Cultivation of Rock Plants
These are mostly dwarf plants and require finein which to grow and to seed themselves. At the same time they like soil which is well drained, and, therefore, sharp gritty sand is added to the when the boxes are prepared.
The commonest plants of the amateur’scan be roughly divided into groups according to cultural treatment.
First there are spreading, creeping plants, of thick, tangled growth. To this group belong such plants as Arabis, Mossy Saxifrages, Pinks, Thrift, and Cerastium.
Nearly all the plants in this group can be raised equally well from the seed or by division. Seeds do not always come true to type, but seedling plants are always healthy and interesting.
If it is intended to raise them from home-saved seed it should be gathered as soon as ripe, and not allowed to disperse itself naturally.
It can be sown at once, in pans of gritty soil or, if more convenient, it can be saved until the following spring.
The preparation of the seed pans to be used for Alpine seeds is on the same lines as for ordinary, except that, as a rule, shallower pans are used and the seed compost must have an extra large proportion of sharp, gritty sand.
Drainage should be well provided for by means of broken crock and rough material at the bottom of the seed pan. The seeds are covered only with a line dusting of sand.
Seedlings are picked out as soon as they are large enough to handle, and grown on under cool conditions, as most of these plants are perfectly hardy, even in the seedling stage.
While the seeds are germinating, the seed pan is covered with a sheet of glass, and until theappear it is shaded also, by laying over it a sheet of paper. The glass will be lifted daily and wiped, otherwise surplus moisture collects on it and may drop on to the seed and set up the disease known as “Damping off.”
As soon as the seedlings appear, the glass can be dispensed with entirely. Water should be given as required, and the best plan is usually to immerse the seed pan in a bowl of (rain) water, so that the water soaks upwards without washing out the seeds.
The vigorous-can also be increased by or division. When the plants have flowered, it is usual to cut them back rather severely, as they spread rapidly and may easily choke rarer Alpines.
The pieces that are cut off will all root if dibbled into sandy soil. They can be set in rows close together in any shady, moist part of the garden. With these rampant growers there is usually no necessity to use a propagating frame. Usually, any that are found difficult to rear in the open garden can be readily rooted if they are inserted in a frame or under a bell glass (which helps to keep the atmosphere moist round them until the roots have formed). In the case of Arabis or of Mossy Saxifrages, some of the crowded green tufts are usually just pulled off and dibbled into the soil without any further preparation.
Another group of rock plants consists of those compact Alpines (frequently rosette-shaped), which do not readily spread, and are therefore not so easily increased by cuttings or divisions.
In such cases it may be essential to increase by seeds, which are sown as already described for the carpet plants.
It is natural for rock plants to seed themselves, and for the seeds to be self-sown immediately they are ripe. In some cases the seed does not germinate quickly, but may lie in the soil for months or even years.
The common practice amongst rock gardeners is to follow Nature and to gather the seed as soon as it is ripe. In the case of rare Alpines they sow one-half immediately, and reserve one-half for sowing the following spring.
If the seed seems slow in germinating, and no signs of the seedlings are visible, when snow weather occurs, the seed pans are allowed to stand in the open and to be smothered with snow; this appears to help germination.
Pans containing seeds of rare Alpines should not be disturbed until there is no possibility of further germination taking place, as the seeds frequently germinate in somewhat erratic manner.
Cuttings under Glass
Propagation by cuttings is a simple method of increase with any type of rock plant from which cuttings or non-flowering rosettes can be removed. To be more certain of success with difficult subjects a propagating frame is used, and in this the cuttings are rooted, in pure silver sand. This medium is ideal, since it does not encourage “ damping off,” and if the atmosphere of the propagating frame is kept moist almost any rock plant cutting can be rooted there satisfactorily.
It is probably a mistake to “make” cuttings too carefully. The majority of rock plants root easily if pieces are pulled out and inserted as cuttings without any further preparation.
Where preparation seems advisable, the lower leaves are stripped off (as in the case of larger cuttings) and a clean cut made just below a joint.
One other method of increase is common amongst rock plants; that is, by. Any rock plant which has thick fleshy roots can probably be increased in this way.