Question of the Week: April 8, 2021

Sunscald in young live oak trunk.

“What has caused this damage to my oak tree’s trunk, and will it kill the tree?”

I’ve been asked this question repeatedly over the past week or two. Here are the quick facts to explain what has happened.

This was the photo that caused me to put up a Garden Tip on sunscald. As of e-gardens time, right at 120,000 people have read it, and hundreds have reacted that they had similar situations.

Sunscald of trunks…
Trees are grown close together in nurseries. They shade one another.
When we buy them, we plant them out in the open. For the first times in their lives they’re exposed to full sun.

I’ve never seen more graphic examples of sunscald than on these red maples along US380 in McKinney a couple of summers ago. Most have since been removed.

Species with thin bark when young (for example, Shumard red oaks, Chinquapin oaks, live oaks, Chinese pistachios, red maples) don’t get much protection.
Their bark on the west and southwest sides overheats. It then cracks and dries.
Over time it splits and peels away from the trunk.

Forest Pansy redbud may have suffered sunscald years earlier. Now trunk has become dried and is cracking.
On closer inspection evidence of borer invasion is shown by the rounded-out voids in the wood and the remains of shallow tunnels. This is probably work of red-headed wood borers. They are “secondary invaders,” meaning they move into redbuds, maples and other trees that are in distress due to other insects, diseases, neglect or old age.

Peeling bark carries with it the phloem (cylindrical tissue that carries manufactured sugars from leaves down to the roots, also the cambium layer).

Empress tree has either been victim of sunscald, or possibly a mower wheel or line trimmer damage. It has tried to form a roll of new bark to cover across the wound, but that bark has also dried. Decay will probably soon set into the trunk.

Continued Below

Tree will try to heal across the wound, but if it encircles more than half of the trunk, or if internal wood of the trunk splits and dries, entire tree may be lost.
If the tree recovers, limbs on affected side (west or southwest) may be impacted until tree can reestablish new conducting tissues.

Paper tree wrap applied from the ground up to the lowest branches and left in place for 2-3 years will protect against sunscald and therefore, subsequent invasion of borers.

To prevent this damage…
Apply paper tree wrap or plastic snap-on plant guards from ground up to lowest branches immediately after planting. As far as I am concerned, this is a non-negotiable practice.
Leave wrap in place for 18-36 months (until tree develops enough shade to protect its own trunk).
Tree wrap adheres to itself if wrapped tightly. Burlap and other fabrics are not good substitutes, but you can use plastic snap-on wraps.
Paper tree wrap is available online and in some local nurseries and hardware stores. One source is A.M. Leonard in Piqua, Ohio. A 150-ft. roll is more than $20 plus $12 shipping. That seems like a lot until you figure it will wrap a dozen or more tree trunks. That brings the per-tree cost down to around $3. For a tree that cost $150-300, that’s a pretty good bargain.

Note: There is no point in wrapping a trunk that already is showing signs of cracking.

Posted by Neil Sperry
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