Renewal pruning should be done in late winter or early spring before.
May 24, Pruning actually stimulates growth. Pruning and thinning shrubs reduces disease pressure (examples - Macrophoma and Volutella leaf spot of boxwood and black spot of roses) by increasing air circulation and allowing foliage to dry faster.
Old overgrown shrubs can be renewal pruned to improve their appearance and reduce their size. Pruning trees and shrubs is important for safety and overall appearance and health.
Late winter is the optimal time to prune trees and shrubs since the plant is dormant and there is less chance for disease and pest damage. Late winter is December-mid March; it is not quite too late as long as pruning is completed as soon as possible.
Fruit trees are pruned to increase new growth and fruit formation. Pruning an old plant can help rejuvenate it and encourage new growth. Pruning has its primary place in training a new tree, shrub or vine. When this is done properly it eliminates virtually all pruning later in the life of most trees, and many shrubs. Pruning and training young trees and shrubs helps to encourage the development of strong branches and an attractive, balanced framework.
If done properly at the nursery and during the early years in the landscape, this formative pruning will minimize the amount of time and labor required to.
Neglected shrubs may call for a more drastic approach: hard pruning. Most deciduous shrubs that respond well to renewal pruning can also take hard pruning, as will a handful of broadleaf evergreens, such as privet. Using loppers and a pruning saw, cut back all stems to within an inch of the ground during the plant's winter dormancy.