Lindley said many years ago: "If well directed, priming is one of the most useful, and if ill directed it is among the most mischievous, that can take place on a plant." It is too common to wait until undesirable habits of growth are apparent and then set a time for a general thinning out and shaping of trees and shrubs. The effect of this severe pruning is to secure a heavy growth of new wood in moist climates, and in dry interior climates it often starts blight, sap-stagnation, and sun-scald on the south side. The general rule for all pruning is simple: it should begin as trees or shrubs are received and heeled in (119. Securing and Caring for Nursery Trees) and be continued lightly each year until they reach, in the case of apple-trees, the "heading-back period" (144).
Always keep in mind the aphorism of Professor Bailey: "Trees which are alternately neglected and heavily pruned are kept in a condition which is apt to be fatal to the best productiveness."
After setting grafts in nursery, if more than one bud starts from the scion rub off the surplus as soon as observed. As growth advances, clip the lower side branches, but in no case rub off the leaf-bracts or small clusters of leaves with which the stem is clothed. Budded trees on strong stocks may be permitted to form a head the first season where low-stemmed trees are desired. In this case keep the side limbs clipped on the stem, but keep in mind that the stem leaves give stocky growth and the large stem a top-heavy growth. The second year the top is formed on root-grafted trees, using care to secure a central ascending stem with radiating branches (27. Proper Mode of Branching) without forking. The second season also the leaves should be left on the stem to give the required stocki-ness. The needed sprouting at the crown and cutting away the branchlets that appear on the stem, together with shaping the top, are the main essentials of pruning in nursery.
Where light pruning is done in nursery or young orchard the best time is when the leaves are about two thirds grown. This begins the period of active cell-growth and favors the rapid healing of the wound. While it is true that pruning in the season of growth is theoretically a check on circulation and subsequent growth, yet on young, excitable trees under cultivation the light pruning at one time amounts to little in checking growth and is far more than offset by the rapid and smooth healing of the wounds. But the best time for pruning varies as applied for varied purposes.
The dormant season is essential for cutting back growing wood severely, removing large limbs, or where stunted trees are cut back to secure vigorous new growth, and where stunted branches are cut back to secure active growth and larger leafage. Summer pruning checks growth and tends to lower vitality. Yet it often is practised to hasten the fruit-bearing period of orchard trees and to give more bearing wood on shrubs that blossom on the preceding year's growth.