I have been getting quite a few questions about pruning evergreens. As a general rule, there are two periods that evergreens are typically pruned. These are during the dormant season and the growing season.
Dormant season, which extends from late fall through spring, is the best time to reduce the size of evergreens like arborvitae, juniper and yew. Late winter to early spring is the ideal time, so the pruning is done close to the initiation of new growth. If the shrubs are up against a structure or overlapping hard surfaces that will interfere with snow removal, it is fine to do some in the fall. Pruning to reduce the size of evergreens thins them out considerably, so pruning close to when the growth flush occurs is better for the health of the plant and is more aesthetically pleasing.
Reduction pruning is effective for evergreen shrubs that have not been sheared heavily. If the shrub is pruned to its natural form, it should have green foliage well into the interior that will fill in the shrub after it has been reduced. Evergreens that are heavily sheared create a dense layer of foliage that shades out the inner growth. These shrubs that are void of green growth on their interior cannot be reduced in size with the exception of yews that may produce new growth inside the canopy after thinning.
Summer pruning is typically done to shape the plants and reduce new growth back to a more desirable length to maintain an appropriate size. This is best done shortly after the summer growth flush. A loose shearing at this time of year is appropriate with most shrubs.
Evergreen trees such as spruce, pine and fir cannot be reduced appreciably in size. Pruning on these species is typically limited to removing double leaders, removing diseased or dead wood and removing lower branches when desired to raise the canopy. This type of pruning can be done any time of year. Winter pruning does reduce the potential to spread disease and removes disease inoculum prior to the next growing season when the disease can spread.
In general, late winter to early spring is suitable for pruning all evergreens to reduce size or to remove dead or diseased wood. Some evergreen pruning is not time-sensitive and can be done any time of year, but there are limitations during certain periods of the season which I am not able to address in this column.