The rubble stone wall of field boulders is most satisfactory, and, when partly covered by vines, is highly picturesque. The dry wall may also be used to enclose the garden, especially in locations where good rock is to be had on the ground. When used for this purpose they should batter or break back from each side, vines to be planted along the full length at irregular intervals. The vines should not be allowed to cover the entire wall. Rather, for reasons of contrast, and to show decidedly the limitations of the garden and the formidableness of the retaining and supporting walls, quite good stretches of it should be left uncovered.
Rubble walls (Fig. 121) are particularly good where a retaining wall is required to maintain an embankment. The dry wall is less expensive than one laid in mortar and gives a greater latitude for ornamental treatment. Quarried stone or stone gathered on the property may be used for this purpose. The larger the stones the better. If the stones are from a quarry they should be as long as it is possible to secure them.
The dry wall should have a batter of not less than one inch to the foot, and where it is proposed to use Alpine plants in the interstices it is better to have a batter of three inches to the foot. The building of a dry wall for plants is given in greater detail in the chapter on Rock Plants.
The hedges of various plants are much less expensive and fulfil many requirements as a dividing line between lawn and garden. The Privet hedge is the most popular, as its quick growth and dark green leafage form an excellent background in a short period of time. The California Privet (Ligustrum ovalifolium), which is most frequently used, is not hardy in some latitudes; the tops are occasionally killed to the ground in Philadelphia, and instances are reported of the same damage being done in Kentucky. For cold latitudes the variety Ligustrum Ibota is more satisfactory. The Ligustrum Regelianum is an excellent hedge plant where it is desired to have a more picturesque enclosure. This plant is most attractive as a boundary to a wild garden, the lights and shadows being highly contrasted, giving a pleasing variety to this formal feature more in tune with naturalistic surrounding. The variety Ligustrum amurense is much the best variety to use south of Washington; it rarely loses its leaves during the Winter and, in the Carolinas, Tennessee and Georgia it is evergreen.
Fig. 121. - Retaining walls in the garden. How much more effective is this treatment than the slope generally adopted when the garden is on more than one level. - See pages 144, 146.
The Hemlock Spruce (Tsuga canadensis) (Fig. 122) hedge has been little used of late years, probably on account of its costliness, certainly not because it lacks beauty of outline or texture. The color is excellent as a background and, after growing to the required height, it is much more formidable in appearance than the deciduous hedges. For quick effect the Arborvitae (Thuya occidentalis) is most valuable. It is practicable to secure specimens of this variety of any height up to seven feet, which is an advantage for instances where it is desired to have an immediate effect. The color is not so good for a background as plants of a darker shade of green but, nevertheless, is recommended as a hedge plant of merit.
The general character of the garden will be improved by using piers at the corners and entrance. It adds dignity to the scenes and defines the outline more clearly. In large gardens, where a long line of hedge is somewhat monotonous and at times irregular in alignment, it is well to construct piers at regular intervals, for variety, and to maintain a more regular line than is otherwise possible.