Here’s a quick quiz – when do you prune your shrubs? When they get too big for the space they are growing in, when you think about it or when they look shabby? For many of us, the answer is all 3. Pruning your shrubs in a horticultural sense depends on 2 things: when they bloom and their age.
For summer bloomers and non-bloomers, early April is a great time to prune. Summer bloomers such as Japanese spirea, potentilla, rose of Sharon, smokebush, blue mist spirea, roses and butterfly bush produce flowers on new, current growing season stems, so pruning now won’t remove the color that adds so much appeal to the landscape. Similarly, plants that don’t bloom or have small inconspicuous blooms such as burning bush, euonymous and barberry can be worked on now without the potential for loss of aesthetic appeal.
Spring bloomers such as viburnum, lilac, cotoneaster, dogwood and forsythia should be pruned after they bloom, so that the color in the landscape is not lost and while there is plenty of time left in the growing season to control height and get rid of dead or diseased stems.
Now that we know when to prune, the next question is how to prune. The cane-producing plants listed here are best thinned rather than sheared. Thinning involves removing older, entire stems at the ground level, not leaving any growth above ground. Really? Yes. Getting rid of this growth keeps the shrub healthy by removing old, less productive and likely pest infested or damaged stems, which allows more light to penetrate to the center of the canopy encouraging new stems to grow and replace the old ones. In fact, overgrown shrubs that have not been pruned for several years are best cut to the ground to renew the growth and increase the appeal and thriftiness of the plant. Thinning also reduces the overall height by a third on average.
As in other areas of gardening, there are exceptions. The most common one is when to prune yews and other evergreens. For information on these, see: https://grobigred.com/2018/07/05/pruning-yews-shearing-or-thinning/
and for hedges: https://grobigred.com/2021/06/02/hedges-are-bad-but-if-you-must/
and for a good summary: https://grobigred.com/2021/01/19/pruning-planning-for-2021/.
