Sunday, April 26, 2015

Time for grafting

    The sun was shinning and the temperature was perfect for grafting pecan trees yesterday. I spent much of the day putting scions on seedling trees using the 3-flap graft, arrowhead graft, or bark graft. The method I used depended on the size of the stock tree and the diameter of the scionwood I had on hand.
    Last year, I posted photos of a graft that froze back during the winter of 2013-2014. Rather than re-grafting the tree right away, I decided to let the tree recover and sprout out a new shoot during the summer of 2014. The photo at right shows how the tree looked this spring (2015) with the dead 2013 graft still attached and a stump sprout that developed in 2014 growing out and around the old graft union.
   In this post, I'm going to show you how I re-grafted this tree while giving a few grafting tips along the way.
   The first thing I wanted to show you is how I carry my scions to the field (photo above). I start by filling a couple of 2-liter pop bottles with water and freezing them solid (make sure to leave some air space in the bottle to allow for ice expansion). I use plastic storage boxes to store my scions in refrigeration, making sure to mark the cultivar name clearly on the box. The plastic storage box fits neatly on top of the frozen pop bottles and I'm ready to go to the field and start grafting. 

   Now let's take a closer look at this problem tree (photo at left). You can clearly see where I stapled in a bark graft back in 2013 (left hand branch) and the stem died back to a point just above the new stump sprout.
   My first step was to cut off all the dead wood. A good, sharp pruning saw is a must for a making a cut that angles away from the stump sprout (photo at right).
   In re-grafting a tree, I decided to place my next graft on the fast-growing, stump sprout. This sprout is now the tree's central leader and I can take advantage of the push I'll get by grafting at this apically dominate location .

   Bark thickness was another key factor in my decision to place the graft on the stump sprout. Picking up the parts of the tree I had removed in preparing the tree for grafting, I photographed (at right) the cut surfaces so you can see differences in bark thickness. The smaller diameter limb was cut from the stump sprout while the larger piece was cut when I trimmed out the dead wood on the main trunk. It is much easier to get a tight fit around the scion when the bark is thin and naturally more pliable.
    Sure, I could have cut the tree off below the stump sprout but grafting on a trunk with thick bark takes a little more time to prepare. With a thick-barked stem,  I recommend shaving down the bark with a wood rasp to make the bark more flexible for grafting. 

   On this tree I decided to use a bark graft. The first step in making a bark graft is to make an incision in a straight line down the stock about 2.5 inches long (photo at left). Notice how crooked the stock is on this tree. When making the incision, make sure to draw a line straight downwards ignoring the bends and twists in the stock.

   Next, I moved on to preparing the scion (photo at right). I pulled a piece of Kanza wood out of my cooler and searched for a spot to make smooth, straight cut. The piece I found had a slight curve at the base the scion that I removed with my clippers. After trimming off the bottom of the scion I was left with a good 2.5 inches of straight stem (highlighted by yellow arrow).

    It was now time to start carving my scion. I rotated the scion around to find the straightest place to make my deep cut. Turns out, I made the first cut just above a bud, removing that bud from the grafting zone. I took several more passes with my knife to pare away wood until I got down past the pith (photos at left).
   One detail to notice at this point is that the scion has a uniform thickness but the tongue of the scion has a slight curve to it. We will find that this curve will reappear as a slight problem when we staple the scion into the stock.
    I turned the scion over and started to make the backside cuts (photos at right). First was the angled cut--slicing off just enough bark to expose the cambium and removing another bud in the process. I finished off scion preparation by shaving a 90 degree cut along the wide edge of the scion then carving a chisel point on the end.  (For more a detailed description of scion preparation for bark grafting, see this post).
   I quickly inserted the scion under the bark of the stock along the left side of the incision I made previously (photo at left).  Here's where the curvature in the scion's tongue comes into play.
    After inserting the scion, I had a tight fit along the bottom portion of the graft. However, the yellow arrow points to an air gap between scion and stock caused by the scion's curvature. To close that gap, I used my staple gun. I always start by placing a staple at the bottom of the graft and work my upward. When I got to the air gap, I pressed the scion firmly against the stock and put in an extra staple to hold the scion in place.

    To remove air gaps along the left edge of the scion, I placed staples vertically to press the stock's bark firmly up against the scion. Notice how the bark has been forced to bend inwards to form to the scion. This is why having a nice pliable bark on the stock tree is so important to graft success.
    Once I had the scion in place, I wrapped the graft union with aluminum foil and placed a plastic bag over the whole thing to hold in moisture. The final step was to attach a bird perch to the tree (photo at left).
    I might have a crooked looking tree for a few years, but eventually the trunk will straighten out and I'll be harvesting bucket loads of Kanza pecans.