Five tips to winterize your trees

By Denise Ruttan

Your trees may still need attention even in the coldest days of winter.

In the life cycle of a tree, winter is the time when trees go dormant and growth slows down, said Paul Ries, an urban forester with the Oregon State University Extension Service. The strongest parts of a tree are its trunk, branches and roots, so they normally survive winter weather quite well.

In the spring, new growth emerges in the form of twigs, buds and leaves.

But the success of this new growth depends in part on the tree over-wintering well, Ries said. That makes winter a great time to take action on pruning and other maintenance tasks – particularly for deciduous trees, he added.  

"It's a good time of year for winter tree care because Oregon doesn't have the same freeze and thaw cycles that happen regularly in places like the Midwest," Ries said. "Frequent thawing out can be hard on trees."

In the various ecological regions around the state, trees face different challenges in winter. Coastal trees endure the force of strong winds and salt damage. Strong enough winds can cause trees to adapt their growth to deal with the extreme conditions, Ries said. That's why you'll see trees surviving in a bent position in windier environments.

In eastern Oregon, trees are fairly well adapted to lengthy cold spells – provided they're native trees or landscape trees that can tolerate tougher conditions, Ries said. But the same winter care tips apply to trees on the eastern side of the state. Tasks such as mulching become even more important, Ries said.

Below are some tips on caring for your landscape trees during winter.

· Pruning – Now is a good time to prune to develop an appropriate branch structure on trees that you have planted in the last three or four years. Remove any crossing branches, broken branches and branches that are touching or too close to other branches. Also pay attention to "double leaders," branches growing in competition to the main trunk. If you're nervous about cutting, Ries advised that you should hire a certified arborist. Be aware that trees don't "heal" like humans do; they don't regenerate tissue but instead "wall off" a wound to keep the damage from spreading, Ries said. If you open up a cross section of a tree that was damaged years ago, the old scar will still be there.

· Mulch – Place bark or wood chips around your tree in early winter to help retain water and reduce temperature extremes. A thin layer of mulch acts like a blanket and helps give the tree's roots extra winter protection. But be careful not to overdo it. Too much material can cause other problems, such as providing a hiding place for rodents.

· Remove stakes – Now is a good time to remove any stakes still in the ground if you planted new trees within the last few years. Leaving stakes too long increases the risk of damage to the trunk. The tree can grow around the guide wires.

· Water – Give your trees a drink. Winter droughts can be as severe as those in summer. When there's not enough rain or snowfall, an occasional watering during the winter can be a lifesaver for a young tree. But water only when the soil and the trees are cool, but not frozen.

· Plant – When the ground isn't frozen, winter is a great time to plant trees. In western Oregon in particular, winter rains will help a new tree get established. Plant trees suited for the cold hardiness zone in which you live.

"Oregon is a very diverse state ecologically and it's important to pay attention to ecological zones when deciding what to plant," Ries advised. Find your plant hardiness zone on a map from the U.S. Department of Agriculture

For more information, see the OSU Extension guide "Selecting, Planting and Caring for a New Tree".  For more tree care advice, visit the International Society of Arboriculture's website.