One of the most charming features of our northern landscapes, especially through the northeastern United States where such a great variety of deciduous trees are indigenous, is the autumn colouration of the foliage. During the normal growing seasons, when ample rains have fallen and the trees are full with sap at the end of the season, the autumn colours are much more vivid than during a dry season.
It is extremely important in designing landscape plantings that thought should be given to the fruiting effects during the fall and winter months and also to the autumn colours of the foliage of our trees and shrubs. A small touch of colour in the border shrub plantings of the average lawn is usually the difference between an uninteresting and an interesting planting at that season of the year. Every planting should have introduced into it a few plants which, if not valuable for their fruiting and flowering characteristics, will produce a touch of colour as accents to relieve the monotony of the dead greens and browns during late September and October.
Autumn colouration is the result of an interesting physiological function within the plant. Contrary to the average opinion that autumn colour is the result of sharp freezes, the presence of frost serves but in a slight degree to hasten this colouring, because the degree of cold necessary to produce a freeze further retards the flow of any sap. The production of the vivid autumn colours is caused by a devitalized or increasingly dormant condition of the plant. The lessened flow of new sap to the leaves, caused by the formation of corky tissue at the extreme base of the leaf to cover and protect the leaf scar during the winter months, results in a chemical reaction of the acids within the leaf itself. The results of this chemical reaction are evident in the autumn colours. The factors determining the kind and degree of autumn colouration is the presence of different acids within the leaf. These acids are present constantly in their respective types of plants.
It is a peculiar fact that in some trees autumn colouration is much more vivid than in other trees, because of the presence of certain acids the chemical reaction of which produces these vivid colours. The autumn colouration of foliage ranges from the dull browns through the yellows and orange to the brilliant red and scarlet. Some of our trees and shrubs such as the red maple, Judas tree, sourwood, sumac, and sassafras, develop their autumn colours during the early part of the fall. In these trees growth [stops early in the season and the wood matures more quickly. There is one group of plants such as the Virginia creeper, Thunberg's barberry, red oak, high-bush cranberry, and arrow-wood, in which the autumn colouration is produced shortly after the early trees have shown their autumn colours. The American beech, golden bell, scarlet oak, and burning bush, and some others mature and ripen last of all and are included in the group with late autumn colouration. With the great opportunity for selection of trees and shrubs from these three well-defined groups a succession, if it may be so termed, of autumn colouration may be obtained, which will supplement and add interest to the fruiting effects produced by a well-selected group of plants valuable for the characteristics of their fruit.
One has only to observe our native trees and shrubs in the northeast to appreciate the wonderful colour effects which are produced during the autumn months and are evidenced on all of our hillsides. So seldom are shrubs selected for landscape grouping with a.specific intention of emphasizing their value because of autumn colouration, that we find there are few good illustrations, and those at scattered intervals, showing the autumn colours and their values in the settings of our lawns.