Wednesday, April 16, 2014

Frost damage and tree height

    The frost that settled over the Neosho River floodplain Tuesday morning was a so-called radiation frost. These type of frosts occur on clear, calm nights when the earth radiates heat back into the atmosphere. During a radiation frost the temperature near the soil surface will be colder than temperatures higher up in the canopies of pecan trees. The temperature gradient that is created by a radiation freeze can end up creating a distinctive kill line in the canopy of pecan trees. Buds below the kill line are exposed to temperatures capable of killing green tissue.  Above the kill line, temperatures are slightly higher and pecan buds can escape damage. Yesterday, I used our hydraulic lift to see if I could find evidence of a kill line.

   My first stop was a Greenriver tree. From the ground, I could tell that this cultivar had suffered major freeze injury. Bud development was well advanced making the exposed green tissues very susceptible to cold. Since my hydraulic lift limits me to reaching just 25 feet above the ground, I decided to collect samples at 12 and 25 feet. Looking at the branches I collected from thee two heights, it looks like Greenriver was damaged well up into the tree's canopy (photo at right).
      It wasn't until I cut open the terminal bud of these branches that I found that height does indeed impact the amount of cold injury. At 12 feet above the ground the emerging vegetative bud is nearly all black (photo at right). At 25 feet, you can see that the outer portion of the new shoot was burned but the inner core remained green. In looking at these buds it became clear to me that the bud at 12 feet was exposed to killing temperatures for a much longer time period than the bud at 25 feet. This make sense because as the earth looses it's heat to the night sky, the critical kill temperature line creeps higher with time. Then suddenly the sun pops out, air temperature rapidly increase and the freezing of plant tissue ceases.

   I also looked at Kanza.  Buds at both 12 and 25 feet looked in pretty good shape. Buds at the outer scale split stage of development may not be as cold resistant as fully dormant buds but they are far more cold hardy than buds that have started to elongate (like the Greenriver buds above). The real test would come when I split them open with a razor blade.
    In the photo below, you can see that Kanza buds taken from both heights are still nice and green. Looks like our Kanza trees will be largely unaffected by this freeze.