Is it harmful to prune trees in the winter?

An interesting question arose last week: "Is it too cold to prune trees?" Since the temperatures have dropped at night sometimes to single digits and only warmed to the 30s to low 40s late in the day, I wondered if small branches might be frozen and too brittle to prune because the tissue is so hard.

These temperatures are definitely cold for the person doing the pruning, but can pruning during these low temperature spells damage trees? I wasn't sure of the answer so I asked our state horticulture specialist, Heidi Kratsch, for her thoughts.

Heidi replied, "No, the cold temperatures don't really affect pruning (except the pruner who will need to dress appropriately). Temperatures in the 40s are not that cold anyway."

She mentioned that there is a lot of talk on the Internet about pruning during cold hurting trees. She suspects it's an old myth without any scientific basis. She checked with the International Society of Arboriculture (http://isa-arbor.com/) and other certified arborist websites, and the consensus is that pruning technique including sharp tools and making proper pruning cuts is really the most important thing.

Where temperature is more critical is in the Southern states, where freezes occur this time of year when the plants are NOT dormant - that's when pruning after a freeze can be damaging.

If pruning is done correctly, cold weather shouldn't be a problem. This is actually a good time of year to prune because there are no leaves to block the tree architecture and the trees are dormant. In addition, many species such as crabapple produce their flowers on new wood so flowering or fruiting will not be affected by winter pruning. As a general rule, growth is maximized and wound closure is fastest if pruning takes place before the spring growth flush.

As always, do not remove more than one-third of the branches to minimize stress to the tree. Pruning during the dormant season reduces the possibility of diseases such as fireblight in apples, crabapples, pears and hawthorns or cytosphora canker and wetwood in elm, poplar, aspen, maple, birch and beech.

Pruning practices that do harm trees include topping, which is the pruning of large upright branches often done to reduce the height of a tree, making it look amputated. Improper pruning cuts injure trees and can cause bark ripping down a trunk. Cuts that leave stubs do not allow a tree to close a pruning wound.

If you want to know more about pruning, contact your local University of Nevada Cooperative Extension office; call 887-2252 or e-mail skellyj@unce.unr.edu.

• JoAnne Skelly is the Carson City/Storey County Extension educator for University of Nevada Cooperative Extension and may be reached at skellyj@unce.unr.edu or 887-2252.