Robert Pavlis, Guelph
The standard advice is to prune spring flowering shrubs right after flowering and to prune summer flowering shrubs in winter or early spring. Pruning spring flowering shrubs after flowering ensures that the shrub has time to develop next year’s flower buds. Summer flowering shrubs develop flower buds in spring on new wood so they are pruned before flower buds are formed.
This is not bad advice for maximizing flower production, but is it the best advice for the vigour of the shrub?
The shrub responds to pruning differently depending on when it is done. Pruning during dormancy (i.e. late fall to early spring) removes dormant leaf buds and may also remove dormant flower buds. Pruning during dormancy does not cause the shrub to initiate new growth. Pruning in mid or late spring after flowering has a different effect. Pruning removes newly formed leaves in addition to wood. The shrub has just grown the leaves and in many cases they are not yet fully developed. The process of growing these leaves requires significant food reserves – food that was stored the previous year. The leaves have not yet paid the shrub back for using these food reserves.
The shrub’s reaction to losing leaves is to activate more dormant buds and many shrubs tend to over react by activating more buds than they really need. This drains even more of the food reserve. Pruning in mid to late spring weakens the shrub by depleting extra food reserves. Most shrubs will survive this situation, but it is not really the best thing for the shrub.
The advice to prune spring flowering shrubs after flowering is good for flower production, but it is not the best advice for the health of the shrub.