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| Frost burn on Kanza |
At first I didn't think it got cold enough last Saturday morning (18 April 2020) to give cause for concern. Our thermometer, read 30 degrees F at dawn. That shouldn't be cold enough to injure emerging pecan shoots. From past experience, 28 degrees F (or below) seems to be the temperature that freezes pecan tissue.
But as temperatures began to warm early this week, I started to notice signs of frost damage (photo at right). It is now clear to me that temperatures out in my pecan grove must have dropped lower than 30 F for at least a short time during the early morning hours of April 18th.
I looked at a lot of pecan trees in my orchard and made two important observations: (1) Damage appeared more severe on lower limbs and was nearly absent on upper limbs, and (2) outer leafs were damaged but the shoot's main growing point remained still green and viable.
I cut limbs from a Kanza tree at two heights above the ground. In the photo at left, the shoot on the left was cut from a branch only 8 feet above the ground. In comparison, the shoot on the right was cut from a limb 20 feet up into the canopy. Note that the catkins on lower shoots are brown and dead, while catkins are healthy higher up in the tree. Both twigs showed evidence of viable shoot growth, however, the twig gathered from a lower limb looks like it is struggling to survive.
I would not be surprised that, when the 2020 harvest season rolls around, the majority of my nut crop will be held in the upper 2/3 of the trees' canopy.
The photo at right is typical of the growth I have observed on many of my pecan trees. The first few leaves created by a new shoot shows signs of frost burn. But if you look carefully, you will see the apex of the new shoot is undamaged and ready to unfurl new leaves. As the season progresses, pecan trees will probably abort all frost damaged leaves.
The frost we experienced last Saturday is called a radiation frost. The earth radiates its heat into a clear cold sky. This results in temperatures that are colder near the surface of the soil as compared temperatures at higher elevations. So when I checked on some pecan seedlings that I was planning to graft this year, I found total freeze kill to emerging buds (photo at left). This will not kill the tree, but the shock of freeze injury means that I will need to wait until secondary buds start to grow before even attempting to graft.
