As regards aspect, one that is built on each side of a narrow path running north and south, does very well, but as this may be impossible in a small garden, a corner rockery built high in the form of a triangle and facing south-east, can be made extremely pretty, as I know from experience. Where the rockery is in the shade, no overhanging trees must be near, if choice Alpines are expected to live there.
The material may be either slabs of grey stone as at Kew, or the more easily obtained “clinkers.” Clinkers are really bricks spoiled in the baking, having all sorts of excrescences on them which unfit them for ordinary building purposes; they should always be ordered from a strictly local contractor, as carriage adds considerably to the cost.
The soil should be a mixture of peat, sand, and loam; no manure should be incorporated, the “pockets” for special favourites and plants that have individual wants can be filled in at the time of planting. One advantage pertaining to a rockery is that many plants which quite refuse to thrive in a border will grow and flourish there, and the attention they need is less troublesome to give; in fact, it is a delightful form of gardening, especially for a lady, as there is no fear of the feet getting dirty or wet, and a trowel, not a spade, is the chief implement used. A small piece of turf, just a few feet wide, at the bottom of the corner style of rockery, is a great set-off, and a vast improvement on a gravel path.
SUITABLE PLANTS FOR A ROCKERY. The following are some of the best flowers for a rockery. The aubrietias are very pretty little plants, having creeping rosettes of greyish-green leaves, and a perfect sheet of mauve or lilac bloom about April. The effect is greatly enhanced when planted so as to fall over a stone or brick; indeed, it is for those things which are so easily lost sight of in a border that a rockery comes in; they can be closely inspected there without much stooping.
The arabis is a pretty plant, somewhat like the aubrietia in habit and time of flowering; hence, where only a small selection can be made, it might be left out, as it is a trifle coarse. Such a term could never be applied to the androsaces, which may be numbered among the élite of rock plants; they are evergreens, and do not exceed six inches in height; they bear tiny but very bright flowers, varying from rose in some species to lavender in others.
APENNINE GEMS. Some of the alpine anemones are lovely, notably A. appennina, which has sky-blue flowers that open out flat on very short stalks, surrounded by pale green denticulated foliage. A. blanda is much the same, save that it flowers a month or so earlier; they are spring-blooming plants, and like moisture and shade, and will not do at all if subjected to much hot sun. These and many similar plants can often be planted on a rockery facing south-east (which aspect suits so many sun-loving plants), by arranging bricks, stones, or small shrubs, so as to shelter them from its hottest rays. Aquilegias, mentioned in the list of border plants, look quite as well on a rockery, if moisture can be given them, as their flowers are so delicate, and the leaves so fragile and prettily coloured, especially in the early spring. The blue and white A. cærulea, from the Rocky Mountains, is a gem, and the scarlet kinds are very effective.
For forming close green carpets, arenaria balearica is most useful; it creeps over rocks and stones, covering them completely with its moss-like growth, and hiding any hard, unlovely surfaces. The campanula family is a host in itself, many of the smaller varieties looking better on a rockery than anywhere else. Some of these tiny bell-flowers have, however, the very longest of names! C. portenschlagiana, for instance, is only four inches high, and a charming little plant it is, and flowers for months, beginning about July. The blossoms are purple-blue in colour, and continue right into November, unless very hard frosts come to stop it. C. cespetosa is another variety well suited to rock-work, as it is even smaller than the last.
The alpine wall-flower, cheiranthus alpinus, is a very choice little plant; it has creamy-yellow flowers, borne on stalks a few inches high, and, though each individual plant is biennial, they seed so freely that they are practically perennial. A light, dry soil and a sunny situation suits them; they will even grow on old walls, and very picturesque they look perched up on some mossy old ruin.
An attractive rock plant, though rarely seen, is chrysogonum virginianum; its flowers are creamy-yellow, and grow in a very quaint manner; this plant blooms the whole season through. Plants of this character should be noted carefully, as they help to give a rockery a well-furnished appearance, so that one always has something to show visitors.
For warm, dry, sunny nooks rock-roses are the very thing; where other plants would be burnt up, the cistus flourishes, for it requires no particular depth of soil. C. florentinus (white) and C. crispus (dark crimson), are two of the best.