Rhododendron (syn Azalea) heath family (Ericaceae)

Rhododendron

The Rhododendron is an evergreen flowering plant that belongs to the heath family. There are more than 1000 species of rhododendron, densely branched it can grow to 5m in height.

also see Trees & ShrubsFlowering Trees & Shrubs

Rhododendron, often called “rhodies”, is a genus of 1,024 species of woody plants in the heath family (Ericaceae), either evergreen or deciduous, and found mainly in Asia, although it is also widespread throughout the Southern Highlands of the Appalachian Mountains of North America. It is the national flower of Nepal. Most species have showy flowers which bloom from late winter through to early summer. Azaleas make up two subgenera of Rhododendron. They are distinguished from “true” rhododendrons by having only five anthers per flower.

Invasive species

Some species e.g. Rhododendron ponticum are invasive in the UK as introduced plants, spreading in woodland areas and replacing the natural understory. R. ponticum is difficult to eradicate, as its roots can make new shoots.

 en.wikipedia.org · Text under CC-BY-SA license

Video

Learn more about growing and maintaining Rhododendrons in your garden. Videos include container growing, suggested varieties and best growing conditions.
Prefers acidic soil with a level of ph 5.5 / 5.7

The following are listed by AD Webster in his Flowering Trees and Shrubs the date is Webster’s date of introduction to Britain listed after the place of origin.

RHODODENDRON.

RHODODENDRON ARBORESCENS (syn Azalea arborescens), from the Carolina Mountains (1818), is a very showy, late-blooming species. The white, fragrant flowers, and noble port, together with its undoubted hardihood, should make this shrub a general favourite with cultivators.

R. CALENDULACEUM (syn Azalea calendulacea), from North America (1806), is another of the deciduous species, having oblong, hairy leaves, and large orange-coloured flowers. It is of robust growth, and in favoured situations reaches a height of 6 feet. When in full flower the slopes of the Southern Alleghany Mountains are rendered highly attractive by reason of the great flame-coloured masses of this splendid plant, and are one of the great sights of the American Continent during the month of June.

R. CALIFORNICUM.—California. A good hardy species with broadly campanulate rosy-purple flowers, spotted with yellow.

R. CAMPANULATUM (syn R. aeruginosum).—Sikkim, 1825. A small-growing species, rarely over 6 feet high, with elliptic leaves that are fawn-coloured on the under sides. The campanulate flowers are large and showy, rose or white and purple spotted, at the base of the three upper lobes. In this country it is fairly hardy, but suffers in very severe weather, unless planted in a sheltered site.

R. CAMPYLOCARPUM.—Sikkim, 1851. This has stood the winter uninjured in so many districts that it may at least be recommended for planting in favoured situations and by the seaside. It is a Sikkim species that was introduced about forty years ago, and is still rather rare. The leaves are about 4 inches long, 2 inches wide, and distinctly undulated on the margins. Flowers bell-shaped, about 2 inches in diameter, and arranged in rather straggling terminal heads. They are sulphur-yellow, without markings, a tint distinct from any other known Indian species.

R. CATAWBIENSE.—Mountains from Virginia to Georgia, 1809. A bushy, free growing species, with broadly oval leaves, and large campanulate flowers, produced in compact, rounded clusters. They vary a good deal in colour, but lilac-purple is the typical shade. This is a very valuable species, and one that has given rise to a large number of beautiful varieties.

R. CHRYSANTHUM is a Siberian species (1796) of very dwarf, compact growth, with linear-lanceolate leaves that are ferruginous on the under side, and beautiful golden-yellow flowers an inch in diameter. It is a desirable but scarce species.

R. COLLETTIANUM is an Afghanistan species, and one that may be reckoned upon as being perfectly hardy. It is of very dwarf habit, and bears an abundance of small white and faintly fragrant flowers. For planting on rockwork it is a valuable species.

R. DAHURICUM.—Dahuria, 1780. A small-growing, scraggy-looking species of about a yard high, with oval-oblong leaves that are rusty-tomentose on the under sides. The flowers, which are produced in February, are purple or violet, in twos or threes, and usually appear before the leaves. It is a sparsely-leaved species, and of greatest value on account of the flowers being produced so early in the season. One of the hardiest species in cultivation. R. dahuricum atro-virens is a beautiful and worthy variety because nearly evergreen.

