Saturday, April 30, 2016

Grafting a pasture pecan tree

    When I stopped cutting hay on a portion of my farm, I discovered several volunteer pecan trees growing up through the grass. These trees had been mowed off at ground level every year for over a decade. So when I finally allowed the trees to grow, up came multiple stems all growing from the same root (photo at right). The growth rate on this multi-stemmed tree was impressive--over six feet of new top growth in just two years. These make great trees for grafting.
    My first step was to trim down the thicket of pecan stems down to a single tree trunk (photo at left). I kept the stem with the largest trunk diameter and cut all other stems of at ground level.
    I then cut the remaining trunk off at about 18 inches above ground level to prepare the stock for inserting a bark graft (photo at right). I have discussed how I make a bark graft in a previous post (click here) but in grafting this tree I had to make several adjustments to my technique to ensure grafting success. If you are unfamiliar with my bark grafting technique, I would suggest reading my previous post before continuing with this article.

    The diameter of the tree I choose to graft was relatively small for a bark graft (1.25 inches) but I found a flat side on one side of the stock that would make a good spot for inserting a scion. I always place my scions on the left side of the incision in the stock's bark so I made my cut on the right edge of the flat spot (photo at left). 

    In choosing a scion for grafting, I always make sure I have two large, plump buds near the top of the stick (photo at right). When carving the scion to fit under the bark of the stock, I always make sure handle the wood in such a way as to not break off either of these two very important buds. 

    Before taking my knife to the scion I always take a quick look at the base of the scion. Making the cuts in the scion and then fitting the scion into the stock is always easier when the wood is straight. However, many times when you pick up a piece of scionwood, the wood seems to grow in a zig-zag pattern, changing direction at each bud. To find a straight portion of the scion to make my cut, I simply rotate the scion until I find a straight side. In the series of photos at left, you can see how I rolled the scion around until the scion magically straightens out.

    After finding the strait side of the scion, I carved the deep cut into the scion. In making this cut, I noticed a bump near the base of the stick (yellow arrow at right). This bump turned out to be leaf scar tissue at the base of a bud (lower photo at right). A bud and leaf scar at the base of the scion does not render this scion useless but it does mean I'll need to be careful when making the next two cuts on the scion.
   I start by removing the bud near the base with my knife (photos at left). This will allow me to make the shallow cut on the back side of the scion without running into a protruding bud.

    I carved the shallow cut on the back side of the scion just as I usually do (photo at right). After making this cut, you can still see an obvious bump sticking out of the side of the scion. This bump is the remnants of the leaf scar  that was just below the bud I previously removed in the photos above. 

    When it came time to make the 90 degree cut on the edge of the scion I made sure to straighten out the edge of the scion and remove part of the leaf scar bump. In the photo at left, you can see that I cut deeper into the wood at the point of the leaf scar  but now the straightened scion will fit snugly against the bark of the stock.

    The leaf scar bump didn't disappear completely. I inserted the scion into the stock and everything fit according to plan. However, the leaf scar bump is still visible as it peeks out from under the split in the stock's bark (photo at right). I placed staples both above and below the bump to make sure the bark of the stock formed around the scion. 

    Before inserting the scion into the stock, I noticed that the cut stock tree was "bleeding".  Since trees don't contain blood, this phenomenon is actually called excessive sap flow and can cause graft failure if left unchecked. In the photo at left, sap flowed out of the cut surface of the stock tree, down a bark fissure, and then created a wet spot on the side of the stock.
    To prevent graft failure from excessive sap flow, I cut sap relieve grooves on either side of the stock tree using my pruning saw. Each groove is cut half way around the stem and the grooves are made 1.5 to 2 inches apart vertically (photo at right). The grooves are cut just deep enough to enter the sap wood.  

   By cutting sap relieve grooves, the excessive sap flows out of the groves and will not reach the graft union to inhibit callus formation. In the photo at left, you can see how much sap has leaked out of the sap relieve groove  just during the time it took me to insert the scion, wrap the graft, and attache a bird perch.
    In 2-3 days, sap will stop flowing from the sap relieve grooves and the tree will begin to callus over the wounds. At the same time, the tree will also begin forming the callus tissues needed to make a successful graft union. 

    Before leaving my new graft, I placed a deer cage over the tree (photo at right). This cage will prevent browsing deer from destroying the new buds as they emerge from the scion. With all the stored up energy in the root system of the pasture pecan tree, I will expect this graft to take off and grow 4-5 feet this summer. It will be fun to watch!