Pruning Trees and Shrubs in the Fall  

When to prune trees and shrubs is a common question among many homeowners and gardeners. October is often when we finally receive some nicer weather to work on outdoor tasks, so is early fall pruning a good idea or not?

We know that pruning in August can stimulate new growth that is less hardy during the winter.  But what about pruning in October?  Here is a little biology to consider: woody plants move sugars and other materials from the leaves to storage places in the woody portions of the plant just prior to leaf fall and we would like to maximize those stored energy reserves. 

Even pruning later in the fall (October) can at times cause a problem by reducing the cold hardiness of woody plants.  Dr. Rich Marini at Penn State Extension has written , “Based on everything that has been published we can conclude that woody plants do not attain maximum cold hardiness when they are pruned in the fall. Trees and shrubs are affected more by heavy pruning than light pruning.” 

This does not mean that woody plants pruned in the fall will necessarily suffer winter damage.  In most cases we can still safely use the old adage of “prune whenever your pruners are sharp.”  But pruning with caution in mind is a good idea.

Cause for Caution

Hardiness of the plant being pruned: The main reason damage can occur from fall pruning is if a sharp drop in temperature occurs before plants are completely hardened off for the winter. If the plant being pruned is marginally hardy for Kansas (plants rated hardiness zone 6 or 7) than that plant will be much more susceptible to winter damage, especially if pruned in the fall. 

Severity of the pruning needed: Heavy pruning on a plant in the fall can increase the risk of winter damage while light pruning generally will not. Consider pruning to be “light” if 10% or less of the plant is removed. Dead wood does not count in this calculation.  Keep in mind that even light pruning of spring-blooming shrubs such as lilac and forsythia will reduce flowers for next year. We normally recommend that spring-blooming shrubs be pruned after flowering in the spring.          

Here is the takeaway. Though light pruning and removal of dead wood are fine this time of year, you may want to delay severe pruning until late winter and early spring.

Shrub pruning reminders 

Shrubs differ in how severely they can be cutback. Junipers do not break bud from within the plant and therefore should be trimmed lightly if you wish to keep the full shape. Overgrown junipers should be removed.

But there are certain shrubs that can be pruned back severely during the spring. Rejuvenation is the most severe type of pruning and may be used on multi-stem shrubs that have become too large with too many old branches to justify saving the younger canes. In rejuvenation pruning, all stems are cut back to 3- to 5-inch stubs. This works well for spirea, forsythia, pyracantha, ninebark, Russian almond, sweet mock orange, shrub roses, flowering quince and others. Just remember that spring is the correct time to do this, not now.