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.
We have found the simplest way to remove the green moss or Marchantia is with the flat, sharpened end of a thin piece of wood, such as an ordinary plant label. By holding the pot in one hand, and the flat wooden tool between the finger and thumb of the other, so that it does not deeply penetrate the soil, the coating of green slime or moss can very quickly be removed. A few small pieces of grit or a little dry sandy loam should then take its place.
Another thing to guard against is the 'coddling' of Alpines in warm or ill-ventilated greenhouses. They cannot have too much air, and most of them do far better in the open than under any glass. This somewhat artificial method of cultivating hardy plants may be necessary sometimes, but it frequently tends to produce a less robust race, with many individual specimens drawn out in quite an unnatural manner.
It is well known that one of the most characteristic features of nearly all the higher Alpine species is their dwarf habit, with stems frequently only a few inches in height, often with a rosette of leaves at the base, and very long fibrous roots which sometimes penetrate several feet into the soil. By this means they are prevented from being blown away by the furious blasts of wind so frequent in high mountains, and at the same time the long thick fibres absorb moisture and nourishment from the soil through which they pass.
Most Alpine plants can be grown from seed, and many are better obtained from this source than from established plants, because much of the material sent to England by collectors arrives with insufficient root. It is very difficult to get up the complete fibrous tap root of such things as Anemone alpina, Campanula barbata, Gentiana ascleftiadea, etc. Again, many plants die in transit to a distant country. Moreover, when seed is collected, rather than roots, there is less chance of disfiguring the mountain side. Seed should be sown as soon as possible after its collection. In saying this we merely follow Nature; and it has been proved that many seeds refuse to germinate after a certain limited time.
Although many Alpines may be sown in a suitably prepared bed in a sheltered position out of doors, some of the rarest and best kinds should be sown in pots or boxes in the following manner. The pots should be well drained and filled either with a mixture of sifted loam and sand, or with sifted sandy heath soil. The best preventative against weeds is either to bake the soil or soak it in the prepared pots with absolutely boiling water, which will destroy the seeds and spores of weeds and kill all insects at the same time. After the boiling water is used the pots must be left for a day or two before sowing the seed. The seeds should be sown on a firm level surface and have a very light and shallow covering, and some of the very small ones might be lightly covered only with triturated moss. The pots or boxes should then be gently watered with an extremely fine rose, placed in a frame or cool greenhouse, well shaded and kept at a uniform moisture. As soon as the seedlings appear they should be gradually hardened and exposed to the air, but not allowed to grow too lengthy, or nothing can be done with them. It is astonishing how rapidly some seedlings become too tall to be manageable in this delicate stage if proper care be not exercised.
A few hours' sun might draw them up to such an extent that they cannot be watered without being beaten down like a field of mowing grass is after a heavy storm, which would make the subsequent operation of pricking out into larger pots or boxes one of great difficulty. This pricking out is a tedious business. The young seedlings are put in rows an inch or two apart, according to their size, kept again at a fairly uniform moisture, and more or less shaded from the hot sun, until large enough to be planted in the open. More usually, however, they are planted singly or a few together in pots, and placed in a frame and kept rather moist. In the winter they may be more freely exposed to the air and kept dry and clean. When the weather permits in spring, they can be transplanted into the open and treated like the old plants. It is important that every Alpine plant should be planted firmly, and the soil or stones should be pressed well round the plant.
Alpine plants are also propagated by cuttings, by division of the rootstock or by layers. This is best done in spring, before growth begins, or in the autumn after it is completed. Most rock, Alpine, and sub-alpine plants and nearly all the Saxifrages, Semper-vivums, Primulas, and Androsaces can be easily increased by division.
Saxifrages are among the easiest of all Alpine plants to grow, and many will thrive under ordinary conditions in an English garden. Special care should be taken that all, except a few water-loving species, such as Saxifraga Hirculus, S. aizoides, and 5. stellaris should be in rather dry, sunny places and not under trees where water drips. However, such species as S. rotundifolia and S. cuneifolia like to be in the shade of trees and especially among big boulders. S. cuneifolia is particularly useful for quickly covering up rocks and ugly banks with its great tangled mass of pretty foliage.
Most of the 'encrusted' section will benefit by top-dressing with gritty loam in early autumn. Some should be wedged tightly between stones; and in dealing with small kinds, such as caesia, diapensoides, Burseriana, etc., care should be taken that they do not get washed out of their place in winter. The small rosettes of encrusted Saxifrages may be transplanted at any time, and it strengthens the flowering shoot if the offshoots are removed. They may be planted in ordinary pots filled with sandy loam. S. longifolia may be placed between a couple of more or less upright rocks, so that water cannot collect in the large rosettes. It takes two or three years to come to maturity, and after flowering and seeding it dies. 5. florulenta is another handsome species to be treated in the same way, but it prefers a vertical position, and has a still greater hatred of surface moisture. It should, however, have plenty of moisture in the soil.