The wall need
not be a straight line; it is better that one end should describe a
curve, and rocks at the base may give it further irregularity. Yet it
can never quite lose the air of man's handiwork. The prime object of the
gardening on it is to reduce this air to a minimum.

The way to make a wall garden is to build a dry wall of rough
stones--that is, a wall without mortar. Instead use soil and pack it
tight in every crevice as well as behind the stones, which should be
tilted back a little to carry water into the soil. This tilting may be
accomplished with small stone wedges. The best kind is a five-foot
retaining wall, as there is then a good body of soil behind to which the
roots can reach out through the crevices. But a double-faced wall may be
made, if the situation demands it, by constructing parallel lines of
stones and filling in solidly with soil.

[Illustration: Planting plan of dry wall, the dark portions representing
the chief earth-filled crevices. The plants are: 1--_Arabis albida_;
2--_Alyssum saxatile_; 3--House leek (sempervivum); 4--_Viola tricolor_;
5--_Armeria maritima_]

[Illustration: A wall garden planted in colonies--the better way. If not
too vigorous of growth, vines may be planted as shown here at the base]

Although the face of the wall in either case may be strictly
perpendicular, it is better that each layer should recede a bit.
Construct it after the manner of the rock garden, laying the stones so
that the top will be level, or approximately so.

[Illustration: Dry wall for retaining bank. Cross-section, showing
crevices, soil runs and tilting of rocks]

In planting also, follow the same rules. It is better to plant as the
work progresses. Either plants or seed may be used. If it is seed, press
carefully into the soil in the front of the crevices. Small seed may be
mixed in thin mud and this plastered on the soil. For a tiny crevice
make a pill of the mixture.

[Illustration: Double-faced dry wall. A few rocks are used with the soil
filling and here and there one on top of it]

The range of reliable plants that do not call for special care is not
great so far as the crevices are concerned. All the stonecrops, the
house leeks, _Arabis albida_, red valerian (_Centranthus ruber_),
aubrietia, _Alyssum saxatile_, snapdragon, wallflower (_Cheiranthus
Cheiri_), Kenilworth ivy, _Viola tricolor_, _Dianthus plumarius_, and
_Dianthus deltoides_ are all very serviceable. Behind the wall,