WINTER on the West Coast is one of the best times of the year to do several seasonally relevant tasks in the garden including mulching, transplanting, liming the soil and, above all, pruning many trees, shrubs and vines.
The advantage of dormant-season pruning is the invigorated growth response that occurs in the spring immediately following a winter prune, which is used to rebuild branch structure and form. Invigoration occurs in deciduous woody shrubs, vines and broadleaved evergreen shrubs when they are pruned during dormancy because sap reserves are stored in the root system for winter and as sap flows up to the branches in spring, there are more sap reserves than branch network and the excess sap reserve is directed to new invigorated growth where the pruning cuts were made.
Knowing that invigoration occurs in specific types of plants we can prune such plants during winter to direct, control or modify their growth. Plants that respond predictably to winter pruning include forsythia, spirea, cinquefoil (Potentilla species) dogwood shrubs (Cornus species), hydrangea, woody vines like kiwi and several broadleaved evergreen shrubs like laurel. Prune deciduous shrubs now by removing the oldest, longest and most misplaced branches at their point of origin, preferably at the ground, to obtain new branch production from the ground in the spring. The advantage of this type of pruning in winter, known as thinning, is the production of new basal-growing branches that can rebuild the structure of the plant and replenish and invigorate overall growth.
Any gardener who grows hydrangeas has seen old branches flower well for many years and then gradually decline; those are the branches that benefit from winter pruning the most, but they must be cut at the ground for the technique to achieve its best effect.
When it comes to pruning woody vines like kiwi (Actinidia species), grape ( Vitis species), wisteria and several other woody vines, the purpose of dormant pruning is to develop multiple spur placements (flowering/fruiting branches) to increases flowering and fruit production.
Another advantage of pruning vines in during dormancy is access to the normally robust and tangled mass of branches that are normally obscured by summer leaves.
The aforementioned vines produce flowers on branches that are at least two years old or older, not on young branches. Young branches are not physiologically mature enough to reproduce (produce flowers) and like children, young branches are all energy and with no wisdom or maturity.
Knowing that we need to keep older branches to produce flowers and fruit on our vines means we can prune the young growth hard or train it to grow where we want it to grow, while retaining the small, short, older branches that have produced flowers (fruiting spurs).
On a typical wisteria, for example, to prune during dormancy to build structure and train the plant to grow into new areas, train any long whips into new areas you wish to cover and tie them into place and remove any dead, diseased or damaged wood as you go. Then prune back the remaining long whips back to two or three buds from their point of attachment. Mulching the root zone and pruning is done for the winter.
There are also many broadleaved shrubs such as laurel (Prunus species), viburnum, (Viburnum species) sweet box (Osmanthus species), strawberry bush (Arbutus species), holly (Ilex species), firethorn (Pyracantha species) and others that benefit from periodic hard pruning during dormancy to restructure their height, width or overall form.
The invigoration benefit of winter pruning will allow any hard pruning cuts to be completely covered with new spring growth.
As an example, a large laurel hedge that has outgrown its space can be pruned hard during dormancy by as much as half the length of its branches to a shape that suits the space. The laurel hedge may look a little skeleton-like after hard pruning, but the hedge can be made smaller, shorter or narrower to suit the garden's space and new invigorated growth will cover the cuts.
After any hard pruning I strongly recommend cleaning out debris under the plant and mulching the entire root zone with mulch to protect roots and foster a good growing environment.
It is important to remember that only healthy plants can respond to hard pruning. If you have some old dog of a hedge growing out against the lane and the poor thing is abused by neighbourhood kids and dogs, the chances of regrowth will be reduced.
Some plants should not be pruned during winter, including Japanese maples, dogwood trees and Mediterranean shrubs, which don't respond to invigoration pruning or they succumb to stem diseases caused by dirty pruning tools and wet weather.
And not all plants respond the same to hard pruning, so know your plants before you prune.
Todd Major is a journeyman horticulturist, garden designer, writer, consultant and organic advocate. For advice contact him at stmajor@ shaw.ca.
