The Best Time of the Year to Prune Trees
Pruning trees and shrubs in late summer is NOT recommended because such pruning causes new soft growth that can’t harden off before winter. You will find this advice everywhere, but is it correct? Listed below is why we believe pruning trees is not recommended for summer and alternative ways to complete this task.
The woody stems of trees and shrubs go through an annual process of maturation. In spring new growth starts and continues into early summer. The growth at this point in time is called ‘softwood’ because it is soft and pliable, and can be bent to a large degree without breaking. If you bend it too much it will snap and break but in some species you can actually tie the stem into a knot without breaking them. This softwood is not yet mature, and it can’t withstand freezing.
As the summer progresses, the plant slowly converts the softwood into hardwood. The bark thickens and and the color changes from green to brown. The hardwood becomes much stiffer and does not bend very well. Along with the physical changes there are internal chemical changes that make it more resistant to low temperatures. It is getting ready for a freezing winter.
Pruning trees and shrubs stimulates new growth. If pruning is done in spring, the new growth has time to mature and become hardwood before winter starts and it can then survive the cold of winter. However, if pruning is done too late in the season, there is not enough time for the new growth to harden off–it runs out of time to become hardwood. When winter comes, it will freeze and die off.
To combat the problem of winter die off, it is commonly suggested that you do not prune trees and shrubs in late summer or early fall. If you don’t prune at this time, you will not cause new growth, and winter will not damage your woody plants.
But ….how early is too early to prune late in the season?
This question was discussed on social media a few months ago, and it was suggested that pruning in mid August might not cause new growth. The following experiment was carried out to determine if it is OK to prune in late summer.
Why we think this
It is possible that a more major pruning (ie removal of a large number of stems) may result in new growth. It is also possible that the results of this test are unusual given the cold summer weather. Since pruning mid August caused very limited new growth, the mid September pruning was not carried out.