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.
Notwithstanding their superlative claims upon all who have an interest in horticultural pursuits, and their admitted value as decorative plants, whether as arranged in masses in the flower-garden or shrubbery, or singly as specimens on lawns, it is somewhat remarkable that as yet Rhododendrons are neither so extensively nor so carefully cultivated as they deserve. In many establishments where they might be expected to be found occupying a prominent place, they are either absent altogether, or have the most inferior position assigned to them; and their sickly, stunted appearance too often contrasts most unfavourably with the more robust and less fastidious shrubs with which they are associated.
This state of things, we are convinced, arises not so much from a want of appreciation of their merits, as from a popular mistake as to the difficulty of providing the necessary soil and conditions for their successful cultivation. Peat is not found in every garden, nor even in its immediate vicinity, and in many places the expense of procuring it in sufficient quantity is considered an insuperable barrier to the introduction of even a limited collection. That peat, or a combination in which the elements of which it is composed are largely present, is their natural soil, and that all known Rhododendrons thrive luxuriantly in peat, is undoubted; but that, at the same time, it is possible without it to create an artificial soil containing all the constituents which it supplies really necessary for their growth and development, and this with materials to which most gardeners have ready access, has been again and again demonstrated.
On examining the root of a Rhododendron while in active growth, we find that it consists of what is commonly termed a ball or mass of roots, netting in a quantity of the soil in which it has been growing. Round the outside will be seen an innumerable quantity of short hairlike fibres, white and transparent, so extremely soft and brittle that it is difficult to handle them without breaking some off: these are the young roots, and the only feeders by which the plant imbibes its food. If growing in peat, they are found pretty equally diffused over the ball; but if in mixed soil, they are invariably in greatest abundance on that side which is most in contact with any fragments of peat or other decomposed vegetable matter. Incapable from their extreme delicacy of penetrating stiff hard soil, and peculiarly susceptible of injury from dryness, particularly while in a state of activity, these fragile rootlets soon wither and die when so exposed, entailing a serious loss upon the plant, and that at a time when it requires all the assistance it can get to enable it to perfect its growth, and form flower-buds for the succeeding season.
From these facts, as well as from experience of results, it seems obvious that a soil to be suitable for Rhododendrons must be soft and spongy in its texture, capable of retaining moisture, and possessed of a large percentage.of vegetable matter.
Supplying all that is necessary for their sustenance, as far as soil is concerned, peat should invariably be used when attainable, in preference to any artificial compost; and when a choice can be had, we prefer that which is found in bogs or heathy moors, as being less decomposed, more fibry, and consequently richer than that from higher and more exposed situations. The surface-turf only should be taken, cut not deeper than 6 inches, and chopped down with the spade sufficiently fine to allow the largest pieces to pass through a 3-inch sieve; after the addition of a moderate allowance of manure, which has been laid up at least twelve months - say 1 ton to 6 - with a similar quantity of clean sharp sand, the whole turned over and thoroughly incorporated, it may at once be transferred to the beds, and the planting commenced forthwith.
As we have already indicated, a limited supply of peat, or even its entire absence, need not deter any one from attempting the cultivation of Rhododendrons; the materials for forming an artificial compost which will adequately supply all their requirements, exist in a separate state, and may be found in abundance in every district in the country. Loamy turf from old pastures, cut just deep enough to include the fibre, of which it cannot have too much, with about one-half of its bulk of rotten leaves and old cow-dung, and more or less sand, according to the character of the loam, the whole mass chopped down with the spade, not too fine, and well mixed together, will form a compost which any Rhododendron will duly appreciate, and grow in with the greatest luxuriance. Charred garden refuse - such as prunings, weeds, and old tan-bark - forms a valuable supplement to such a compost, and may be used liberally, when it can be had, with great benefit to the plants. In no other form should these substances be introduced, as unless they are so thoroughly decomposed as to be scarcely distinguishable from fine mould, they are not only worthless, but highly pernicious. This applies specially to old tan, which has sometimes been recommended.
We have never seen the young roots working freely amongst it in any state; but very often when the decomposition was but partial, the ball was found to be covered with white fungus, and the plant in a sickly condition.
In connection with the making up of such composts, it may be noticed that, from some cause which we have never heard satisfactorily explained, Rhododendrons have the greatest repugnance to calcareous soil, and refuse to grow where lime or chalk is in immediate contact with the roots. Along with suitable soil, much depends for the successful cultivation of Rhododendrons upon the selection of a proper situation. Although the great majority of what are termed hardy sorts can bear any amount of frost they are ever subjected to in this country, they should invariably be planted in sheltered situations, as, particularly during the flowering and growing season, they are liable to damage from cold dry winds. When so exposed, their extremely delicate flowers become prematurely blighted, and the tender growths so much injured, as not only to prevent the formation of the buds, but often to cause a second growth, which is generally destroyed by autumn frosts.
Shade and drip from trees should also be avoided; for while the plants will grow vigorously provided they have sufficient moisture, they require a full exposure to the sun to enable them to ripen their wood sufficiently to withstand the winter's frost, and to form flower-buds. Even when they do flower in such circumstances, they develop neither their form nor colour to perfection. In preparing the beds for the compost, the best plan in most cases is to remove the old soil altogether, particularly if it is either stiff hard clay or worn out by long cropping. It is not requisite that this should be done deeper than one foot, as the tendency of Rhododendron roots is to spread out near the surface rather than downward, when the subsoil is stiff and retentive: nothing more is required than to fill in the compost; but in cases where it is loose and gravelly it is of importance, with a view to prevent its absorbing the surface moisture too quickly, that two or three inches of peat or leaf-mould should be forked in, and afterwards beat down firmly.
Although it is possible to transplant successfully all over the year, not even excepting the flowering and growing seasons, we would recommend that it should be confined to the autumn and winter months, beginning in October, by which time the buds are developed, and the young wood ripe enough to bear removal without injury. They should never be planted deeper than to allow the top of the ball being covered with about two inches of soil, which should be trod as firmly over and around it as possible.
In excessively dry summers, such as we had last, a slight mulching with short grass manure, or some similar material, will be found beneficial, by preventing evaporation and keeping the roots cool; while an occasional copious watering during the blooming and growing seasons will contribute largely to the vigour of the growth, and assist materially in the formation of large, sound flower-buds.
Hugh Fraser. (To be continued).