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.
Of all our common shrubs I often wonder that the commoner varieties of the Rhododendrons are not far more extensively used. Many of the common shrubs are well adapted for giving shelter, while, as to ornament and general appearance, they are by no means superior to the common sorts of Rhododendrons. I have in the course of my practice planted many shrubs, and whenever I could I used a good number of the common Rhododendrons. They have many desirable qualities - not that there is by any means the least objection to the better sorts of Rhododendrons, when a few shillings, as to price, is of very little consideration. The general idea that Rhododendrons will not live in any soil except that of a peaty nature, may be greatly qualified according to many local constituents in the soil. There are two things which Rhododendrons are very impatient of, stiff adhesive clay, and chalky soils: perhaps as much as any kind of shrubs, the Rhododendron dislikes a bottom where the water cannot pass away freely.
I have seen the common sorts grown about shrubberies and grounds from self-sown seed by the thousand on a poor hungry pebbly soil having only a few inches of a peaty nature on the surface, principally from the decay of vegetable substances in the course of ages; and when this soil was trenched about a foot deep, nothing could do better in it than Rhododendrons.
Rhododendrons are a surface-rooting plant, seldom, even in old well-established plants, having strong, thick, extending roots. Several years ago we had to do with some very old and very large plants: a portion of them had had a trench dug out for them of about 18 inches deep, which trench had been filled up with peat, and this had become a compact mass, just such as would make good fuel when well dried. The roots had scarcely gone over 6 inches deep into it, and as the leaves had accumulated upon the surface in the course of many years (they had been planted perhaps 100 years), immediately under these leaves the roots were nearly all found in a compact mass of small fibres. Another lot of plants close by, where the soil was well mixed with small pebbles, sent their roots deep into the soil. Some years ago I planted a good many hybrid Rhododendrons in borders. The soil generally was of a good loamy texture, what might be termed good wheat land. It was well trenched. Other shrubs were planted behind the Rhododendrons, and flower-borders formed in front of them.
Selecting nice bushy plants of Rhododendrons, I hollowed out a place for each plant, into which was put a small barrowful of peat formed from the decayed fronds of Pteris aquilina and oak-tree leaves, from where possibly it had been accumulating for many ages. This peat was placed round the small ball of the young plants, and in two years' time they had many hundreds of little rootlets ramifying through the peat in all directions. Nothing can be so easily removed when thus managed, for either forcing or to rearrange in the borders, clumps, or elsewhere. G. Dawson.