After studying a compilation of this kind the great range of plant adaptations becomes evident. There is a large group of materials which are hardy under almost all conditions. There are many other types of material, however, too numerous to mention in detail, which are adapted only to specific sections of the country.
The question of the adaptation of evergreens is perhaps the one which may cause the most serious discussion. Evergreens are a group of plants which possess widely varying characteristics, especially of hardiness under different climatic conditions. The evergreens which will flourish in the humid and less severe atmosphere of Long Island will not grow through the windy regions of the Middle West, nor in the extreme exposure of the water-front conditions of the North Atlantic and the Great Lakes Region. Neither will evergreens which flourish in the sandy soils of Michigan grow upon the clay soils of the Middle Western States. Therefore, the question of evergreens has been carefully considered, and an attempt has been made to differentiate the groups and the requirements of each. This subdividing of evergreens for various locations is based entirely upon the normal protection. Many evergreens, as with other plants, will thrive under abnormal conditions if they are nursed and protected during the severe winter months. On the other hand, many evergreens which will withstand the exposures of winter conditions will not continue to thrive when placed in the clayey soils of some of our Middle Western States, where they are subjected to the severe baking and drying out of these soils during the summer months.
The question of whether plants for landscape use should be grouped according to height or according to other characteristics, such as season of bloom, character of growth, soil adaptations, etc., is one that has been discussed to a considerable extent. The writer feels that in the selection of plants as a part of any landscape design, the question of height is of secondary consideration as compared with the natural characteristics and adaptations of the stock under consideration. The selection of plants for specific purposes such as background planting, undergrowth planting, windbreaks, ground cover, etc., implies that, in addition to other important characteristics of hardiness, compactness, and qualities of fruiting and flowering, such plants fulfill the requirements of height automatically imposed by some few groups. Height of plants means little to the landscape designer as compared with the many other important requirements of foliage effects, character of fruits and flowers, soil adaptations, hardiness and habits of growth and their response to pruning operations.
Plate II. This map shows those portions of the United States which because of climatic environment and geographical location possess similar planting seasons. Investigation of zones of similar planting seasons has, to date, not provided complete information to the landscape architect in his planting work. Numbers refer to numbered section on Plate III.
(See page 14, also chapter II (Planting And Seeding Seasons. General Considerations))
In the selection of garden perennials questions of height are entirely secondary to the ability of the plants to produce flowers and foliage of varying qualities, so essential to the success of the garden picture during different months of the growing season.