R. FERRUGINEUM.—Alpine Rose. Europe, 1752. This dwarf species, rarely exceeding a yard in height, occurs in abundance on the Swiss Alps, and generally where few other plants are to be found. It is a neat little compact shrub, with oblong-lanceolate leaves that are rusty-scaly on the under sides, and has terminal clusters of rosy-red flowers.

R. FLAVUM (syn Azalea pontica).—Pontic Azalea. A native of Asia Minor (1793), is probably the commonest of the recognised species, and may frequently, in this country, be seen forming good round bushes of 6 feet in height, with hairy lanceolate leaves, and large yellow flowers, though in this latter it varies considerably, orange, and orange tinged with red, being colours often present. It is of free growth in any good light peaty or sandy soil.

R. HIRSUTUM.—Alpine Rose. South Europe, 1656. Very near R. ferrugincum, but having ciliated leaves, with glands on both sides. R. hallense and R. hirsutiforme are intermediate forms of a natural cross between R. hirsutum and R. ferrugincum. They are handsome, small-growing, brightly flowered plants, and worthy of culture.

R. INDICUM.—Indian Azalea. A native of China (1808), and perfectly hardy in the more favoured portions of southern England, where it looks healthy and happy out of doors, and blooms freely from year to year. This is the evergreen so-called Azalea that is so commonly cultivated in greenhouses, with long hirsute leaves, and large showy flowers. R. indicum amoenum (syn Azalea amoena), as a greenhouse plant is common enough, but except in the South of England and Ireland it is not sufficiently hardy to withstand severe frost. The flowers are, moreover, not very showy, at least when compared with some of the newer forms, being dull magenta, and rather lax of habit.

R. LEDIFOLIUM (syns Azalea ledifolia and A. liliiflora).—Ledum-leaved Azalea. China, 1819. A perfectly hardy species. The flowers are large and white, but somewhat flaunting. It is, however, a desirable species for massing in quantity, beside clumps of the pink and yellow flowered kinds. Though introduced nearly three-quarters of a century ago, this is by no means a common plant in our gardens.

R. MAXIMUM.—American Great Laurel. North America, 1756. This is a very hardy American species, growing in favoured localities from 10 feet to 15 feet high. Leaves oblong-lanceolate, slightly ferruginous beneath. Flowers rose and white, in dense clusters. There are several handsome varieties that vary to a wide extent in the size and colour of flowers. R. maximum album bears white flowers.

R. MOLLE (syn Azalea mollis), from Japan (1867), is a dwarf, deciduous species of neat growth, with flame-coloured flowers. It is very hardy, and a desirable acquisition to any collection of small-growing shrubs.

Cultivars

There are now over 28,000 cultivars of Rhododendron in the International Rhododendron Registry held by the Royal Horticultural Society.

Horticultural divisions

Horticulturally, rhododendrons may be divided into the following groups:-

  • Evergreen rhododendrons: the main default category
  • Vireya (Malesian) rhododendrons: epiphytic tender shrubs
  • Azaleas (section of generally small-sized, small-leaved and small-flowered shrubs):
    • Deciduous hybrid azaleas:
      • Ghent (Gandavense) hybrids – Belgian raised
      • Knap Hill-Exbury hybrids – English raised
      • Mollis hybrids – Dutch & Belgian raised
      • New Zealand Ilam hybrids – derived from Knap Hill/Exbury hybrids
      • Occidentale hybrids – English raised
      • Rustica hybrids – sweet-scented, double-flowered
    • Evergreen hybrid azaleas:
      • Gable hybrids – raised by Joseph B. Gable in Pennsylvania, USA
      • Glenn Dale hybrids – USA raised complex hybrids
      • Indian (Indica) hybrids – mostly of Belgian origin
      • Kaempferi hybrids – Dutch raised
      • Kurume hybrids – Japanese raised
      • Kyushu hybrids – very hardy Japanese azaleas (to -30 °C)
      • Oldhamii hybrids – dwarf hybrids raised at Exbury, England
      • Satsuki hybrids – Japanese raised, originally for bonsai
      • Shammarello hybrids – raised in Northern Ohio, USA
      • Vuyk (Vuykiana) hybrids – raised in the Netherlands
  • Azaleodendrons – semi-evergreen hybrids between deciduous azaleas and rhododendrons

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Post Author: LondonArborist

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