Wrapping Young Trees: Sunscald and Rodent Protection

Two things kill young trees over a northern winter that most people never see coming: the sun, and rabbits. Both do their damage in January and February, and by the time you notice in spring, the tree may already be done. A few dollars of protection in the fall takes care of both.

Sunscald: the long crack on the south side

On a clear winter afternoon, the sun hits the south and southwest side of a young tree's trunk and warms the bark well above the air temperature. The tissue under the bark thaws and starts to wake up. Then the sun drops behind the houses and the temperature crashes. That tissue freezes fast and dies.

The result is a long, sunken crack or a dead strip of bark on the southwest side, often called southwest injury. Insects and decay get into it, and a young tree can struggle for years or die.

Trees most at risk are young, thin-barked ones, especially in their first few winters after planting:

• Maples (especially red and sugar maples)
• Lindens
• Honeylocusts
• Fruit trees (apple, pear, cherry)
• Ash, mountain ash, and young oaks

Older trees with thick, rough bark rarely get it.

Rodent damage: chewing under the snow

Rabbits chew bark when other food is scarce, usually at snow height and a foot or two above it. Voles tunnel under the snow and chew the bark right at the ground line. If they chew all the way around the trunk (girdling), the tree can't move water and sugars between roots and leaves, and it dies.

You'll often find this in spring, when the snow melts and reveals a ring of chewed bark.

What to use

For sunscald:

• White plastic spiral guards or paper tree wrap reflect the sun and keep the bark from heating up. Wrap from the ground to the lowest branches.
• Light-colored or white trunk wraps work on the same principle. Some orchards paint trunks with white latex paint thinned with water for the same reason.

For rodents:

• Hardware cloth (quarter-inch mesh) formed into a cylinder around the trunk, a couple of inches away from the bark. Bury the bottom 2 to 3 inches into the soil or mulch to stop voles, and make it tall enough to reach well above your usual snow depth, since rabbits stand on the snow.
• Plastic spiral guards help against rabbits too, but voles can get under them at the base.

Many people use both: a spiral guard for the trunk and a hardware-cloth collar at the base.

When to put it on and take it off

• On: late fall, after the leaves drop and before the first snow. Around Thanksgiving is a good reminder across most of the northern US.
• Off: in spring, when growth starts. Leaving wraps on through summer traps moisture against the bark, gives insects a place to hide, and can choke the trunk as it grows.

Hardware cloth can stay on year-round if it's loose and you check it every year so it doesn't cut into the trunk.

Check what you've already got

If you have a tree that's been wrapped for years, pull the wrap off and look. I've seen plenty of trees with old wrap or a nursery tag wire cutting into the bark. Remove anything tight.

Other fall steps for young trees

• Water deeply before the ground freezes. A well-watered tree handles winter stress better.
• Spread 2 to 4 inches of mulch over the root zone, keeping it a few inches away from the trunk so voles don't nest against the bark.
• Don't fertilize trees in fall; new growth won't harden off in time.

Protect a young tree for its first five or so winters, and it'll outgrow the need on its own.

Related: Planting Trees in Fall in the Northern US · Emerald Ash Borer: Treat or Remove Your Ash Tree?