The variation in adaptation of evergreens has come to be a much-debated question among horticulturists and landscape architects who are called upon to use them. An interesting fact is discovered after some study of evergreens. The hardy types are not perfectly hardy under the varying conditions of climate and exposure in various sections of the country. Therefore, evergreens should be selected for landscape plantings with a considerable knowledge of their ability to withstand local conditions.
The factors which are evidently most injurious to the normal development of evergreens as a group are: (i) sudden variations in climatic conditions; (2) condition of the soil; (3) exposure; and (4) atmospheric conditions in the vicinity of congested city districts. A short discussion of these factors, as bearing upon the growth of evergreens, may be of value.
1. Climatic Conditions. It is a safe assumption that evergreens which are indigenous to sections of country where the climatic conditions are severe will withstand similar conditions in any other section of the country unless the other factors, of soil, exposure, and atmospheric conditions are extremely adverse. An interesting fact is seen in the repeated endeavour to acclimate evergreens, which otherwise are hardy, to the sudden changes of temperature experienced along the shores of the Great Lakes. Evergreens which are perfectly hardy in the severe climate of New York State and New England, and even in Michigan, will not prove hardy when used close to the shores of the Great Lakes.
While no one has given a definite reason for this, it is presumed that the sudden changes of climatic condition are responsible for the failure of many plants. In general, in the selection of evergreens, the more tender varieties should not be far removed from climatic conditions in which they are known to be perfectly hardy, unless the person using such plants expects the inevitable loss which will be experienced during a very severe winter.
2. Condition of the Soil. The question of soil conditions with reference to the planting of evergreens is more important in the clayey sections of the Middle West than in most of the other sections of the country. In the northern portions of the Middle West this is not so evident, because the summer months are not as hot and dry as they are farther south. Even the more hardy evergreens will not withstand the stiff clay conditions of the Middle West during the hot summer months, at which times this clay bakes very hard. Evergreens, to be grown most successfully, should be planted in a loose, sandy-loam or clay-loam soil, and should not be planted directly in a soil the predominating portion of which is clay. Therefore the soil conditions should be examined closely before it is definitely decided to use evergreens, and the list of evergreens shown under IX-B are those which are most hardy and best adapted to the climatic conditions and soil conditions of the Middle West, as proven by years of experience and careful observation.
3. Exposure. If the more tender types of evergreens which do not normally grow under the most severe climatic conditions are selected for use in a section where the climatic conditions are severe, then such plants should be so located that the exposure from the prevailing winds of winter is greatly reduced. It is possible to use most of the evergreens shown in Group IX-A in any section of the country, provided a proper exposure is selected and the proper winter protection is given. Many times this is impracticable, because evergreens are selected to be of equal value during the winter months and during the summer months. There is no windbreak or screen as yet developed for the protection of evergreens against exposure which in itself does not detract to a great extent from the beauty of these plantations during a period of the year when their foliage should be most effective.
4. Atmospheric Conditions. Many evergreens are used in the vicinity of our congested city districts, or in locations where the prevailing winds surround them with an atmosphere polluted with dust, smoke, and gases, all of which are very injurious to most evergreens. Only the most hardy evergreens should be used under such conditions, and in order to keep them in a normal growing condition it is essential during the cooler hours of the day, and at frequent in-tervals, to spray these plants to wash off much of the soot which collects from the atmosphere. While the leaves of evergreens are heavily covered with a layer of cuticle, the breathing pores are very susceptible to clogging from the dust of a polluted city atmosphere.
Plate XVI. Plume-like cypress, naturally a small tree, can be maintained as a compact and a very formal low hedge if given plenty of skillful pruning, and protection in winter. (See page 123, group XII-A-a)
Plate XVII. The Canadian hemlock, when grown from the northern seed and_when well established, forms one of the best windbreaks and barriers for the protection of the garden or orchard wherever drifting snow must he overcome and seclusion also attained. (See page 125, group.XII-C)
Plate XVIII. Upon a spacious lawn effective use can occasionally be made of trees and shrubs possessing symmetrical habits of growth and fine flowering and fruiting qualities. The deutzia is a shrub with these capabilities; but it is seldom seen as a specimen plant. Restraint, however, must be observed and a dotted effect avoided. (See chapter XIV (Accent And Specimen Trees And Shrubs))
Plate XIX. The extended lawn area often requires specimen trees to lend scale and colour to the picture, and it also offers opportunity to display the natural beauty of many of our fine specimen trees. (See page 133)
So much for the discussion of the adaptation of evergreens to these conditions. A list of evergreens is shown in Group IX-C, found through experience to be not generally recommended for use in the Middle West.
Another condition often arises in the use of evergreens. Material is desired for undergrowth planting in shady wooded areas. There are very few kinds of evergreens which will produce anything like their normal density of foliage where they are deprived of a great portion of sunlight. This group of material is shown in Group IX-D.
It is often necessary to use groups of low-growing and refined types of evergreens to provide mass effects on private lawns and about residences, to be of value during the winter and summer months. A list of evergreens available and adapted to this use has been generally outlined. They must be of the more slow-growing types if overcrowding within the first two or three years after transplanting is to be avoided.