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.

Tuesday, April 15, 2014

Freezing temperatures injure emerging pecan buds

  During the pre-dawn hours of April 15th, temperatures dropped to 26 degrees F at the Pecan Experiment Field. At 26 degrees and lower rapidly expanding green pecan tissue is frozen and killed. A few trees had already developed visible catkins and were pushing out the first new leaves on new shoots. These trees were frozen back completely (photo at right). Fortunately, most trees had hardly started to push out new growth and escaped the freezing temperatures with little or no damage.
   To really appreciate the extent of freeze damage we suffered Tuesday morning, I spent some time up in our hydraulic lift to survey bud health.  Let's take a look.
   The first thing I noticed was an amazing degree of variation within a single tree's canopy. Just look at the photo at left. These two branches are growing on the same Greenriver tree and are even attached to each other a few inches below the view of the photo. The branch on the left is fully green and shows no indication of cold injury while the branch on the right was burned by the frost.  Why or how this happened has no easy explanation.
   However, lets look at the kinds of variation I found within the canopies of some other trees to see if we can learn how this weather event will effect the 2014 pecan crop.
    I stopped at the original Chetopa tree to see how the emerging buds looked. Once again, I found everything from healthy green buds to obviously dead buds (photo at right). Fortunately, the number of green buds on the Chetopa tree far outnumbered the damaged buds.
    You can check the condition of your buds by slicing through a bud with a knife or razor blade. Healthy buds will be green while frost killed buds will be black (photo at right).
    When I looked at a random native tree, I found similar variation in frost injury (photo at left). Once again I cut open some buds to check on their viability. However this time I found something fascinating.
     Once again, the healthy buds were green inside while the frozen buds were black. However, look carefully at the bud in the middle of the photo. This bud, like all terminal pecan buds, comes in three parts. The center vegetative bud is flanked by two, catkin-containing sexual buds. In the photo, the sexual buds are black inside, indicating they were killed by the cold. However, the central vegetative bud is still green indicating live tissue. It will be interesting to follow all our tree over the next several weeks to see how the April 15th freeze impacts pecan flowing in May.

Friday, April 11, 2014

Watching pecan bubreak

  The past couple of days have been sunny and warm. Pecan buds are swelling and I have even noticed the early stages of catkin development.   Today, I collected terminal branches from Kanza and Major to show you how bud development differs between protogynous (type 2) and protandrous (type 1) cultivars.
   Back in 2012, I photographed some terminal branches that had already started vegetative shoot elongation. By that time, the differences between cultivars that shed pollen early (type 1) and those that shed late (type 2) were very obvious. The photo I took today shows pecan growth at a much earlier stage of development (photo above, right). Here the differences between protandrous and protogynous cultivars are a little more subtle.Let take a closer look.
   The shoot on the right was cut from a Major pecan tree. Once the outer scale splits opens and falls off you will see what looks like 3 buds inside (see red arrow). The bud in the center will develop into a new vegetative shoot that will eventually terminate in a cluster of female flowers. The green buds that flank the central bud will develop into catkins. Note how plump and large the catkin producing buds are on the Major twig as compared to the thin, less-noticeable, catkin-containing buds that surround the vegetative bud on Kanza. To produce pollen early in the flowering season, Major catkins need to start developing early, starting right at bud break. In fact, the yellow arrow points to Major catkins starting to emerge from under the inner bud scale.
    In comparison, late-pollen-shedding Kanza will expend most of its early growth energy creating new shoots capable of producing mature pistillate flowers quickly  and ready to receive early flying pollen. Catkins will develop on Kanza at a much slower pace and will not begin to shed pollen until after pistillate flowers have pollinated by a protandrous cultivar like Major.

Wednesday, April 9, 2014

Grinding out stumps

  Over the past few weeks we have been thinning trees to give the remaining trees in our pecan orchards more room to grow. Even though we can cut the tree stump down fairly close to the soil surface with a chainsaw, that chunk of solid wood that still sticks up can still play havoc with our mowers.
   Yesterday, we brought out our stump grinder to cut all our tree stump down to a level about 3 inches below the soil surface (photo above). Ours is a 3-point hitch model stump grinder that uses a spinning saw blade to grind the wood into chips. Hydraulic cylinders move the blade's arm up and down, back and forth, and right to left.
   It takes about 5 minutes to grind down a 10-12 inch diameter pecan tree stump.  Later this year, we will need to move soil into the hole that becomes noticeable after the remaining portion of the tree's root system starts to rot.