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!

Thursday, April 21, 2016

Pecan buds push open and reveal catkins

City Park, protogynous
  Yesterday, the sun broke through the clouds so I grabbed my camera to photograph emerging pecan buds. My goal was to concentrate on recording the growth stage and flowering habit of a dozen pecan cultivars that we haven't gained a lot of experience with at the Pecan Experiment Field. When you look over the photos published in this post,  you can see that some cultivars have popped open earlier than others. In addition, you can now distinguish protandrous flowering cultivars (early pollen) from protogynous cultivars (late pollen). Protandrous cultivars have short thick catkins that emerge at the same time as new shoots. The catkins on protogynous cultivars emerge after leaf brust and are long and thin in appearance. I have captioned each photo in this post with the cultivar name and the flowering type for that cultivar. Remember, you can see an enlarged version of each photo by simply clicking on one of the images.

Goosepond, protogynous

Hark, protandrous
KT143, protandrous

Lucas, protogynous

Mullahy, protogynous
Oswego, protogynous

Surecrop, protogynous

SWB617, protandrous

Waccamaw, protandrous
Warren 346, protandrous

Yates 68, Protandrous

Tuesday, April 19, 2016

When is it time to graft pecan trees?

    We have a small planting of seedling pecan trees that I plan on grafting this spring. All the trees in this planting are open-pollinated Kanza seedlings. This morning, I looked over the field and noticed that some trees in the planting had entered the leaf burst stage (photo at right).  Since I'll be using a bark graft to establish selections from our breeding project in this field, seeing trees in the leaf burst stage has made me anxious to start carving scions and placing grafts.  You see, leaf burst is the signal I use to determine when the bark of the tree will slip enough to allow scion insertion.
     However, looking across the entire field I noticed that bud development varied widely among seedling rootstock trees. Right next to a tree in leaf burst, I found a tree in the bud enlargement phase of bud development (photo at left). If I tried to place a bark graft on this tree, I'd probably find that the bark on the main trunk would not slip and I would have trouble inserting a scion. A tree at this stage of bud development is just not ready for bark grafting.
    Moving down the row of seedling trees I found several trees in the inner-scale split stage of bud development (photo at right). If I was planning to use a three-flap graft, this would be the perfect stage of bud development to start grafting. However, the trees in this block have all grown much too large in size to effectively use a three-flap graft. Since the bark graft is the preferred method to use on trees with trucks that range from 1 to 4 inches in diameter, I will need to wait a little longer before attempting a bark graft on the tree pictured at right.  
     Even though a handful of trees in this block of Kanza seedlings have leafed out far enough for effective bark grafting, I'll probably wait another week before grafting. This will allow the other trees in the planting to catch up in terms of bud development and allow me to graft every tree in every row all in a single day. That way I'll be sure to get every tree grafted and grafted to the right breeding project selection.

Thursday, April 7, 2016

Why we call it sapwood

   Last week, we removed a couple of large native pecan trees that were shading the growth of some young trees in an adjacent planting. After cutting the trees, we left the stumps in place for a couple of days while we hauled off the brush. One day after cutting the trees down, you could see large amounts of tree sap flowing from the outer growth rings of the trunk (photo above). In every case, the sap flow was limited to the outer 4 to 5 growth rings. The vast majority of the wood in the tree stump remained relatively dry.
    This high flow of sap from the outer-most layers of wood is the origin of the term "sapwood". However, the term sapwood means something entirely different to those that cut pecan trees for lumber. The sapwood refers to the lighter, almost white colored wood on the outer portion of the trunk. The heartwood of pecan trees is found in the center of the tree and is typically brown in color. You can see darker colored heartwood at the far right in the photo above.

     Next time you cut down a pecan tree, look to see if you can identify the different tissues that make up a tree's trunk. In the photo above, I've labeled the important tissues.  Starting on the left you can see several annual rings of wood tissue. This photo was taken just moments after the tree was felled and you can already see wet spots developing around the largest pores in the outer-most growth rings. Botanically, the wood of a pecan tree is xylem tissue which is responsible for conducting water from the roots upwards to the leaves. As wood tissue ages, becoming increasingly buried under new layers of wood (annual growth rings), the pores in the wood becomes clogged with lignin. Lignin blocks water flow but increases wood strength and rot resistance.
    Outside the wood is a narrow band of cells called the cambium (red arrow above). Every spring these cells become active creating new wood cells on one side and new bark cells on the other. The activity of the cambium layer is responsible for the tree's annual increase in diameter.
   On a mature pecan tree, you will find two distinct layers of bark. The inner bark or phloem functions to transport carbohydrates from the leaves downward to all other portions of the tree (branches, trunk, roots). The outer bark provides protection for the living tissues underneath. The outer bark acts as a vapor barrier to prevent moisture loss and as insulation against heat and cold.

Tuesday, April 5, 2016

Pecan tree starting to push

    During yesterday's this beautiful spring weather, I took a short walk around my pecan plantings and noticed that some trees were starting to push open new buds. The photo at right was taken yesterday and shows the emerging buds of a Faith pecan tree. The inner scales have split open and dropped to the ground to reveal a large, green vegetative bud surrounded by smaller buds containing catkins.
    During my walk-about, I also found a Jayhawk tree with bright green buds (photo at left). Seeing green buds this early in April makes me a little nervous. I've already loss a large part of my peach crop and part of my pear crop to late-spring freezes last month and we are not statistically past the danger of additional cold weather. Pecan buds at this of development can withstand temperatures of 26 degrees F (-3 C) and thankfully the 10 day forecast looks like we'll stay above the freezing mark.
    Other pecan cultivars were just starting to split open their inner scales. Kanza buds look like they are wearing little helmets as the inner bud scale pops free from the expanding bud beneath (photo at right).  Gardner trees looked to be at the same bud stage as Kanza.
     Greenriver buds have swollen but the inner bud scale was still covering the entire bud (photo at left). Hark and Lakota buds appeared much like Greenriver, still fully covered by the inner scale.