Time of Pruning by A. D. C. Le Sueur 1949
“The usual period for pruning is carried out between the months of October and March when the circulation of sap is at a minimum or has ceased altogether. But with most trees pruning can be carried out safely at all times of the year except, perhaps, from April to June in the case of young trees and others in which excessive ‘bleeding’ takes place. As regards the danger to the tree this appears to be somewhat overrated. Beech wounds on trees of about 30 years in age, cut in May have bled for days without ill effect and, in fact, the wounds healed* faster than others made in winter.
Summer pruning seems to check the new growth to some extent, when dealing with street and other trees requiring frequent pruning. This is perhaps and advantage. The presence of leaves however, is apt to make the operation rather more difficult. Winter pruning, preferably on mild days, is quite sound practice, although it is realised nowadays that as healing* takes place more quickly during the growing season, pruning during the period April to September has certain advantages.”
Today, healing as a term has been replaced by occlusion of a wound.
The real advantage of summer pruning is that tree defences are active and the spores of fungal pathogens less abundant than in Autumn or winter.