When a rock garden is constructed on a dry hill it should be provided with a sub-irrigation system, as many Alpine plants require a deep, moist soil. This is very much more important than the shade or partial shade so often thought necessary to their well doing. Such a system of irrigation may be economically installed by running a two-inch agricultural tile along the top of the slope, twelve inches below the surface. The bottom of the trench should be inclined toward the rockery and filled with crushed stone or clean cinders, placed around the tile. The tile should be connected with a rubber hose at the faucet. The use of valves is thus done away with, such as would be necessary if the line was directly connected with the water supply system. Where the rock garden is close enough to the house, water may be applied directly by hose, but the irrigation method is to be preferred.
An interesting use of rock and Alpine plants is in the planting of dry walls (Figs. 172 and 173) and particularly when such walls are built as retaining walls in the flower garden. A great variety of plants may be had for such a purpose and the list greatly enlarged if a water supply is near by to help out in very dry periods.
When it is purposed to plant the interstices in dry walls, the walls should have a batter (Fig. 174) of three inches to the foot, or one foot in a wall four feet high. The stones should be set at a right angle to the inclined line. The pockets left for plants should continue directly or indirectly through the wall so that the soil will be in direct contact with that at the back of the wall. These soil pockets should be filled as the wall progresses and the soil held in place by tough pieces of sod until ready for planting.
For wall gardens it is advisable to install a sub-irrigation system (Fig. 174) to supply abundant moisture to the wall plants; this may be done by installing a perforated wrought iron pipe along the top at the rear of the wall; the perforation should be a thirty-second of an inch in diameter, spaced at intervals of one inch. The pipe should be placed with the holes at the bottom, on a bed of crushed stone, seven inches below the surface, and covered with three inches of cinders, allowing four inches of top soil above. The water supply should be controlled by a valve set flush with the grade, in a neat box, and located at a convenient point.
ANOTHER FORM OF ALPINE GARDENING.
Fig. 175. - Rustic garden steps such as these should have six-inch risers and treads of not less than fourteen inches. The larger stones should be used for the base, and the soil well rammed to prevent settling. The treads should be tied into the cheek walls. In rustic work the cheek walls may be left without coping.