Before you spend too much time wondering about the answer to this question, let me tell you that it is a trick question. 

The easy answer to the question: best not to do any pruning back of shrubs at this point of the year. 

I know you must be thinking, “But I always do my pruning in the fall and nothing bad has happened.” 

That may be true, but fall is not the proper time. Here is a bit of science to explain my reasoning.

As we go into autumn and start seeing plants go into dormancy and the leaves start to turn beautiful fall colours, what we don’t see is what is going on inside of the plant. 

All summer, the leaves have been storing up food from the sun in a process called photosynthesis.

As the fall approaches and the temperatures become cooler, all this stored energy starts to move from the leaves, down the stem and back into the roots. 

In the spring, that same energy then moves from the roots, up the stem and forces out new leaves. That is why, in springtime, we say that the sap is rising in the maple trees, because it is physically rising from the roots and up throughout the plant. 

If this whole process did not occur, a plant, when it dropped its leaves, would lose all its stored energy, resulting in a weakened plant. 

So what does that mean when it comes to pruning shrubs in the fall? 

If you prune your shrubs at this time, before its leaves drop, you are cutting off this stored food, which will result in a weaker plant. 

Will it kill the plant? No, but it will not be perform at its best. 

Also, by pruning in the fall, the cut area does not have time to properly heal up, making it more susceptible to winter die-back.

Pruning also stimulates new growth. Whenever you cut back a plant, its first instinct is to push out new growth. 

Therefore, if you prune early fall, the plant may push out new growth before going dormant. This new growth will not have a chance to properly harden-off before winter and may suffer more damage from cold winter winds. 

This is true for evergreen shrubs as well.

Early spring flowering trees and shrubs, such as magnolia, wisteria, lilac, forsythia, purpleleaf sandcherry and viburnum all set their flower buds on growth from the previous year (usually in late summer). 

Therefore, pruning back these early bloomers in the fall will result in loss of flower buds that would have opened the next spring

If a shrub has grown excessively this year and some of its branches are overhanging walkways or scratching against a wall, you can prune back those branches lightly so they are not problematic — wait until spring to do your final cutting back.

So, when it comes to pruning shrubs, trees and evergreens and this time of year, put down your pruners and slowly back away. 

This is one chore that you can stroke off your to-do list until next spring. You’re welcome.

Joanne Young is a Niagara-on-the-Lake Garden Expert and Coach.  See her website at joanneyoung.ca