Summer Pruning (June – July)
Wilson Nursery recommends summer pruning for controlling shape and size of fruit trees. Summer pruning has the greatest dwarfing effect in June and July.
There are plenty of good reasons to prune in summer rather than in winter, including the fact that the weather in July is more conducive to spending time with your fruit trees.
More to the point, though, summer-pruning increases light on the developing fruit and therefore increases fruit size, colour, and sugars. It also often, but not always, forces fruit buds to form, increasing the yield the following year.
Summer-pruning is also ideal for controlling closely planted trees (cordons) and trees that have “filled their space,” as it reduces vigour. Do not summer-prune young trees that have yet to grow into their space. Instead summer-prune trees that need to be controlled within the space allowed for them.
To explain, think of two similarly sized trees of the same variety of apple. In the winter, when all the nutrients are down in the ground, you prune one-third off the top of one tree. In the spring the nutrients in the roots rise up, but find only two-thirds of the tree. What happens to the excess sap? It forces upper buds to break and the result is lots of “water shoots.” The tree is invigorated and vegetative wood is produced.
The other tree is left alone during the winter. In the spring the sap rises up and fills the whole tree without producing water shoots. In July, you remove one-third of the tree. In the fall, only the remaining two-thirds of the sap moves down into the roots. The following spring, the two-thirds sap rises up and fills the two-thirds remaining of the tree, again with no water shoots. (http://derrysorchardandnursery.ca/p5_pruning.html)