This is a broad subject and yet it has been well covered by numerous bulletins and books. There are a few important questions, however, which should be decided very definitely in the minds of those who are selecting trees for use as street plantings. There is a group of trees most of the types of which are entirely hardy under all conditions. There is another group of trees which should seldom, if ever, be used on streets, and there is a third group of trees which possess some real value for street planting; but they should be selected only with a thorough knowledge of the conditions under which they are to be placed.
In general, trees which are selected for street planting should be symmetrical in character, they should be long lived, and they should not be readily susceptible to injury from insects and smoke.
There is a certain group of trees specimens of which can be safely selected for street planting, either in city or suburban districts. This group comprises such trees as the sugar maple, red oak, European linden, and American elm, the last of which is best adapted to planting on narrow streets because of its high-headed characteristic of growth. Though it becomes very tall its vase-form enables it to reach above dwellings that may be not far back from the street and to leave open a vista down the street axis. The low, compact-headed types of trees, such as the sugar maple, pin oak, and the linden ought to be planted only on wide streets. While the use on narrow streets of such vase-shaped trees as the elm allows for an unobstructed vista, the use of such trees as the pin oak and linden, unless the lower branches are severely pruned, has a tendency to "choke up" and to obstruct the vista.
A certain few trees should rarely, if ever, be used in street tree planting. This list comprises such trees as the horse chestnut, box elder, black locust, willows, birches, and poplars. Birches and locusts are too short lived. The horse-chestnut and the box elder are littering in their habit. The poplars are subject to disease as well as being short lived, and the willows are not adapted to shade tree purposes, even in their habit of growth. In spite of the fact that there are some excellent trees as shown in the first and second lists in this chapter, well adapted to street and avenue planting, and thoroughly tested through a period of years, yet many individuals responsible for the selection of these permanent assets or detriments to the public streets will still select trees such as those which are included in the third list of this chapter.
Trees such as the maidenhair tree, tree of heaven, oriental plane, and pin oak should not be used for street tree planting without a thorough knowledge of the conditions under which they are to grow and the conditions to which they are best adapted. The pin oak and the maidenhair tree are tall, pyramidal trees, which should be used only on wide streets in a heavy soil, and the maidenhair tree should never be used for street and avenue planting except in the less severe climatic conditions. The tree of heaven has a vigorous habit of growth and is an excellent tree in the smoky, congested sections of our cities where shade trees are required. Before trees in this group are used some of the important bulletins and books on our city street trees should be consulted for further information. (See Bibliography.)
Where an avenue is of such length that it passes through two or more radically different soil types care must be exercised or the trees on one soil will not grow as fast or luxuriantly, as upon another soil. This will result in an avenue of uneven height and spread in the tops of the trees and thus spoil an otherwise successful planting.