The selection of plants for hedges forms one of the most interesting subjects in the study of use of plants. More often than for any other purpose trees and shrubs for hedges are selected either from an economic or an aesthetic point of view. It may be desired to have a hedge for its beauty, or it may be desired for the purpose of a screen, a windbreak, or as a definite barrier.
Many times it is desirable in the development of hedge plantations, especially those which are more than the average height (three to four feet), to develop a hedge which will retain its foliage during the winter months. This may be desirable for two reasons: first, to provide a barrier and at the same time a screen against objectionable views and to secure privacy, and, second, to lend interest to a winter landscape because of the foliage effect. The only effective hedge barrier which holds its leaves during the winter is one composed of conifers. Broad-leaved evergreens are not desirable for this purpose; mainly because they are not sufficiently compact in their habit of growth to meet the requirements of a hedge, and they do not lend themselves to shearing. The evergreen hedge which is planted for the purpose of providing a complete screen, and requires a normal growth of the foliage, should seldom be planted in the heavy shade of large overhanging trees. Hedge plants which are selected as barriers or screens should be close growing and compact in habit. Many among them are thorny in character, thus making passage through them very difficult.
Hedges which are planted for barriers and which do not hold their leaves during the winter are usually valuable mostly for their summer effect. They are seldom planted for the purpose of a screen, for such a screen is desirable during the months of the year when the foliage is not present.
Many flower gardens, especially large rose gardens, have been much enhanced from a landscape viewpoint by the presence of low-growing, compact hedges which accurately define the outline of the various beds and emphasize the main axial lines of the garden. There is a group of plants from which kinds are selected for hedge purposes, and which lend themselves to frequent clipping and shearing. These are most often used for edgings beside formal garden walks, pools, and beds of planting. The ideal hedge for this purpose is one which requires a very small amount of pruning in order to maintain its close, compact habit. It is therefore necessary to select plants for this purpose with a careful knowledge of the natural habits of growth of the mature plants and to use such plants for hedge purposes, rather than to endeavour by severe pruning to adapt larger growing types to such uses. Such hedges are usually maintained from six to twelve inches in height and should be planted at least twelve inches away from the edge of any garden walk in order to provide ample width for the hedge to spread as it matures.
Hedges for the purpose of windbreaks and solid screens are composed almost entirely of trees which are more or less compact in their habit of growth and will continue to develop while planted in a crowded space. Considerable good judgment should be used in locating a windbreak which is likely to act as a snow trap also, because the great drift of snow which accumulates behind a large windbreak may prove a nuisance in the early spring by lying deeply on the ground long after the land under it should be thawed out and ready to use. This drift may also break down small and brittle trees and shrubs and do more damage than good. For this reason, on the open prairies of the Dakotas it is often found necessary to locate windbreaks as far as one hundred feet to the windward of the buildings or road which are to be protected, because a strip approximately ten times its height is affected by a windbreak. This is shown by the snow lying drifted for this distance to the leeward after a heavy snowfall, accompanied by a driving wind.
Some thirty years ago, L. H. Bailey gave the following rules for planting windbreaks (Garden and Forest Vol. 1, page 46). While primarily intended for orchardists they are well worth considering to-day by anyone who is going to do such planting on a large scale for ornamental purposes.
(1). The windbreak should not obstruct atmospheric drainage.
(2). The windbreak should never be dense enough to force the buds on fruit trees in those localities which are subject to late spring frosts. (3). As a rule, in localities where atmospheric drainage will not be severely checked, the windbreak should have a comparatively dense bottom, formed by undergrowth or low-branched trees. (4). Native trees and shrubs are preferable for windbreaks.
To these rules it might be added that, while a single row of plants is often desirable, it does not take care of the contingency that arises when one or more plants die. It is consequently preferable to plant two or more staggered rows of plants which thus do not require to be planted so closely and for that reason are more likely to survive a long while and retain their lower branches. It is possible to gain a good ornamental effect also by combining evergreen trees with harmonious deciduous ones, such as hemlock or spruce with birches and maples. This type of planting is often called a "shelter belt" and when a considerable number of evergreen trees are used a pleasing effect is secured the year round, and large numbers of birds will be found to be attracted and held, not only through the nesting season but sometimes all the year.
Still another use for hedges is that of providing privacy. Most of the shrubs used in this group should be of the tall types, exceeding five feet in height, and should have a compact, heavy foliage. The natural growth of the shrubs should be close and they should hold their foliage during the late summer and early fall. Some of the shrubs which are best adapted for this purpose are the rose of Sharon, common buckthorn, and the European beech, the foliage of which does not develop until the latter part of the spring.
It is often desirable to select plants which will serve as hedges in the bleak exposures of lake fronts and ocean shores, and also in the Canadian northwest. These plants should be hardy under all severe climatic conditions of the northeast and the Canadian northwest. Most of the plants which have been suggested for this group have been found growing normally under the most severe conditions of climate and exposure.