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.
With a situation sheltered, yet fully exposed to the sun, a competent supply of suitable soil, a good amount of moisture, without wetness, and an occasional watering when exceptionally dry weather occurs during the growing season, Rhododendrons will grow vigorously enough for all practical purposes without being stimulated with manure in any form. Over-luxuriance is neither necessary nor desirable. On the contrary, when the primary object is abundance of bloom, it is rather prejudicial than otherwise, causing them to devote an undue amount of energy to the formation of mere wood, to the detriment of the flower-buds - and often inducing a second growth, which seldom ripens in time to escape damage from early frosts. Any tendency of this kind, whether arising, as is frequently the case, from excessive moisture at the root, or the soil being too rich, should be checked when the young shoots are sufficiently advanced to make it desirable that they should ripen and form their buds; and this may be effected by simply removing the soil from round the ball for a few days, or pruning the roots moderately with the spade.
All the varieties, however, delight in rich manures, and in cases in which the plants, by their weakly growths and sickly appearance, indicate a deficiency of the supply of nourishment, an inch or two of rotted manure, either forked in among the roots or laid on the surface as a top-dressing, with a slight covering of sand or light soil, in the course of the winter or early spring, will be found most beneficial; while the occasional application of a dose of liquid manure of medium strength, when the roots are in an active state, and even when the flowers are expanding, will work wonders in promoting their health and enabling them to make robust shoots and fresh well-developed leaves. We have seen a large collection, in which the plants had been for years in a most unsatisfactory condition - rarely flowering, and producing puny attenuated shoots - materially improved by such means. It may at the same time be observed that such a state of things will not be permanently remedied by mere stimulants, seeing that it is the result of either an exhausted or unsuitable soil; and the only effectual cure consists in lifting the plants and completely renewing the compost.
Considerable diversity of opinion has prevailed among cultivators as to which mode of propagation is best calculated to insure permanently handsome and healthy specimens. Some have contended that plants on their own roots, either obtained from seeds or layers, are superior to such as are grafted. We believe, however, that no universal rule can be laid down on the subject. All the species are easily raised, and make admirable plants from seed, but hybrid varieties will not thus reproduce exactly the same characters as the parents; while layering, except in the case of the few sorts that are naturally dwarf and branching from the roots, such as "Caucasicum" and "Noblieanum," is at once a tedious and expensive process, necessarily limited in its application, and possessing few, if any, advantages over grafting. The plant so operated upon is sacrificed, or at least hopelessly disfigured and, after all, the product in young plants is at the most so trifling, as to afford no compensation for the years of care necessary to make them presentable in the permanent beds among other specimens.
There are, moreover, some of the richest-coloured and most attractive varieties which on their own roots have a rank, robust habit of growth, and consequent shyness in flowering, unfitting them for a place in general collections, but which are modified by grafting, and rendered all that can be desired.
The best and most commonly-used stocks for grafting are free-grown seedlings of the robust form of the common Ponticum, and the future wellbeing of the plants depends largely upon their being well selected. If they are weakly and stunted to begin with, failure and disappointment will be the sure result, and no amount of cultural skill will ever make them effective or creditable specimens. For the first two or three years after grafting, the stocks, if healthy and vigorous, have a tendency to throw up suckers from the root. These materially weaken the graft if allowed to remain, and should be carefully removed; and as it acquires strength to absorb all the sap the root can supply, they will gradually disappear. The finest specimens are produced from plants on single stems, entirely clear of branches for at least 6 inches above the root; when they are intended to stand singly in prominent situations, even 12 or 18 inches will not be found, after a few years' growth, too much. The lower branches soon bend down sufficiently to clothe the stem, while the head acquires a symmetry and uniformity of outline always pleasing, and which can never be attained by plants with an irregular mass of stems emanating from the root.
In ordinary seasons, most of the varieties set their seed freely; indeed, almost all the capsules will be found full; but unless wanted specially, it should never be allowed to ripen, as it entails a severe and unnecessary tax upon the strength of the plant; and the best course is, immediately after the blossom is decayed, to pinch the trusses off, which at that early stage may easily be done with the finger and thumb.
