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For gardens where lime is not found in any appreciable quantity, no more useful genus exists than this, and in very few genera are there so many really ornamental species. It belongs to Ericaceae, and is made up of deciduous and evergreen species, the former, in gardens, being more often referred to as Azalea.

The genus is very widely distributed through Europe, Asia, and North America; by far the larger number being found in the Himalayas and western China. The species in cultivation number over 100, and nearly 100 can be grown out of doors in this country in certain parts, while even in the coldest districts many may be cultivated. In habit they differ considerably, for while some grow but a few inches high, others make lordly bushes, or even small trees, 30 to 40 feet in height, with a very wide spread, and sometimes with trunks 1 foot or more through.

The foliage and flowers likewise show a great diversity of character, the leaves of some being as small as those of the Common Box; while those of others are 12 or 15 inches long and 5 or 6 inches wide. With regard to flowers, some, like Rhododendron ferrugineum, are quite small, and borne but a few together, whilst those of other species are several inches across and in large conical heads. In shape, too, the blossoms differ, those of Rhododendron lepidotum being quite flat, while others, such as Rhododendron Thomsoni, have long, tubular blossoms. Others, again, have wide mouthed flowers with very short tubes.

The colour is as diverse as the shape, for while some have blooms of the purest white, others have rich crimson or blood red flowers, and every intermediate shade exists. Yellow, though uncommon, is found in several species, the deepest coloured being the deciduous Rhododendron flavum; while of evergreens the most distinct is the Himalayan, Rhododendron campylocarpum.

In such a large genus it might naturally be expected that a number of hybrids should arise, and such is the case; for, by crossing and intercrossing, several fine races of hybrids have been produced, of which there are hundreds of named varieties. As a rule, the evergreen and deciduous species do not cross very readily, though there are several very useful varieties from such crosses, as is instanced by Rhododendron azaleoides, Rhododendron Gowenianum, Rhododendron Smithii aureum, and others.

Although many of the species are ornamental plants, they are often excluded from the garden for the sake of the hybrids, many of which are easier to grow. The majority of both evergreen and deciduous varieties have arisen through the intercrossing of a few species only. In the former case the parents have been Rhododendron arboreum, Rhododendron Ponticum, Rhododendron Catawbiense, and Rhododendron Caucasicum principally, while the deciduous ones are from Rhododendron flavum, Rhododendron calendulaceum, Rhododendron nudiflorum, and Rhododendron Sinense.

Through so much intercrossing it is now difficult to trace the parents of many of the varieties. In the evergreen set, Rhododendron Catawbiense is considered the best breeder, as it is stronger and hardier than the other species, and hybrids having a good percentage of Catawbiense blood are to be recommended for cold districts. They are usually distinguished by large, rather blunt leaves and great, conical trusses of flowers, which open during the latter half of May and June. In the deciduous section those possessing Rhododendron Sinense blood are not desirable for cold places, as they blossom early, and are less hardy than the others.

The best situations for Rhododendrons are the sides and bottom of a cool, moist, but well drained valley, shaded from the fiercest midday sun, but with plenty of light and air. The soil may be either peat or loam, but should contain no lime; and light soil is preferable to heavy. When planting, care must be taken not to bury the top roots more than 1/4 inch below the surface, for it is essential to success that the roots should be kept as near the surface as possible; in fact, if the top roots can be covered with decayed leaves so much the better, for in them the air circulates freely and they always keep sweet.

When the surface roots of a Rhododendron show plenty of white fibres it may safely be assumed that the plant is in good health. Although the above mentioned position is most suitable for these shrubs, they are also of the greatest service for shrubberies, specimen beds, and isolated specimens in other parts of the garden: The propagation of Rhododendrons is effected by means of seeds, cuttings, grafts, or layers.

For convenience sake the species are divided into several groups, i.e. those suitable for the warmer counties, tall growing evergreens, dwarf evergreens, and deciduous species. All old flower heads should be removed directly the blooms are over.

Tender Rhododendron Species
Species and hybrids suitable for places not visited by severe frosts:

Rhododendron arboreum is a tall growing, Himalayan species, recognised by its long, narrow leaves, which are silvery or brown on the under surtace, and its dense heads of crimson flowers. There are varieties with rose, pink, and white blooms, and it has been largely used by the hybridist. In Cornwall, at Tregothnan, Carclew, Penjerrick, and Tremough, there are many fine specimens.

Rhododendron arycntcum is a Himalayan species with leaves 1 foot long and 3 to 5 inches wide, with silvery under sides. The flowers are waxy white, tubular, and borne in immense, dense heads. At Tregothnan, near Truro, there is a fine specimen, which in 1902 bore upwards of 900 trusses of bloom.

Rhododendron Aucklanclii - See Rhododendron Griffithianurn.

Rhododendron campanulatum is also a Himalayan plant. It has oval or oblong leaves, covered with a dense brown tomentum on the under surface, and bears trusses of lilac coloured blooms. It thrives in the open about London.

Rhododendron campylocarpum is a yellow flowered species from Sikkim. It grows 4 or 5 feet high, but is more often met with at less than half that height. It is growing outside at Kew, but succeeds better farther south.

Rhododendron ciliatum - Where this will grow it forms a very ornamental plant, growing into a dense bush 4 feet high, and bearing in great profusion, during April, white blossoms faintly flushed with pink.

Rhododendron Falconeri is a striking species by reason of its immense, deep green leaves, which are covered on the under surface with a dense brown felt. The flowers are white, and borne in large, compact trusses. At Tregothnan there is a very large bush which has been known to carry more than 1,200 trusses of flowers at one time. It comes from Sikkim.

