This section is from "The Horticulturist, And Journal Of Rural Art And Rural Taste", by P. Barry, A. J. Downing, J. Jay Smith, Peter B. Mead, F. W. Woodward, Henry T. Williams. Also available from Amazon: Horticulturist and Journal of Rural Art and Rural Taste.
We are aware that many advocate pruning in the spring, especially when increased vigor is desired to be given to the tree; but we confess that careful observation for many years has taught us that we really gain little on that score, and that in ninety-five cases out of every hundred we induce a cankered diseased wound, a tendency to water-sprouts, or great increase of sucker shoots. Very late fall or winter pruning generally results in a wound that dries and cracks, and requires two or more years to heal over, and in extreme cold sections we believe renders the whole system of the tree more sensitive.
The month of July, however, is one in which the tree is making a rapid but healthy growth, with fully formed foliage to elaborate its sap, and a wound then made rapidly heals and is soon covered by the well-digested layers of new bark and wood which the tree is then in the natural course of its life increasing. Again, at this time the drain of foliage on the roots, in case of a little drought and by reason of the heat and rapid evaporation, is lessened by the removal of a part of the surplus foliage, and hence the whole health and life of the tree is increased and added to rather than lessened or rendered diseased as is the fact when spring pruning is practiced.
 
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