Dormant pruning happens during winter for one primary reason; dormancy. For the same reason, most alternating canes pruning should happen during winter. The results of such pruning will become obvious during the following spring or summer. New growth should be more vigorous. Bloom and any subsequent fruit production should be more abundant.
Alternating canes is similar to coppicing, but not as drastic. Coppicing entails pruning all growth down to the ground. Alternating canes involves pruning only old and deteriorating stems to the ground. Younger and more vigorous stems remain with more space to grow and bloom. The process essentially promotes constant growth of more productive stems.
Pruning grape vines with this technique is a bit different. It entails pruning old canes only back to their permanent trunks, rather than to the ground. In that regard, it is more similar to pollarding, but not as drastic. Pollarding entails pruning all growth back to a trunk or a few main limbs. Small grape vines might support only a pair of canes after such pruning.
Most, but not all, alternating canes pruning should happen during winter dormancy. Lilac and Forsythia bloom profusely for early spring, but generate no fruit. Therefore, for them, alternating canes pruning may occur immediately after bloom. Delay of such pruning can allow old canes to bloom one last time prior to their removal. It maximizes floral potential.
Some species that benefit from alternating canes pruning happen to be evergreen. Their pruning can happen whenever it is convenient for them, even if not during winter. Abelia, for example, may be rather sparse after such pruning. It remains sparse for longer during early winter than late in winter. Oregon grape and Heavenly bamboo are also evergreen.
Proper dormant pruning for several types of plants involves alternating canes by default. These include, but are not limited to, roses, hydrangeas, elderberries, and cane berries. Some types of filbert, and some types of cotoneaster are less common examples. Witch hazels, like forsythias and lilacs, should bloom prior to pruning. Bamboos are conducive to alternating canes whenever they appear to get unkempt.
Tony Tomeo 