A Lost Hope?

Posted by: Curtis Utley, Jefferson County Plant Diagnostic Clinic

On a recent tree evaluation visit I discovered more fallout from the historic polar vortex freeze event in November 2014. Many of my contemporaries said we would continue to see more damage in otherwise apparently unaffected species and sure enough, now two seasons later, green ash trees are pushing off large sections of damaged bark. 

Bark loss from Callery pear observed in 2015

This damage and the resulting recovery is similar to what we are seeing on ornamental pears but the symptoms have been delayed do to the difference in bark thickness of green ash. The South or Southwest facing sides of green ash trunks and branches had not gone dormant when the freeze event occurred and these portions of the vascular cambium were killed. 

Green ash with loose bark on the south side of the tree.

White ash trees were not affected but green ash trees growing in warm locations, in this particular case street trees growing on a south facing slope show considerable damage. 

Thinning canopy and dieback on ash from the November 2014 polar vortex freeze event.

The marginal cambium that did not freeze is now trying to grow over the damaged section of trunk wood and is pushing the dead bark off in its attempt to close the wound. Another concerning symptom that is explained by this freeze damage is the thinning canopy and decreasing growth increments in affected trees. 

Ash tree attempting to close wounds from freeze injury.

We have had two cool wet springs that should have resulted in fantastic growth in green ash and to see the opposite has had me concerned. The elephant in the room anytime we discuss symptomatic  ash trees is of course the possibility of finding emerald ash borer, especially in trees that are losing chunks of bark, have thinning canopies, and dying crowns. The difference between freeze damage induced bark loss and bark loss as the result of the exotic emerald ash borer is the absence of a zig-zag or meandering pattern visible both on the loose pieces of bark and exposed wood caused by EAB larval feeding. Learn more about emerald ash borer at: eabcolorado.com

Emerald ash borer feeding (photo courtesy of University of Minnesota Extension)