The habit of growth of a variety has much to do with its pruning. Some varieties of apples are upright, others are spreading growers. Climate and locality greatly affect these habits of growth. So also the habit of a young tree often differs from the habit of the same tree in old age. The tendency is for a tree to continue its growth from its uppermost or terminal buds. Although the heading in of new growth checks this upward tendency and throws the energy of the tree into the development of lateral and dormant buds, nevertheless the pruned tree soon resumes its natural upward growing habit.
Plant food is taken up by the minute tree rootlets in solution and carried to the leaves where it is elaborated and then returned for use to the growing tissues of the tree. Whenever there is any obstruction above a bud the tendency is to throw the energy of the branch into a lateral bud, but if the obstruction is below the bud the branch merely thickens and growth is checked. When too heavy pruning is practiced the balance between the roots and top is disturbed. This usually results in what are commonly known as "suckers." These are caused by an abnormal condition and while they may be the result of disease or injury to the tree, they are often of great value in restoring or readjusting the proper balance between the roots and top.
Pruning a tree is a way of thinning the fruit and a good one. It may sometimes be used to influence the bearing year of trees like the Baldwin, which have an alternate bearing habit, but this is a more theoretical than practical method. Fruit bearing is determined more by the habitual performance of the tree than by any method of pruning, and this is especially true of old trees. It is easier to influence young trees. Conditions which tend to produce heavy wood growth are unfavorable for the formation and development of fruit buds. A quiescent state is a better condition for this.
Reasons for Pruning.—With these fundamental principles in mind we may safely outline a method of pruning an apple tree. As the desired end is different so will the method of pruning a young tree differ from that of an old one. There are five important things for which to prune a young tree, namely:
1. To preserve a proper balance between the top and root at the time of setting out. This usually means cutting off the broken and the very long roots to a reasonable length and cutting back from one-half to two-thirds of the growth of the previous season.
2. To make the top open in order to admit the sunlight freely. In the humid climate of the Northeastern States, it is usually advisable to prune a tree so as to have a rather open top. This is necessary in order properly to color and mature the fruit.
3. To regulate the number of limbs composing the top. Probably three branches well distributed on the trunk would make most nearly the ideal head, but as these cannot always be obtained the best practice is to leave from three to five branches from which to form the top.
4. To fix the branches at the proper height from the ground. This is more or less a matter of opinion, some growers preferring a low and others a high head. The character of the tree growth, the method of culture, and the purpose of the tree whether temporary or permanent greatly influence the height of the head. An upright growing variety should be headed lower than a spreading one. Trees kept in sod or under extensive methods can well be headed lower than those under more intensive culture where it is desirable to carry on cultural operations close around them. Permanent trees should be headed higher than temporary trees. Apple trees should seldom be headed lower than a foot from the ground, nor more than four feet above it. For upright growing varieties intended as permanents, the writer prefers three to three and one-half feet and for more spreading varieties four feet; while for temporary trees eighteen inches should be a good height.
5. To do away with weak crotches and to remove crossing or interfering branches. A crotch formed by two branches of equal size, especially when the split is deep, is a weak crotch and should be avoided. Strong crotches are formed by forcing the development of lateral buds and making almost a right angle branch from the parent one. All branches which rub each other, which tend to occupy the same space with another, or which generally seem out of place, are better removed as soon as any of these tendencies are found to exist.
Ideals in Pruning.—The general method of pruning the old trees and the ideal in mind for it will also influence the pruning of the young tree, especially the shaping of it. Once determined upon, the ideal should be consistently followed out in the pruning of the tree as it becomes older. As the tree comes to bearing age it will be necessary to prune somewhat differently and for other purposes. These we can conveniently consider under six heads: