Tuesday, January 30, 2018

Selecting scionwood

    On long cold winter days, my thoughts turn towards collecting scionwood in preparation for next Spring's grafting season. The best scions are often found growing at the tops of young pecan trees (photo at right). However, I make it a practice to wait until a tree starts bearing nuts before cutting wood from a tree. This way, I verify that the tree is true to cultivar before cutting scions.

   Walking up to the tree pictured above,  I noticed that the lower branches had very short one-year-old wood and that twigs were small in diameter. Last year's wood was only three inches long (length of the blue arrow)--not nearly long enough for a proper scion stick (photo at left).
    The pedicel at the end of the shoot (red arrow) indicates that this shoot held a cluster of nuts last summer.  When a branch is located within the lower portion of the canopy, the combination of partial shade and nut production slows shoot growth dramatically. With limited sunlight, the shoot invests all available energy resources into nut development rather that shoot extension growth.
    Ultimately, short shoots make poor scions because the buds on the stem are too close together and the diameter of the stem is less than that of a pencil.

    High up in the canopy of the tree the shoots were longer. Normally I can find some pretty good scions at the top of a tree but in 2017 this Kanza tree had a full nut crop. Rather than growing a nice thick vegetative shoot, upper branches on this tree set a cluster of nuts (pedicel marked by purple arrow) and then produced a secondary flush of vegetative growth. The result were shoots long enough for scionwood but too small diameter to make really excellent propagation material (photo at left).
    To ensure a plentiful supply of scionwood, we have developed an orchard of trees specifically trained to produce long, thick vegetative shoots. I call it training but what we are really doing is chopping back the tree to keep the tree short (so we can reach the scionwood with our hydraulic lift) and force the growth of new vigorous vegetative shoots. In the photo at right, you can see numerous shoots growing from one of our scionwood trees.

    The photo at left shows a couple of one-year-old shoots that I cut out of the top of a scionwood tree. These shoots are between three and four feet in length. The buds are far apart and the wood is the perfect diameter for scionwood. When I cut scions, I'll break down these long shoots into sticks that contain at least three buds. I'll discard the terminal portion of the shoot, where buds are spaced too close together and the shoot is too thin. 

   One of the side effects of forcing vigorous vegetative growth on pecan trees is the development of stalked buds (photo at right). The red arrows point to stalked buds that look more like short stems than buds. Although stalked buds do not impact a scions ability to sprout and grow after grafting, I make it a practice to prune off these stalks before storing the scions in the cooler. Left on a scion, stalked buds have a tendency to puncture the plastic bags I use for scion storage. I surely don't want to risk having my scions dry out in storage because of a hole in the bag.
    If you plan to graft a large number of trees, it pays to plan ahead. Develop your own scionwood trees this March and harvest high quality scions the following year. Once you finish grafting your orchard, scionwood trees can be left to grow normally and eventually return to become productive nut trees.