This section is from the book "The Gardener V2", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
Among the many novelties which, during the last quarter of a century, have been introduced into British gardens, none have attracted greater attention, or obtained a higher place in popular favour, than the magnificent species of Rhododendrons sent home in the spring of 1850 by Dr Hooker from the Sikkim Himalayas, and a few years later by Thomas J. Booth, Esq., from Assam and Bhotan.
The high expectations created by the glowing descriptions given by these gentlemen, on their introduction, of their beauty in their native habitats, the great diversity among the individuals in habit of growth from the tiny Heath-like shrub up to the stately broad-leaved Evergreen tree - the variety among them in foliage, form, and colour of flowers, and in many instances their remarkable dissimilarity in general appearance from any of the sorts hitherto known - have since then been fully realised; and they have proved invaluable acquisitions to the long list of these gorgeous flowering-shrubs already in cultivation. It is scarcely possible to overrate the importance, in a horticultural point of view, of these brilliant discoveries, either as indoor decorative plants, for which many of them, from their noble foliage, delicate colours, and, in some cases, exquisite fragrance, are peculiarly adapted, or as the parents of a new and distinctive race of hardy sorts; and though, considering the length of time they have been in this country, comparatively little has been effected in this direction, from the difficulty of inducing some of the most desirable to cross with the hardy late-flowering kinds, considerable progress has been made; and there are a number of hybrids which, while possessing many of the features of the Indian species, are at the same time hardy enough for outdoor cultivation.
Much, however, remains yet to be done; and we can conceive of no more interesting and inviting field for the exercise of the hybridiser's skill, and in view of what has been already accomplished with R. arboreum, more likely to yield a rich return for his labours.
Since these first introductions, several no less useful and interesting species have been added by other collectors from the same and similar localities, which, together with the varieties obtained from time to time by hybridising, constitute the now somewhat large group known as Greenhouse and Conservatory Rhododendrons.
Though the great majority of the species have been found able to survive our winters in the open air, and in one sense may be said to be hardy, they for the most part start too early into bloom and growth; and, as a rule, the expanding flower-buds and young shoots are destroyed by spring frosts, and this even in the mildest seasons and most favourable situations. For the full development of their beauty, therefore, the shelter of a greenhouse is absolutely necessary; and they will richly repay their occupancy of the best place that can there be assigned to them, as they are undoubtedly the most interesting plants it can contain, especially during the spring months, either while in bloom or making their new growths, when they are singularly attractive and interesting.
The conditions necessary for the successful cultivation of greenhouse Rhododendrons are at once simple and easily supplied, and few plants are less exacting upon the care and attention of the cultivator.
In common with the outdoor members of the genus, they delight in a rich fibrous peat soil, which, with the addition of more or less sharp sand to make it sufficiently porous, alone should be used for their cultivation in pots.
In the preparation of the soil for potting, the turf should be chopped down to the requisite fineness with the spade, so as to retain all the fibre it contains; and as it must be pressed as firmly round the ball as possible, care should be taken that it be well aerated, and only used in a dryish mellow state. To insure perfect drainage, a thin layer of moss or rough peat should be put over the crocks before the finer soil is introduced.
The size of the shift must always be regulated by the habit and vigour of the individual plant, as well as by the state of the roots. In no circumstances is a large shift desirable; it is far better that it should be moderate, and repeated at such intervals as the growth of the plant and the spread of the roots render necessary. Apart from the importance of having them in as small pots as is consistent with their real wants, there is the possibility of the soil, from the necessity of frequent watering, becoming sour and sodden - a condition most prejudicial to their health, particularly in the case of such as are naturally slow-growing and weakly in their habit. In shifting, the ball should never be disturbed more than is necessary for the removal of loose soil, and on no account should the roots be torn off or mutilated. Of all the seasons of the year we prefer the spring for repotting, immediately after blooming, and when they are beginning to give indications of growth; the roots then at once take to the fresh soil, and the increased nutriment enables them to make rich luxuriant shoots, and consequently full and well-developed flower-buds.
During the growing season, the temperature of an ordinary greenhouse, except in the cases of a few of the more tender kinds, will be found amply sufficient; at this period they require an abundant supply of water at the roots, and a frequent sprinkling overhead.
Such as have not been shifted, and have any appearance of being pot-bound, will be benefited by the application of weak liquid manure once or twice during the progress of their growth. As soon as the shoots begin to give indications of reaching maturity, water should be gradually withheld; and when the growth is complete, they should have no more water than what is barely necessary to keep them from flagging, and as soon as possible be removed to a cool situation out of doors, the great object being to prevent them from making second growths, to which most of the species have a strong tendency. If plunged to the brim of the pots in a sheltered situation, little artificial watering or attention will be necessary during the summer months, at the end of which they must be housed along with the other greenhouse plants.
