Monthly Archives: November 2013

Discover the Cold, Ugly Truth about Sunscald and Frost Cracking

Sunscald discolored the bark on this young nursery tree.  Wrapping the tree in late autumn would likely have prevented injury. (Photo courtesy of Robert Cox)

Sunscald discolored the bark on this young nursery tree. Wrapping the tree in late autumn would likely have prevented injury. (Photo courtesy of Robert Cox)

Now that trees have shed their leaves, you’ll likely notice their architectural splendor.  But you may also notice something else—disfigured trunks from sunscald and frost cracking.

What is sunscald?  “It’s essentially a sunburn, but it’s also got to do with the amount of water and moisture in the trunk,” explains Robert Cox, extension agent—horticulture with Colorado State University in Arapahoe County.  “(Sunscald produces) real obviously discolored bark, particularly on the south or southwest side of a young, non-native, deciduous tree.”

Sunscald-damaged bark may eventually crack or fall off in patches, maiming the tree trunk and leaving it vulnerable to diseases and insects.

Trees prone to sunscald include fruit trees, maples, oaks, ashes, lindens, honey locusts, beeches and willows.

Fortunately, you can reduce the risk of this problem, according to Cox, by protecting your young, deciduous trees that have south, southwestern or western exposure.  Wrap your youngsters with light-colored tree wrap in late fall, and then remove the wrap around mid-April.  Once they’ve grown old enough to produce furrows in their bark, you can then stop wrapping them.

For directions on tree wrapping, visit Colorado State University Extension’s website at www.ext.colostate.edu.

Frost cracking disfigured this stunning maple.  Fortunately, wound wood is forming to cover the damage.

Frost cracking disfigured this stunning maple. Fortunately, wound wood is forming to cover the damage.

As for frost-cracking, here’s how it occurs.  A tree’s bark warms up on a sunny winter or early spring day.  The warmth causes the water in the trunk’s inner bark and in the wood to expand.  As the sun sets, temperatures drop quickly, causing the water to cool and contract. This rapid expansion and contraction can cause vertical cracks in the trunk.

Cox notes that maples, particularly Norway maples, are susceptible to frost cracking.  The only way to prevent frost cracking, he says, is to “plant trees that aren’t prone to frost cracking.  But that’s not really practical,” he points out.  “People are going to plant what they want to plant and deal with the consequences later.”

Trees prone to frost cracking include maples, crabapples, beeches, oaks, lindens, horse chestnuts and willows.

Once a tree has been injured by sunscald or frost cracking, the damage is permanent, though usually not fatal, unless insects or diseases move in and weaken the tree further.

What can you do if your trees fall victim to these two maladies?

“Try to increase its (the tree’s) vigor so that it produces wound wood–that growth you see developing after an injury like that,” explains Cox.  “Fertilize it (the tree) maybe in spring or early summer of the growing season; make sure it gets sufficient water; keep it watered during dry spells in the winter—anything you can do to keep it healthy so it does form that wound wood readily.”

Concerning trunk cracks, he adds, “Certainly, use of a fungicide wouldn’t be a bad idea.  It isn’t going to hurt anything to spray a liquid fungicide in that frost crack periodically just to make sure things don’t happen in there, because water’s going to get in there, snow’s going to melt and rain’s going to come into that wound.”

Leave a comment

Filed under Garden Maintenance