This section is from the book "Sub-Alpine Plants Or Flowers Of The Swiss Woods And Meadows", by H. Stuart Thompson. Also available from Amazon: Sub-Alpine Plants: Or, Flowers of the Swiss Woods and Meadows.
Grit is beneficial to most of the Saxifrages because it checks the evaporation of moisture in summer, and prevents damp from stagnating round the collar of plants in winter. The reason why roots are often found in a network over the surface of a stone is because stones in the soil remain cool and moist, even in summer. It is therefore well to mix a quantity of stone chips with ordinary loam when making a rockery. They tend to keep the soil open and porous rather than sodden and water-logged. The air can penetrate further, and frost exercises its pulverising influence on a broken, gritty soil better than on a dense, compact one.
Saxifrages of the oppositifolia group die away in two or three years and should be taken up and pulled to pieces in fresh soil, and grit and leaf-mould may be worked into the shoots as a top-dressing.
Though in the wild state many Saxifrages grow almost solely on limestone, and a few seem to thrive only on granitic rocks, it has been found in cultivation that they are not so particular, and the great majority will live in an ordinary loamy soil, especially if it contains a fair proportion of lime.
In my former volume 11 did not sufficiently emphasise the fact that in the Alps themselves certain plants avoid calcareous soil in some districts, while they tolerate it in others; and in a few cases I was tempted to generalise about them in a manner which further observations and reading have proved unwise. For example, one of my kindest critics points out that the common Ling (Calluna vulgaris) is not strictly speaking a lime hater, for it is fairly common on the hillsides of the Dolomites, where it grows mixed with Erica carnea, which almost always affects limestone soils. Then again Rhododendron ferrugineum does not invariably grow on primary rocks. I saw it last year on the limestone in Switzerland; though possibly it was an apparent congener and growing there with the other species (R. hirsutum) because a sufficient thickness of peat separated it from the subsoil. Mr. Reginald Farrer has also pointed out in his last volume (Among the Hills) that "no reliance can be placed on rigid assignments of a plant to one stratum or another," and that this applies even to Anemone alpina and A. sulphurea.
But generally speaking the chemical and physical nature of soil is a factor of far greater importance in the distribution of plants in relatively small areas than the physical configuration of the land.
1 Alpine Plants of Europe, p. 5.
Many of the Primulas, such as P. Auricula, P. viscosa Vill., and P. latifolia, which grow on rocks in the Alps, may be planted in deep crevices between rocks; but there must be plenty of good compost made of a mixture of peat, sand, and loam, or leaf-mould instead of the peat. The roots should be tightly wedged between two stones. Mr. W. A. Clark recommends the use of clay instead of loam to wedge the plants in, because it will hold moisture longer.
The roots of Primulas should be well watered in spring and early summer. Some species such as P. glutinosa and P. involucrata do better in a northerly aspect, sheltered from the sun. P. integri-folia will cover damp, flat rocks if the soil is moist and yet well drained. It wants an abundance of water in spring, for it usually thrives best in the Alps just below the melting snow. The tiny P. minima hates lime, and likes sandy peat in a bare, open spot, though neither it nor the rare P. Allioni should be allowed to become very dry. P. Allioni is endemic in the Maritime Alps, and grows most luxuriantly in limestone grottos or small caves. Mr. W. A. Clark suggested the placing of a large stone to hang over this Primula, about a foot above the plant, sloping in towards the bank, so that rain may run off to the roots in a sort of little pit lined with clay and filled with loam and broken limestone.
To propagate Alpine plants by cuttings, a method particularly applicable to shrubs and certain hard-wooded plants, such as Helianthemum, Cistus, Daphne, Rhamnus, etc., half-ripe branches should be cut off in autumn and placed in a shallow pan or bed. The cuttings should not be more than three or four inches long; the lower leaves should be trimmed off, and the shoots must be cut immediately below a joint, whence the roots will spring. There should be good drainage, and the compost of loam and sharp sand can be made more porous by adding pieces of charcoal. Each cutting should be firmly placed in little holes about an inch and a half deep, which are drilled with a stick or ordinary lead pencil, and a little silver sand can be dropped into each hole. After the earth has been firmly rammed down, the cuttings may be placed in a cool frame, or in some shady place covered with a bell-glass, and allowed fresh air daily. They should be kept uniformly moist, but not too moist. When the young shoots have grown some size and become rooted they should be gradually accustomed to the open air until ready to be transplanted.
Many of the Helian-themums, or Rock-roses, can be grown from cuttings in a properly prepared bed in a sheltered position in the open.
Lastly, those Alpine plants which have runners, such as Linncea borealis, can be increased by merely detaching separate pieces and replanting them firmly in shaded beds or in pots. Some of the dwarf Campanulas with a creeping rootstock can be similarly treated. Some hard-wooded shrubs, such as Azalea, certain Daphnes, and Rhododendrons, which don't ripen their seeds in this country, and are difficult to strike from cuttings, can be increased by layering. A suitable branch near the ground is chosen, cut half through near a joint, and then pegged down firmly and covered with a compost of loam and sand an inch or two deep. If by the following spring roots have been formed, the layered portion can then be cut from the old stock.
The greater number of sub-alpine plants which grow in the woods and meadows, and many of the true Alpines, may, if desired, be treated like ordinary garden plants. Even forty years ago this was realised by Messrs. James Backhouse and Son, who said in issuing their Catalogue for 1871: ' A large proportion of the truly Alpine species, which find their natural home in the crevices of rocks at a great elevation, grow with perfect ease in an open border in ordinary loamy soil. And, strange to say, some that succeed with difficulty on artificial rockwork, flourish well under such circumstances, and thus bring within the range of every garden a large and varied amount of beauty.' Many could be planted in turf, and perhaps some of the woodland denizens might be tried under trees, where little else will grow, provided the foliage is not very dense. Some of the Orchids, Butterworts, Parnassia, Primula farinosa, and many others frequenting damp places, should have a peat or bog garden prepared for them at the foot of the rockery.