Only a few years ago the advice given in the fruit books of Europe and America was to thin out the tops of bearing fruit trees to let in the sun. Charles Downing was an innovator when he wrote in 18G9: "Every fruit tree grown in the open orchard or garden as a common standard should be allowed to take its natural form, the whole effort of the pruner going no further than to take out all weak and crowding branches, those that are filling uselessly the interior of the tree where their leaves cannot be fully exposed to the light and sun, or those which interfere with the growth of others."At the present time our advanced horticultural writers say that the necessity for much pruning depends largely upon climatic peculiarities. As an instance, Professor Card says: "The Eastern grower finds it necessary to thin his trees and admit light and air to produce fruit of high color and good flavor. The Western man, however, finds that under his conditions of intense sunlight and low humidity, fruit will develop color regardless of such conditions. The Eastern grower, if he neglects this, may find his fruit suffering from attacks of fungous diseases; but in general fungi are less troublesome in the drier climate of the plain, so this reason loses its force."
Sometimes reasons for varied systems of pruning are found on two sides of a mountain range. As a marked instance, Professor J. A. Balmer, of the State of Washington, gives positive reasons why summer pruning, pinching, and even root-pruning of orchard trees are required west of the Cascade range to check exuberant growth. On the other hand, east of the range winter pruning is advised, as the trees are inclined to premature bearing and consequent stunted growth, which is corrected by the severe winter pruning or cutting back, which favors vigorous growth of young shoots.
In the prairie States fruit trees make quite rapid growth, and as stated by Professor Card fruit will color and develop in the interior of thick-topped trees, and it often happens that orchard fruits of all kinds exposed to the sun are scalded on the exposed side by sun heat. Young trees properly shaped when started in orchard are mainly grown on the let-alone principle, except in the way of cutting out dead twigs and such weak interior growth as cannot have exposure of leaves to the air and sun. The thick-top principle of fruit-growing is sustained by the fact that where the new growth of young orchard trees is cut back in autumn for three or four years in succession the top becomes very thick on the outside. Yet these trees bear finely colored fruit beneath the thick exterior and better average crops than thinner-topped trees of the same varieties. But in quite moist regions of the East and South where moss grows on north-exposed house-roofs, or Spanish moss hangs down from tree branches, the old plan of thinning out to let in the sun will always be practised. 144. Heading Back Old Apple- and Pear-trees. - With the thick-topped plan of the prairie States and the arid regions, or the more open-topped plan of the moister belts, the bearing wood as the trees get old becomes far removed from the center of the tree. The time has now come for severe cutting back of the top, as is practised with old peach-trees. With the apple and pear in the dent-corn belt this severe heading back is an epoch in the life of the tree, and the shock is too severe if the cutting is all done at one time. The safest plan is to cut one half the tree at a time, deferring the work on the other half to the next year, when the growth of new shoots is a relief from the injury liable to occur from sap-pressure. If two years are taken for the severe heading back of old orchard trees in connection with culture and manuring, new growth starts vigorously from the cut-back stubs and the water-sprouts that start in the interior, and the trees may be said to be rejuvenated. During the period of cutting back little fruit will be gathered, but the bearing stage is soon renewed with increased size and perfection of fruit and the longevity and profit of the trees are much increased.