We append a very select list of really desirable sorts, any one of which is worthy of a place in even a limited collection of house plants; and to give a facility in selection, we have arranged them in two classes, the first containing such as, from their dwarf habits and free-blooming qualities, are best adapted for greenhouse culture; and the second, those that are tall and robust, and more suitable for a large conservatory.
Ciliatum (Sikkim), a dwarf bushy species, forms a neat and effective pot plant, blooms freely when only a few inches high, flowers when newly expanded, of a blush colour, turning afterwards pure waxy white.
Countess of Haddington (hybrid), with a general resemblance to the preceding, of which it is a hybrid. This grand variety is more robust in habit and much larger in foliage and flowers; colour blush white.
Edgeworthii (Sikkim), a distinct and magnificent species, with large, pure white, spreading, cup-shaped flowers of exquisite fragrance. The leaves are dark green, strongly reticulated, and thickly coated with a rusty - coloured wool on the under side.
Fragrantissima (hybrid), a hybrid from Edgeworthii, which it resembles, of a dwarfer habit. Flowers pure white, with a slight pencilling of rose on the outside of the florets. A distinct and elegant variety.
Jasminiflorum, a beautiful species, with white tube-shaped flowers arranged in clusters.
Javanicum (Java), a species with bright orange flowers, dwarf habit, blooms freely, requires during winter and spring the temperature of an intermediate house, and to be kept in the greenhouse in summer.
Formosum (Gibsonii) grandiflorum, a free-blooming variety; flowers pure white; a neat-growing and attractive plant.
Multiflorum (hybrid), a very dwarf profuse-blooming variety; flowers at the axils of the leaves; colour pure white, delightfully scented.
Princess Alexandra (hybrid), a pretty dwarf variety, with long tubed flowers of a pure white colour, the stamens delicate pink.
Princess Alice (hybrid), a neat compact dwarf variety, with bell-shaped flowers, white shaded with pink on the outside; delicately scented.
Princess Helena (hybrid), a singularly beautiful variety, with long tube flowers in clusters; colour delicate pink, streaked with a darker shade. Blooms freely.
Princess-Royal (hybrid), resembling the last, but sufficiently distinct, the flowers being of a richly-shaded rose colour.
Virgatum (Bhotan), a species of an extremely dwarf bushy habit; flowers profusely when only a few inches high; creamy white shaded with rose at first, afterwards pure white.
Veitchianum laevigatum (hybrid), a very handsome variety, with large white flowers, yellow blotch in the centre.
Arboreum (Sikkim), the grandest of all the Rhododendrons. Flowers bright crimson or scarlet; its robust treelike habit, however, renders its admission into glass structures of ordinary dimensions impossible; but where it can be accommodated, it is unexcelled.
Arboreum album, a variety of the last species, with white flowers.
Argenteum (Sikkim), a tall-growing species, with large heads of white flowers, somewhat shy in flowering, but much admired for its magnificent foliage; the leaves being from 6 inches to a foot long, and from 4 to 5 inches broad, silvery on under side.
Aucklandii (Sikkim), one of the finest of the Indian species; flowers snowy white, of a waxy texture, bell-shaped; the leaves from 6 inches to a foot long, and from 2 to 3 inches broad.
Blumeii (Java), a beautiful species, with delicate lemon-coloured flowers; blooms freely in a young state.
Dalhousiae (Sikkim), a well-known and splendid species; flowers freely when of a moderate size; the flowers are white, 3 to 4 inches long, as much across the mouth, and fragrant.
Fortuneii (China), flowers of a delicate pinky-white colour, with a bright-yellow throat, cup-shaped, from 3 to 3½ inches in diameter; very sweet-scented.
Falconerii (Sikkim), a noble species, shy in blooming while young, but much appreciated for its noble foliage; the leaves measuring from 1 foot to 18 inches long and 7 inches broad, and beautifully ferruginous beneath.
Longifolium (Bhotan), like the preceding, a fine-foliaged species; leaves from 1 foot to 18 inches long, and from 3 to 5 inches wide; flowers at first a delicate primrose colour, changing to a purer white.
Hookeri (Bhotan), allied to Thomsonii, to which it bears a strong resemblance; flowers dark crimson, bell-shaped; a fine and distinct species.
Thibaudense (Bhotan), a remarkable and handsome species, different in general appearance, as well as in flowers, from any other known species, except Keysii, to which it is allied; the flowers are of a bright-red colour, edged with yellowish green.
Maddeni (Sikkim) resembles Edgeworthii in its large finely - scented white flowers, but in foliage quite distinct; an admirable pot plant.
Nutalli (Bhotan). This noble species, whether as regards foliage or flowers, is unsurpassed among conservatory plants; leaves 6 to 10 inches long by 3½ to 6 inches broad, beautifully reticulated; flowers trumpet-shaped, 4 to 5 inches long; colour white, with tint of rose red, and yellow at the base; delightfully fragrant. Hugh Fraser.