For forcing, to decorate the conservatory in winter, Rhododendrons occupy an important place, and one for which it would not be easy to find substitutes. A great many of the sorts are available for this purpose; and by a judicious selection, along with skilful management, a constant succession may be secured from December till the genial warmth of spring and early summer tempts those that are outside to expand their blooms. They should be potted as soon as possible after the flower-buds are fully formed, and placed in an open sunny situation out of doors until severe frost (to which they ought never to be exposed) necessitates their removal to more comfortable quarters. The earliest lot may be introduced into heat about the beginning or middle of December, according to the time they are wanted in flower, or the amount of heat that can be applied. From two to three weeks will generally suffice in an ordinary plant-stove temperature to expand the earliest varieties, while those that are later will require longer periods, according to their natural season of flowering when out of doors.
In potting, the balls should never be reduced more than is absolutely necessary. It should always be borne in mind that every root cut off inflicts an injury upon the plant, and though they will expand their blooms after being considerably mutilated and cramped into unnaturally small pots, the evil effects of such treatment will be seen for years after.
While forcing, a moist atmosphere should be constantly maintained, the plants twice a-day syringed overhead, and the roots abundantly supplied with tepid water, with an occasional dose of weak liquid manure after the buds begin to swell. After the flowers are fully expanded, and not a day before, as they will be completely checked and make no further progress with a sudden change of temperature, they may be transferred to the conservatory, where the more they are shaded from the sun, the longer they will continue in perfection, adding to its attractions, and eliciting from even the most unimpressionable of its visitors the warmest expressions of admiration.
Forced plants, after their blooms are decayed, should be kept under glass till such time as they can be put outside without danger of suffering from frost, to which, after the heat they have been subjected to, they are peculiarly susceptible; and as they very rarely set a sufficient number of buds the first season to make them eligible for the following winter's work, they should be replanted in the borders as soon as the weather is mild enough to permit its being done with safety.
While all the varieties are available for forcing, and may be induced by strong heat to flower considerably earlier than their natural season out of doors, the subjoined list of the earliest flowerers comprises a selection of the best sorts for forcing, to come in early in winter, or for potting for the decoration of the conservatory in spring, with no other heat than is necessary to exclude frost: those in the first division are most effective in the open air in mild springs, but so early that they can never be depended upon; the others, though blooming in May, in ordinary seasons escape damage from frost.
Albertus superb - Light pink, changing in strong heat to an almost pure white; a free-blooming easily-forced variety.
Caucasicum - Straw colour, changing to white when under glass; of a dwarf bushy habit; a very profuse bloomer.
Caucasicum grandiflorum - Slightly darker in colour, and more robust in habit than the above.
Cinnamomeum - White, with black spots; flowers freely when grafted.
Cinnamomeum hybridum - Pure white, dark spots, dwarf bushy habit.
Dauricum - Rosy purple, very profuse; a deciduous species, admirable for very early flowering.
Dauricum atrovirens - Deeper in colour, semi-evergreen; otherwise like the species.
Diadem - Clear pink, habit of Caucasicum.
Noblieanum - Dark damask, in some varieties light pink; of dwarf bushy habit; forces with great facility; a well-known and popular variety, not yet surpassed for early forcing.
Noblieanum album - White, with dark spots; similar in habit to the above.
Proecox - Lilac, in appearance like Dauricum, of which it is a hybrid, but more compact in habit; a profuse bloomer and a first-rate forcing variety.
Proecox rubrum - Rosy-lilac, same in habit and general appearance as the preceding.
Venus - Pale silvery blush, dwarf and bushy in habit, and free in flowering.
Alta-Clarense - Dark scarlet, finely spotted; a free bloomer when grafted.
Alstroemeroides - Bright rose, spotted on all the petals.
Campanulatum - Delicate blush, dark spots.
Campanulatuni hybridum - "White, reddish spots; dwarfer in habit than the species.
Caucasicum pictum - Blush, beautifully spotted with crimson, fine foliage, dwarf habit, and free bloomer.
Comet - Fiery crimson.
Eclipse - Dark crimson.
Gloire de Gandavensis - White, finely spotted.
Jacksonii - Light rose, dark spots; profuse bloomer.
Lavinia - Rose.
Medora - Light rose, intensely spotted.
Princeps - Light crimson; free bloomer.
Regalia - Crimson, large flowers.
Russellianum - Scarlet, robust habit.
Russellianum superbum - Dark scarlet.
Smithii elegans - Fine scarlet, black spots.
Venustum - Light crimson; free bloomer.
Verschaffeltii - Blush, much spotted. Hugh Fraser.