Michigan State University Extension
Ornamental Plants plus Version 2.0 - 00001628
01/01/98
The least desirable time to prune is immediately after the new growth has developed in the spring. At that time, much of the food stored in the roots and stems has been used to develop new growth, and the new foliage should replace this food before you prune and remove the foliage. Otherwise, considerable dwarfing, dieback and decline of the plant may occur.
It is also advisable to limit late summer pruning, which stimulates new growth on some plants. This growth may not have sufficient time to harden off before cold weather arrives and so may be damaged or killed by low temperatures. Late pruning also removes valuable food reserves.
Prune when twigs, branches and limbs are dry and when the weather forecast calls for dry weather for a week. This is most important in fall and spring, when diseases are active and easily transmitted to vulnerable plants. Whenever possible, avoid pruning the tender spring flush of growth to avoid tearing new bark tissue and opening wound sites for disease organisms to enter.
Spring flowering trees and shrubs should be pruned shortly after flowering to avoid removing flower buds, which form in late summer on mature wood and overwinter. Prune plants that bloom after the end of June in late winter before new growth starts. These plants develop their flower buds during the spring growth period.
Bleeding of pruning wounds can be heavy on certain trees, such as birch, dogwood, sugar maple and elm. Minimize bleeding of susceptible trees by making small cuts -- less than 3 inches in diameter -- and pruning in summer. Bleeding is very likely if severe pruning is done just before growth begins in the spring. Bleeding doesn't harm the tree, but if it's heavy and persistent, it may injure the bark below the pruning cut and cause slow callusing of the lower wound.