.21323 from south-west of Lichiang had orange-yellow flowers. Most remarkable are the forms with heavily spotted flowers, of which one was growing at Kew in 1911 and is figured in the GardenersChronicle for that year (Vol. 49, facing page 350). This was presumably from the Delavay introduction. A similar form received an Award of Merit in 1957 when shown from Exbury on March 19 under the clonal name ‘Polka Dot’, but this plant has a very large number of flowers per truss for R. irroratum and could be a hybrid between it and R. arboreum. The normal white-flowered R. irroratum received an Award of Merit on the same occasion, exhibited from Minterne by Lord Digby.


subsp. irroratum

Synonyms
R. ninguenense Hand.-Mazz.

Ovary and calyx stalked-glandular, not tomentose.

Distribution China (W & C Yunnan, SW Sichuan, Guizhou).

Habitat 2,500-3,350m.

Awards AM 1957 (Col Lord Digby, Minterne); flowers white, faintly tinged pink. AM 1957 (E. de Rothschild, Exbury) to a clone ‘Polka Dot’; flowers white, heavily spotted deep purple, suffused pink.

R. ningyuenense is said to differ from subsp. irroratum in its more hairy leaf stalks and in the more open, unspotted corollas. Plants under that name have been introduced into cultivation recently; this should allow its status to be checked.

subsp. pogonostylum (Balf.f. & W.W.Sm.) D.F.Chamb.

Synonyms
R. pogonostylum Balf.f & W.W.Sm.

Ovary and calyx tomentose and glandular.

Distribution China (Yunnan, SW Sichuan).

Habitat 2,100-3,000m.

There is a complete range of variation from the white to cream-flowered forms with strong flecks and exclusively glandular ovaries and calyces that occur in the north of the range of the species to forms with pink flowers, with few flecks and ovaries that are glandular and tomentose, that occur further south. Some populations contain both forms.

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