While many trees and shrubs in the hands of expert designers and gardeners may be used interchangeably for either formal or informal effects, the fact still remains that there is a group of plants which are best adapted for use to produce the heavier and more compact formal effects. The effect of formality is obtained by emphasizing geometric lines or surfaces. Plants which are upright, slow-growing, and more compact in their habit, are better adapted for this purpose than plants which are more vigorous in their habit of growth, more spreading, and looser in texture and therefore less apt to retain a consistent, definite form.
The expert may select plants from Chapter XVII (Plants For Natural, Informal Effects) and in many instances produce an effect equally as attractive as that produced by plants in this group. The possibilities of failure, however, are much greater, and the subsequent necessity of pruning to retain approximate forms is much greater.
We speak of heavy formality as a contrasting term to plants which are loose. Compact may be a better word. It is often necessary to develop border plantations surrounding lawns filled with a feeling of formality, because of numerous axial lines, and also to surround or border definite formal garden areas with trees and shrubs. To be successful these masses of plants must lend themselves to this formal effect. They must be such as can be kept within definite limits with the normal amount of pruning. The European hornbeam, the red cedar, pyramidal arborvitae, and tartarian honeysuckle are excellent illustrations of this type of material. A so-called formal effect in a border planting is not necessarily produced by the use of trees. It may be equally well produced by shrubs depending upon the scale of the landscape setting. It is not necessary either to resort to the use of evergreens, although there are certain types of architectural details beside which deciduous plantings appear "weak." Climatic conditions being favourable for a normal growth of the type of material best suited, the decision to use evergreens or deciduous material will be governed by the effect which is desired.
Topiary work as a type of planting producing formal effects is the extreme of artificial methods. Trees and shrubs which are selected for this purpose are included almost without exception in the group known as "evergreens and broad-leaved evergreens," such as the yews, holly, and boxwoods. The hawthorn and the beech are the marked exceptions to this general rule, and are plants capable of severe pruning to produce artificial and fantastic shapes. These plants must lend themselves readily to frequent and to severe prunings in order to produce these forms. While topiary work as a matter of design is extremely limited in its application, there will often arise situations in which this extreme and violent treatment to produce the artificial forms in plants is justified. Topiary work has at times been very appropriately termed verdant sculpture. It is nothing more nor less than sculpture in plant forms so far as plants will lend themselves to details of such experiments. All of these plants which are especially adapted to topiary work are extremely slow growing and long lived. While many fantastic forms can be developed from such plants as the privet and hawthorn within a comparatively short period, the rare, more perfect, and permanent forms are usually the result of using the boxwood or yew. Most of these trees which are adapted to topiary work, especially the pyramidal form of topiary work, are upright growing, single-stemmed specimens. Plants which are adapted to these effects must also be of a compact texture with foliage evenly developed to a point close to the ground.
There are many deciduous plants which, while not being adapted to topiary work, are adapted to close shearing to produce formal effects. Many experts do not realize that there are various species of the same genera which lend themselves much more effectively to close shearing in definite forms than other species of that genera. The Japanese privet is much more effective in the lower hedge of two to four feet, because of its tendency to "mat," than the.Amoor River privet, which has a tendency to make long growths. All of the deciduous plants in these groups are comparatively slow in growing habit. They have a tendency to frequent branching and a further tendency to throw out new growths from dormant buds when the ends of the existing branches are removed.
While there is a considerable list of plants which are adapted for growing in tubs, as frequently seen, for accent points in a formal garden or on a terrace, the amateur should best confine himself to the Japanese laurel, the evergreen evonymus, the greenhouse hydrangea, pyramidal arborvitae, and the boxwood. Most of these should be transferred during the winter months, preferably to a cold cellar or to a cold house, and even those which are semi-hardy, if left out of doors, should be carefully boxed and protected.
Plate XXII. To develop a successful rock garden, not only must the stones be well placed, but the plants must be selected to produce an effect in keeping with the scale of the garden; otherwise the effect will be that of a collection of stones which overpower the garden picture, as shown above. (See Chapter XV (Plants Valuable For Use In Rock Gardens, In Japanese Gardens, And In Wall Crevices))
Plate XXIII. To few of us does the term "wall garden" convey a definite impression. Yet how frequently the opportunity comes, even in a small way, to change a wall of rock to a wall of flowers and foliage. In this photograph we see Scotch pinks, creeping phlox, golden tuft, tunica, and other similar plants used to excellent advantage. (See page 139, group XV-C)
Perhaps the most interesting groups of trees and shrubs for formal effects are those which are valuable for use in pleached allees. This feature in the design of large estates has not yet reached its height and will become more popular with the development of landscape design as applied to American estates and gardens. The plants of this group must be resistant to disease and insect pests and they must be able to thrive under conditions of severe pruning. The one most important requisite is that they shall be long lived and not easily broken by winter storms. The texture of branching must be close. To use for pleached allees trees, such as the birches, which are short lived and which always begin to deteriorate at a time when the allee should be most picturesque and at its height, is landscape folly. It takes years, five to eight years, to develop a pleached allee so that the tops will come together. To endeavour to hasten the growth of plants by excessive fertilization during the first two or three years will have a tendency to split the bark and to expose the trunks to severe injury from freezing and rotting. These plants should be of a spreading habit of growth as contrasted with the columnar habit of growth desired for open allees. While these specimens are planted at intervals of eighteen to twenty-four inches in rows, it often becomes necessary to interplant with the smaller specimens which will serve as fillers for the base. The normal distance between rows on either side of a pleached allee is six feet to eight feet. It is most advisable to train these plants to the pleached form by the use of iron pipe and wire. This can be done by a skilled gardener, by constant attention and the frequent use of pruning shears.
Trees and shrubs for open allees must meet the one requirement of being close growing and columnar in their habit. An open allee may be developed with rapid-growing material as well as with slow-growing material, and the time required is less than two-thirds as long as the time required to develop a pleached allee of the same height. Six to ten years may be required to develop an open allee eight to ten feet in height. The scale of the allee, whether wide, with a tall border on either side, or narrow, with a lower border, governs the type of material which should be selected. Here again, with such plants as the thorns and elms, a larger specimen may be used with a high head, and the smaller specimens may be planted between and on either side to produce the mass of foliage at the bottom. An illustration of this is shown in plate No. XXVIII on Page 190. This interesting open allee of thorns and flowering dogwood is planted according to the following measurements. The distance between the middle line of each row of thorns is twenty-two feet six inches. Each row was originally planted with high-headed thorns at a distance of four feet six inches apart in the row. Equally spaced at a distance approximating one foot six inches apart, small specimens two feet to three feet high were planted in a single row at a distance of one foot six inches on either side of the main row of thorns. These small thorns were for the purpose of producing a foliage effect beginning at the ground and extending into the higher heads of the larger thorns. The width between the rows of flowering dogwood is eight feet and the distance between each flowering dogwood plant in each row is eight feet. The width of the walk in this picture is four feet. It is very essential to use types which have a branching habit to the extreme base of the main trunk if a perfect open allee is desired.