TYPES OF STONE TO USE
Limestone has already been mentioned as good rock for a picturesque rock garden. For a long time it was regarded by some as almost indispensable for the making of a first-class rock garden, but this somewhat unreasonable preference has largely disappeared. It is recognized that the kind of rock is actually less important than the manner in which the rock is used.
The chief kinds of rock available are limestone, sandstone, tufa, granite, and artificial cement lumps. Each has certain advantages.
Limestone, if weather-worn, that is exposed to the air in its natural home so that it has become partly overgrown with moss or lichen, or water worn, is extremely picturesque. Because of its porous nature, it forms a congenial plant home, and roots of rock plants frequently penetrate the actual rock. Landscape effects are very easily obtained with such rock, and the greyish colour tones generally with almost any kind of garden. It very quickly assumes a weathered and aged appearance, giving the rock garden an established look after only a few months.
Sandstone, of various colours, is quarried in many districts. It is very porous, and therefore a fine home for plants, particularly in districts where the soil is inclined to be dry. It weathers to an established appearance almost as quickly as limestone.
Tufa, the volcanic rock resembling the pumice stone of commerce, is also very satisfactory on account of its porousness. Its colour is dark grey, but the pleasing striations often found in limestone are mostly absent from tufa. It does not rapidly become moss and lichen covered; nevertheless, it is not unattractive, and many plants like it particularly well. It should certainly be used in the districts where it is obtainable at cheap rates.
Granite is not approved by rock gardeners who grow the rare alpines. It is hard and non-porous, and in large lumps gives an extremely rugged and picturesque air to a garden. Beautiful effects can be obtained by massing granite at the sides of a stream, which are planted with sweeping drifts of showy carpeters. It should not be despised, but it should be employed with discretion.
USE OF CEMENT IN ROCK GARDENS
Cement lumps are looked down on by some. Nevertheless, I have seen excellent rock gardens made for the most part of home-made concrete, which in the hands of an artist can be extremely attractive. From the viewpoint of the health of the plants, nothing can be said against the use of cement, except where certain calcifuges or lime-loving plants are concerned; and from the viewpoint of cheapness and availability everything is in its favour. The owner of a large garden and a small purse may be excused if he prefers to make cement rocks as and when his funds permit, instead of laying out large initial sums on imported rock.
To make useful cement blocks for a rock garden, all that need be , done is this. Take one part of Portland cement to four parts of coarse sand, being careful to mix them well together before any water is added. Then use enough water to make a rather sloppy mixture.
Excavate rough holes in the ground, in some available vacant corner of the garden. Drop in a layer of the mixed concrete, and then set on it any large stones, old tins—these not beaten flat, as for paths, but with the lids on, so that they remain hollow—or any other rough material. Then add more concrete, so that these tins, etc., are quite covered, leaving the surface rough and lumpy.
Gravel or rough small stones can be mixed with the concrete, and if desired, some of the coloured cements can be used. Colour should not be too lavishly employed, however, or it will add to the artificial appearance, which is what you want to avoid.
Leave the lumps untouched for a few weeks, then dig them out and make more. It takes very little time, just a few odd minutes now and then, to make enough lumps to build a small rock garden.
If just a limited amount of real stone is available, it often pays to use such cement lumps in association with the more picturesque rocks. They can usually be built up so that only the natural stone is visible after a short time, quick carpeting plants being set wherever the cement is visible, so as to hide it. In fact, as remarked previously, the manner of building is actually much more important than the kind of rockery stone used.