This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
Shrubs and trees requiring a peaty or boggy soil are commonly known as American plants, whether natives of that continent or otherwise. They include all, or nearly all, of the Ericaceae and members of several other families, such as Berberis Bealii, and other species, Calycanthus spp., Escallonia, some of the Daphnes, Magnolia glauca, Spiroaea spp., and numerous other evergreen shrubs, will flourish better in peaty soil, or with an admixture of leaf-mould. We might also mention that many herbaceous plants succeed best in a peaty soil. Reference is made to the fact under those species growing naturally in boggy places.
Although all of these plants prefer a prepared soil, there are some of them that will flourish in ordinary garden soil, if properly drained. And in the absence of peat, good leaf-mould and sharp sand mixed with the natural soil will answer for most species. Varieties of Rhododendron Ponticum, Ivalmia latifolia, Azalea Pontica, Erica carnea, etc., etc., and Arbutus Unedo are the least exacting in regard to soil.
To cultivate peat plants successfully two conditions are indispensable; these are, climate and soil. In default of these conditions, all efforts will remain fruitless. With regard to climate, it should be remembered that all the species of this group are not equally hardy; hence the necessity of choosing such as are suitable for the situation where it is wished to grow them. But temperature is not the only element of a climate. Humidity of the atmosphere and light are other elements which must be taken into account. Broadly speaking, we may say that all the Ericaceae demand a certain amount of moisture in the air surrounding them. This moisture may have its source in the mists which frequently envelop the mountains where they grow; or it may be due to the vicinity of the sea, lakes, marshes, or water-courses. The site and aspect for these plants are also of great importance. For those species loving a marshy habitat, a low, flat, spongy soil is naturally the best; but these species are few in number and less generally cultivated than the others. All the other species prefer a sloping bank of eastern or northern aspect, but it should be sheltered from the winds. If, therefore, there are natural or artificial mounds in a garden, the shady side of these should be selected for planting clumps of Rhododendrons, etc. In the absence of small hills, raised beds can be substituted; but if the natural drainage be insufficient, a layer from six to nine inches thick of coarse gravel or some other open material will be necessary in a low situation.
The site having been selected, and the form of the plantations decided upon, the natural soil should be taken out to a depth of eighteen inches or two feet; and if the subsoil is of a heavy impervious nature, a layer of draining materials from six to nine inches thick put in at the bottom and filled up with peat, roughly broken but not sifted. This earth is thrown up into a mound high enough that after sinking it will still be a little above the general surface of the ground. The thickness of this bed will vary according to the size of the species it is intended to plant. It will readily be understood that a greater depth of soil will be necessary for the large-growing Rhododendrons than for those of smaller growth, and such as Heaths and Vacciniums.
In some parts of the country there is a difficulty in pro-curing peat, and, besides, a great deal of it is too poor to use alone with advantage. It may be enriched by the addition of thoroughly rotten leaf-mould with a little sharp sand, or a com-post may be substituted for it consisting of equal parts of sand and vegetable mould. But the fibre of the peat being one of its important elements, no substitute can equal it. The top-spit of earth in an old wood is usually rich in humus and very light, and mixed with sand is perhaps preferable to leaf-mould. Or it would serve well to improve the fertility of poor peat. In all cases the introduction of even the smallest quantity of farm-yard manure should be avoided, for of all plants these are the most susceptible, and contact with animal dung-is either fatal or very injurious to the majority of them. Otherwise the more substantial the soil is, other conditions being equal, the better the plants will flourish in it. Nearly all the trees and shrubs coming under this category may be propagated from layers or suckers; but the garden varieties of Rhododendrons, etc., are usually grafted on the commoner forms. As a stock for Rhododendrons, R. Ponticum is generally employed, being very hardy and easily raised from seed or layers. R. Catawbiense is also occasionally used, but the hybrid varieties do not take so well on this as on the common one. The principal objection to R. Ponticum for the tender varieties is its earliness; but as only a few species, such as R. campanulatum, and R. argenteum, are said to succeed well on the other, it is generally preferred.
As an artificial soil in most cases is necessary, it is a common practice to plant these shrubs in masses, and from their bushy habit this method is very suitable and effective. After a bed is once established, little care is required. The removal of weeds and dead wood, and the yearly application of a thin layer of leaf-mould, are the principal operations. In very dry seasons, and especially where recently transplanted, copious waterings will be very beneficial.