Fall is here and winter is just around the corner and I'm anticipating what global warming will have in store for us this winter. Perhaps another +20 Celcius March Break or a snowstorm in May? If it's anything like last winter, my staff and I will be busy pruning trees all winter.
Mild weather and no leaves on branches makes for ideal pruning conditions, both for us climbing around the canopy and for the trees.
I've always answered the question of" when is the best time to prune trees?" with "it's not when you prune trees that's so important, it's how you prune them." In most cases quality tree pruning can be performed at any time of the year, spring summer, fall or winter. There are some exceptions for instance we don't prune elm trees during the growing season so as not to attract Dutch Elm Disease spreading bark beetles.
The fall, winter and spring seasons, collectively referred to as "the dormant season" is a fantastic time to get some maintenance pruning and structural training done.
With no leaves on the trees, it's far easier to see the branch structure and to either move around in a tree or maneuver tools inside a canopy from the ground. Since plants, fungi and other critters are generally dormant, there is a decreased chance of infection to new pruning cuts. There's always exceptions to the rule but generally, new tissue will grow over pruning cuts faster if pruning is completed in the dormant season.
When we undertake structural training on medium sized trees, often we need to remove large sections in order to avert future branch failures. This can leave gaps or holes in the canopy that will fill in more readily if pruned in the dormant season.
If you have a schubert cherry tree that is riddled with black knot fungus, pruning must be completed during dormancy to avoid spreading spores and to make sure that all the knots are visible to the pruner.
Some shrub pruning work must be accomplished during plant dormancy. For example, reducing a large forsythia or a cotoneaster hedge back to ground level must be done when the leaves are off. This will ensure that carbohydrates stored in the root systems will be maximized to provide energy to the new shoots that will arise in the spring.
That advice on cutting back large forsythia doesn't scale up to pruning back a Manitoba maple or a poplar. There are no occasions where cutting back a tree, ie. topping, is a good idea for a tree. True, if you are topping a tree, to avoid outright killing it, it must be done in the dormant season. This is why along with the "when" questions should always come the "how" question.
If you are interested in learning more about how to prune trees well, look into the City's Citizen Tree Pruner Program to be offered again this coming spring. Otherwise you could apply for a job with my company and apprentice as an arborist for the required 6000 hours. That's a lot of learning!