Young Tree Trunks Run 77 Degrees Warmer on the Southwest

On a sunny winter afternoon, the southwest side of a young tree’s trunk can be as much as 77 degrees warmer than the north side, according to Tom Kalb, an extension horticulturist at North Dakota State University. That swing between sun-warmed and shaded bark is the reason extension services recommend wrapping young trunks in fall, and Iowa State Extension puts the date at around November.

What the Afternoon Sun Does to Bark

Kalb describes the injury in steps. Winter sun heats the trunk and activates cells under the bark. When the sun sets, the trunk cools quickly, and those cells freeze and burst, causing the bark to crack. His example is vertical cracks on the southwest side of maple trunks, a problem NDSU calls sunscald or southwest injury.

Michele Warmund, a horticulturist at the University of Missouri, describes the same pattern in a University of Missouri article on tree wraps. Southwest injury, or trunk scald, occurs on the lower portion of the trunk, she writes, and trunk cracking or splitting on the south or southwest side is a common symptom.

The injury peaks late in the season. Bruce Spangenberg of University of Wisconsin-Madison Extension writes in Winter Damage Concerns for Trees that the sun grows stronger in late winter while nighttime temperatures can still fall to single digits or lower. That daily swing drives the injury, and the bark warmed by day cools quickly at night and can pull away from the wood beneath it.

Which Trees Need a Wrap

Not every tree needs one. Kelly Feehan, a Nebraska Extension educator, writes in a Knox County column on avoiding winter wounds that young or thin-barked trees are wrapped during winter to reduce frost cracks and sunscald. She also notes that avoiding trunk wounds may matter as much as wrapping, since frost cracks often begin at wounds.

Kansas State University’s Wild West District, in a piece on preventing winter sunscald on trees, lists the most susceptible as thin-barked species such as honeylocust, fruit trees, ash, oaks, maples, lindens and willows, with young trees facing the greatest risk. Spangenberg likewise names smooth or thin bark on the south or southwest side of young trunks, with fruit trees and maples among the most susceptible.

Kalb gives the length of the commitment. Trees should be wrapped for at least their first two winters, until their bark develops texture. Sensitive trees such as linden, mountainash and apple benefit from protection for their first five winters.

New plantings are the usual candidates, and Lovely Garden Roots covered the other end of the same job in a post on planting a new tree in November.

Wrapping From the Soil Line to the First Branches

Warmund’s instructions begin at the soil line. She says to spiral the paper around the trunk up to the first branches, overlapping each layer. The material should be light-colored, such as waterproof kraft paper, tree wrap or vinyl tree guards. Vinyl guards cost more, she notes, but can be reused for several years.

Spangenberg agrees on light colors, which reflect heat, and on starting at the bottom. He adds that the wrap should overlap like shingles so water runs off, and be secured at about the first set of branches.

Kalb offers two options: kraft paper from the base up to the first major branch, or white plastic tree guards around the trunk to reflect sunlight. He advises against black guards because they absorb heat. The Kansas State piece describes commercial wraps as a flexible heavy plastic strip coiled around the trunk or heavy white paper, and says homemade wraps are an option.

A related post on the site covers protecting young trees from winter cracking.

When the Wrap Goes On and Off

Iowa State’s page says wrapping is usually unnecessary during the growing season, and that a wrap belongs on in fall, around November, and should come off the following spring, around April. Iowa State adds that wraps are beneficial through winter on young trees to prevent rodent damage or sunscald injury.

Kansas State says the job is best started in November and removed the following March, and that wrapping at any time may be better than leaving susceptible trunks bare all winter. Warmund says as a rule wraps and guards should be applied by December, but it is still not too late after that. Spangenberg says to wrap in late fall and keep the wrap on until late spring.

Removal matters as much as installation. Warmund says to take wraps and guards off in early spring, because leaving them on too long can restrict growth and shelter insects and disease. Kalb says to unwrap after the last frost so the trunk can expand and insects do not move in. Feehan makes the same point: wraps need to come off each spring to avoid damaging the trunk as it expands.

This article was produced with AI assistance and edited for accuracy against the sources linked above.

Image: Hameltion – CC BY-SA 4.0/Wikimedia Commons