Rhododendron Griffithianum - In many gardens this is better known under the name of Rhododendron Aucklandii. It is from Sikkim, and is recognised by means of its large, green leaves and its pure white blossoms, which are often 5 inches across. It has been used by the hybridist, and a number of first rate varieties have been raised, of which Pink Pearl, Glory of Penjerrick, Kewense, and Beauty of Tremough are among the best.

Rhododendron Thomsoni - Although this Himalayan species sometimes grows more than 12 feet high, it is usually met with 3 or 4 feet in height. The flowers are tubular, waxy in texture, and blood red. Others are Rhododendron Indicum varieties, Rhododendron glaucum, Rhododendron niveum, and Rhododendron barbaturn. Rhododendron Indicum amoenum is the hardiest of the Indicum set, and does well about London.

Evergreen Rhododendron species
Evergreen species which succeed about London and farther north are:

Rhododendron Californicum - A Californian species growing 6 or 8 feet high, with rosy purple flowers, 2.5 or 3 inches across.

Rhododendron Catawbiense, introduced from the United States in 1809. The leaves are oblong, 3 to 5 inches long. The flowers are in shapely trusses, and are lilac purple in colour. It grows 12 feet or more high, is very hardy, and has been one of the parents of many of our best hybrids.

Rhododendron cinnabarinum - A very interesting Sikkim plant, introduced in 1851. It grows 5 feet or more high, has ovate leaves 2 to 3 inches long, and nodding, tubular flowers which may be orange, scarlet, or brick red in colour.

Rhododendron Dauricum - This is a semi-evergreen or sometimes deciduous species from Siberia. It is of loose habit, and bears small, rosy purple flowers in January and February.

Rhododendron ferrugineum is found in the European Alps, and is known as the Alpine Rose. It grows 1.5 to 2 feet high, bears small leaves and tiny red flowers, the latter appearing in May and June.

Rhododendron Fortunei - This is a tall growing, bushy plant from China, which bears, in May, pale rose, fragrant blossoms. Introduced in 1859.

Rhododendron lepidotum is a curious, dwarf species from the temperate and Alpine Himalayas. It grows about 1 foot high, and bears curious, flat, purplish blooms.

Rhododendron Ponticum is probably the commonest of all. It is found in Spain, Portugal, and Asia Minor, and has been of great service to the hybridist. It grows 15 feet high, and bears purple flowers in June.

Rhododendron racemosum is a dwarf plant from western China. It grows from 1 foot to 2 feet high, and has small leaves and pretty, pink tinted blossoms, which are borne in great profusion. It is an excellent subject for the rock garden.

Rhododendron rubiginosum is another Chinese species. It grows 2 to 3 feet high, and bears rosy red flowers in April and May.

Rhododendron Smirnowi was introduced from the Caucasus in 1886. It has large leaves covered with a dense white tomentum on the under surface. It grows about 3 feet high, and flowers freely when quite young; the blossoms are pinkish purple, and borne in May.

Rhododendron Yunnanense was introduced from Yunnan about 1894: It grows 22 feet high, and beats white, red spotted flowers in April and May. In addition to these, there are many other species, and a number of botanical varieties and first hybrids. Some of the best of the latter are Cunninghami, Kewense, Luscombei, myrtifolium, Nobleanum, and Russellianum.

Deciduous Rhododendron Species (Azalea)

Rhododendron calendulaceum - This grows 4 to 6 feet high, and bears fragrant, yellow, red, orange, or copper coloured blossoms in May and June. Introduced from North America in 1806. It has been used largely by the hybridist.

Rhododendron flavum, is a native of the Caucasus, introduced in 1793. It grows 6 or 7 feet high, and produces fragrant, yellow flowers. It has been used largely by the hybridist.

Rhododendron nudiflorum - This is the Pinxter Flower from Canada and the United States. It grows 6 feet high, and has white, pink, or red flowers. There are a large number of hybrids. Introduced in 1734.

Rhododendron Occidentale is a white or pink flowered species from California. It blossoms in June and July.

Rhododendron Rhodora, from North America, has rosy purple blossoms.

Rhododendron rhombicum is an April-flowering, Japanese. species, with rosy purple flowers.

Rhododendron Sinese - In gardens this is better known as Azalea mollis. It is a native of China, and Japan, grows 4 to 5 feet high, and bears large yellow or red flowers in April. It is one of the most ornamental of all species, but unfortunately it blossoms early, and is often damaged by spring frosts. It is an excellent subject for forcing.

Rhododendron Vaseyi is a beautiful species from Carolina, with pink or white flowers.

Rhododendron viscosum is the Swamp Honeysuckle of North America. The flowers are white, very viscid, and borne in July.

Evergreen Rhododendron varieties

Alexander Dancer, rose; Amphion, pink; Auguste van Geert, rosy purple ; Caractacus, crimson ; Charles Bagley, red; Doncaster, scarlet; Duchess of Connaught, white, with yellow blotch; Everestianum, rosy lilac; Gomer Waterer, white; Helen Waterer, white, crimson edge; Hermit, scarlet; James Nasmyth, rosy lilac; John Waterer, crimson; Kate Waterer, rose, yellow centre; Lady Clementina Mitford, peach; Lady Eleanor Cathcart, rose; Lady Grey Egerton, blush; Lord Palmerston, rosy crimson; Madame Carvalho, white; Michael Waterer, crimson; Minnie, white; Mrs. John Clutton, white; Sappho, white, with dark blotch; Pink Pearl, pink; and Snowflake, white.