Shrubs usually do not require any formative pruning, but only maintenance pruning and renewal or rejuvenation pruning.
This type of pruning is done periodically to keep a shrub healthy.
- Remove any dead, broken, damaged or diseased stems.
- Remove any inward-growing, weak, misplaced, crowded, interfering or excess branches. Any long, straggly stems should be trimmed.
- If you want to remove faded blooms, do so immediately after the shrub has finished flowering. Leave them on the shrub if it produces ornamental or edible fruit.
- For a grafted shrub, remove any water sprouts and suckers that form under the graft point.
- For a variegated shrub, any stems with all-green leaves should be removed. Such stems are very vigorous and can quickly become dominant.
Renewal and rejuvenation pruning
As they age, most shrubs tend to either become bare at the base or produce many stems that flower poorly. They can be reinvigorated with renewal or rejuvenation pruning. This sort of pruning encourages the growth of new shoots to replace old stems.
Gradually remove the oldest stems by cutting them back very close to the ground (for shrubs that produce many stems at the base), to the junction of a younger stem or where they meet the trunk (shrubs with a single trunk). For vigorous species, including Siberian peatrees (Caragana arborescens), honeysuckles (Lonicera spp.), mockoranges (Philadelphus spp.) and common ninebarks (Physocarpus opulifolius), you can remove 25% to 30% of the old wood each year.
This type of pruning involves cutting all stems back to within 10 or 15 cm of the ground. It is suitable for shrubs that have been neglected for a number of years and older shrubs that are bare at the base.
This method can also be used to accentuate the colours of shrubs with coloured leaves or stems, like willows (Salix spp.) and dogwoods (Cornus sericea), and to enhance the blooms of some plants that flower on the current year's growth, like wild hydrangea ‘Annabelle' (Hydrangea arborescens ‘Annabelle'). You may wish to do this kind of pruning every year, or even every two or three years.
After rejuvenation pruning, shrubs should be fertilized with compost or natural fertilizer.
Careful! Rejuvenation pruning is not suitable for all shrubs.
Only those shrubs that readily produce new stems from the base can be cut back severely. The list includes dogwoods with coloured stems (Cornus alba and C. sericea), smoketrees (Cotinus coggygria), wild hydrangeas (Hydrangea arborescens), shrubby cinquefoils (Potentilla fruticosa), mockoranges (Philadelphus spp.), common ninebarks (Physocarpus opulifolius), alpine currants (Ribes alpinum), willows (Salix spp.), Japanese spireas (Spiraea japonica), some lilacs (Syringa vulgaris and S. x hyacinthiflora) and elders (Sambucus spp.).
When to prune deciduous shrubs
For best blooms, shrubs that flower on the current year's growth should be pruned in early spring. This group is essentially made up of summer-blooming plants, including wild hydrangeas (Hydrangea arborescens), shrubby cinquefoils (Potentilla fruticosa) and Japanese spireas (Spiraea japonica).
Shrubs that flower on the previous year's growth should be pruned immediately after their blooms have faded. Most of these are spring-blooming plants, like forsythias (Forsythia spp.), mockoranges (Philadelphus spp.) and lilacs (Syringa spp.)
You can remove dead, diseased, broken or damaged branches whenever you spot them. Water sprouts and suckers may also be removed at any time.
Renewal and rejuvenation pruning
The best time for renewal or rejuvenation pruning is early spring, before the buds burst. Shrubs can also be pruned in fall, after the leaves drop. Remember that less-hardy species could suffer frost damage, however.
Illustrations: Espace pour la vie/Audrey Desaulniers