Wednesday, January 29, 2014

Cutting quality scionwood

    Even though it's freezing cold outside today, it won't be too long before we will need to go out into the orchard and collect some pecan scionwood. The other day I cut down a young pecan tree (a really poor cultivar!) and it gave me the opportunity to cut off some pecan branches so I can show you how to find and select quality scions.

   Let's start by looking closely at a pecan branch. The best scions come from last year's new growth. In the photo above, you can see 2-year-old wood on the left and one-year-old wood on the right. Two-year-old wood is generally more grey in color and all the primary buds are missing. The reason 2-year wood has missing primary buds is because last spring, these buds opened and produced  catkins.  Two-year wood ends in what is called the annual growth ring which is actually a ring of closely spaced bud scars. One-year wood has a more brown or reddish-brown color with primary buds still firmly attached to the stem. Note that the first few buds on the 1-year wood are small and you don't see a prominent primary bud until about two inches up the stem. When I cut scionwood from a tree, I take only 1-year wood and make the first cut just above the annual growth ring.

     The best scions are collected from one-year-old shoots that are at least 2 feet in over-all length. After I cut the shoot off the tree, I cut the wood up into individual scion sticks that are each 6 to 7 inches long and contain at least 3 buds (photo at left). I try to make sure that the upper most bud on each stick is at least one inch below the top of the scion. I always discard the terminal piece of wood (stick at far left) because it is too small in diameter for the grafting techniques I use.

   Once a graft is made, the vigor of emerging buds from a scion is largely dependent on the amount of energy stored in the wood of the scion stick. In the photo at right, I've cut into each stick pictured above to reveal the amount of wood inside. The scion cut from the base of the one-year wood has the thickest layer of wood and the greatest reservoir of stored energy. As you go up the stem, you'll note that pith seems to become more prominent and the actual woody tissue more narrow.  The base of last-year's growth is the oldest portion of the stem and has had an entire growing season to lay down wood. As a stem grows in diameter the wood also compresses the pith into a narrower band of corky tissue.
    When I'm grafting, I can always tell if a scion is going to make a successful graft based solely on the ratio of wood to pith. The scion with a lot of wood and tiny pith will always out-perform a scion that's mostly pith. In cutting scionwood I always discard the terminal piece for two reasons; the buds are too close together and the stick is too pithy. In looking at the scions above, I'd choose to graft with the two sticks on the right. The second stick from the left is marginal and I'd use it only if I've run out of other scion choices.
   Remember, when you are cutting scions, you don't need to slice into each piece to check the wood to pith ratio. Simply look at the cut surface at the top of each scion and note the diameter of the pith as compared to the overall diameter of the stem. The pith diameter should be no larger than 1/3 the diameter of the entire stick.