By Kim Ellson
I have some bushes that have gotten far too large. I've heard that cold weather is a good time to prune them back. But I'm not sure if that applies to all. I have hydrangeas, roses, forsythia, butterfly bushes, Korean spice bush, spireas, and some lilacs. When should I cut them back?
Answer
When deciding when to prune your shrubs there are a few factors to take into consideration. Do your shrubs bloom in spring and do they bloom on new or old wood? If you have early blooming shrubs like Forsythia and Korean Spice (Viburnum) they should not be pruned in autumn as the flower buds for next year's floral display are already developed. Autumn pruning will remove these buds and therefore spring blooms. Summer or autumn blooming shrubs will not necessarily be affected by autumn pruning.
The next consideration is; do your shrubs bloom on new or old wood? Old wood is simply last year's growth whilst new wood is the current year's growth. Shrubs that bloom on old wood, regardless of when they bloom, can be affected by autumn pruning. If they do not have old wood in the following year there will be no blooms. Examples of this would be Lilac (Syringa) and some Hydrangeas. Shrubs such as Butterfly Bush (Buddleia) bloom on new growth and are not only unaffected by a pruning, they actually benefit from a pruning to increase the amount of new wood.
Be aware that different species of one plant can have different bloom times and pruning requirements. Hydrangeas can be split into 3 groupings based on the wood they bloom on; old-, new-, and both old- and new- wood. Mophead (H. macrophylla), Lacewing (H. macrophylla) and Oakleaf (H. quercifolia) Hydrangeas bloom exclusively on old wood and should only be pruned immediately after blooming, allowing enough time for bud set prior to winter. For those not familiar with these names, Mophead include all color Hydrangeas. Pee Gee and Annabelle Hydrangeas bloom on new wood and can therefore be pruned in autumn without compromising next year's blooms. 'Endless Blooming' is a Hydrangea that will bloom on both new and old wood.
The same applies to Spirea; these can be divided into groups based on bloom time; spring and summer. Spring flowering Spirea including Baby's Breath should be pruned following blooms, whilst summer flowering Spireas such as Japanese Spirea can be pruned in autumn.
Roses too have various groupings however as a general rule roses are generally pruned in early spring prior to new growth forming.
The timing of any pruning can also be decided by the type and severity of the pruning that is carried out. If I am removing old individual canes within a shrub I may not need to consider bloom time as I will still have ample bloom material. If I wish to remove dead or deceased wood this again would not affect subsequent blooming. And sometimes pruning itself is not even necessary, so prior to pruning always ask yourself, why you are pruning a particular shrub. The answer will help you decide how and when to prune your plants.