Tuesday, April 29, 2014

Changing pecan cultivars by top-working

   After some warm days and some much needed rain, pecan trees are finally starting to show a burst of new growth this week (photo at right). When you see new leaves just starting to unfurl, the tree is giving you the signal that it is the best time to start bark grafting.
    For an in-depth look at the bark grafting method I use to top-work pecan trees, check out this previous post. In today's post, I wanted to show you how I attack the top-working process while giving a few tips along the way.
   The tree I chose to top-work was a beautiful example of a well trained tree that had previously been grafted to Jayhawk (photo at left). Over the past several years we have noticed some serious kernel quality issues with Jayhawk so I decided to regraft this tree to a selection from our pecan breeding program.
   My first cut was on the central leader about 18 inches above the lowest whorl of side branches. This cut is marked by the uppermost red line. I also removed a small side limb on the left side of the tree just below the cut I made on the central leader. I also removed the upward-growing portion of a lateral branch on the right side of the tree to make sure my new graft will get plenty of light. These cuts are also marked by red lines on the photo.

    The photo at left shows you how the tree looks after I finished pruning. Notice how open the center of the tree looks. This openness will allow full sun to reach the graft and will promote rapid regrowth. Although chopped back to one half its original height, the central leader still holds a prominent position in this tree's architecture. Leaving 18 inches of central leader above lower branches will help draw energy and nutrients toward the new graft and increase the chances for graft success.
   Now let's focus our attention on selecting the location to insert a bark graft. After cutting the central leader, I looked at the shape of the cut stump (photo at right). Notice how the left side of the stump seems to have a flat spot and is not as highly curved as the rest of the stem. This is where I will place my graft.
    I then inspected the health of the bark on the flat side of the trunk. Right within the grafting zone I found a prominent bud scar (in yellow circle at left).  I have often found that the cambium layer under this kind of bud scar is dead causing the bark to adhere to the wood making scion insertion difficult. In addition, a dead zone in the cambium of the stock within the grafting zone limits overall graft success.
    In this case, I was able to make the incision in the bark far enough to the right of the bud scar to avoid any potential problems. After carving the scion, I inserted it between the bark and wood of the stock and slid it into position perfectly.
    The photo at right shows the scion in place and the bark flap stapled tightly against scion's cut surfaces. Note that I was able to avoid sliding the scion under any portion of the bud scar. Time to wrap the graft union in aluminum foil and cover with a plastic bag.
      Before leaving this newly top-worked tree, I attached a bamboo stake to the trunk of the tree (photo at left). The stake will provide a bird perch to prevent bird damage to the scion. As the new graft emerges, I can then tie the new shoots to the stake to prevent wind damage. Last but not least, I labeled the tree with the name of the scion cultivar.
   When I moved on to the next Jayhawk tree, I found a not-so-perfectly-shaped tree (photo at right). At first glance the tree was growing every-which-way. The tree had at least one "V" crotch that I had tried to suppress in previous years with pruning. As soon as I approached this tree, I knew this tree was going to need some major pruning along with my planned top-working.
       As I came in closer to look at the tree limb structure, I quickly noticed a major limb wound on the main trunk. I also found no clear place to position a bark graft so it would grow without direct competition from lateral limbs. This tree was going to need radical pruning.
  I then decided to remove most of the upper portions of this tree. My first cut was to remove entire left side (larger diameter side) of the tree's forked trunk. The smoother bark and smaller diameter of the right side fork would provide a perfect grafting spot once the limb was stubbed back. Fortunately, this tree a several small, lateral branches growing out of the trunk below my pruning cuts. These branches would help shade the trunk and prevent any sunscald that might occur following such radical pruning.  
     My neighbors are sure going to shake their heads when they drive by this tree (photo at right).  Some might even think I was trying to kill the tree. However, once I insert a new scion and that scion takes off this summer, I will have a graft that will grow tall and straight providing me a great central leader tree.
   The photo at left shows the branch stub that will receive my scion. As I said before, I like to place the graft on the flat side of the stock. When grafting forked trees, I have always noticed that the flattest side will be facing the inside of the fork. That is exactly where I inserted the scion in this tree.
   The completed graft is shown below.