Unequally yoked: grafting hickory on pecan and pecan on hickory.

    Pecan and hickory are closely related, both belonging to the genus, Carya.  Pecan and hickory are also graft compatible meaning scions from one species can be grafted successfully on a tree from another member of the Carya genus. The question becomes, should these type inter-specific grafts be made?
    Pecan is the fastest and largest growing species in the hickory family. Because of this fact, I often recommend that hickory nut enthusiasts graft their favorite hickory cultivars onto pecan roots. The vigor of the pecan rootstock pushes the hickory scion to grow faster and to start producing nuts earlier.
    The photo at right shows two hickory cultivars (smooth bark) grafted on to a single forked pecan tree. Note how the pecan portion of each graft union has already grown larger in diameter than the hickory tops.
    One word of caution. If this were my tree, I would have pruned out the fork in this tree long ago and kept just a single hickory cultivar.  As this tree grows larger, the chances of this tree breaking out in a wind storm only increases.

      I do not recommend grafting pecan onto hickory rootstock. Slow growing hickory will actually slow down the growth of a pecan scion and inhibit nut production. The photo at left shows what happens when a pecan scion is placed on a hickory root.  Once again the pecan portion of the tree out-grows the hickory portion. The tree becomes top heavy and the hickory roots have a hard time providing enough water and nutrients for the massive pecan top.
   It is tempting to graft pecan on to hickory seedlings because several species of hickory can tolerate drought-prone soils found on upland sites (as in the photo). Pecan, being a flood plain species, would struggle if planted directly in upland soil conditions. Unfortunately, grafting pecan onto hickory tree seedlings, already established on a upland site, will not make nut production from the pecan top any easier. The hickory root and the upland soil will never be able to provide the water needed by pecan scion to grow and fill out a nut crop. 

Wood growth in a forked pecan tree

    Back when I was thinning trees out of our pecan breeding plot, I cut down a tree that four years earlier had been a forked tree. The photo at right shows the basal portion of the removed tree and you can see the tree wound left by removing the fork in the tree.

    Cutting down this tree gave me the opportunity to see how pecan wood growth is effected by a tree having two main trunks. In the photo at left the red arrows point to the centers of the two trunks that began growth back when this tree was a small sapling. The yellow arrow points to the bark inclusion that developed between the two trunks.
    Four years ago I pruned out one of the two main trunks. The bark inclusion stopped spreading and four years of solid wood grew all around the tree's circumference. 

    A bark inclusion forms between the two halves of a forked tree largely in response to wind. As wind moves the upper portion of the tree, the two halves of the tree are pulled apart, splitting open the wood.
    If you look closely at the growth rings of this tree, you will note that the rings are identical on both sides of the bark inclusion (photo above). This tells me that the growth ring was formed first then split apart later.

    It is interesting to look closely at the wood growth patterns of original two stems of this tree (photo at right). At first the two stems grew independently, each forming their own growth ring. As the two grew in diameter, the stems grafted together and developed a single, shared growth ring. However, as the tree grew taller and branched out to form a larger canopy, the wind started to pull on the two trunks taring them apart.
    If you ever see a young tree develop a forked trunk, prune the tree to a single stem as soon as possible. Once pruned, the tree will grow new growth rings over any bark inclusions that may have started to develop.
    
       

Thinning a pecan orchard in a single year.

      When to thin is one of the most difficult decisions a grower must face during the life of a pecan orchard. Over the past several years, I've documented our gradual approach to thinning a block of Kanza trees. But last week, we completed a more typical "once-over" tree thinning operation in a 4 acre cultivar trail (photo at right).
    This cultivar trial was originally planted at a 30 by 30 foot spacing back in 1991. After 24 years of tree growth the limbs of adjacent trees were getting close to touching and very little sunshine penetrated to the orchard floor in mid-summer. In fact, several portions of the orchard were actually past due for tree thinning. 
    Before cutting a single tree, I mapped out the entire orchard recording the locations of weak trees or trees badly injured by wind or ice storms. I knew I would be cutting down 50% of the trees during this once-over thinning operation. However, with map in hand, I could choose a thinning plan that would maximize the removal of "problem" trees.

    In thinning the grove, we removed odd-numbered trees in the first row. Even-numbered trees were removed from row two. In row three, we were back to removing odd-numbered trees. We continued this pattern across the entire block of trees.
    Once all the trees were cut, I could look across the field at a 45 degrees from the tree rows and see an entire diagonal row of trees had been cut (photo at left).
    After removing felled trees from the field, the orchard looked wide open with plenty of space for the remaining trees to grow wide canopies. The remaining trees are now spaced 42.4 feet apart.
   

Orchard thinning yields great scionwood.

    February is a good month for thinning pecan orchards. It is also a good month for collecting pecan scionwood. So today, we took advantage of the sunny weather to collect scions from the trees we cut during a orchard thinning operation (photo at right).

   Once a tree is cut down, you can ready see why it is so hard to find good scions from nut bearing trees. On lower and mid-canopy limbs, new shoot growth is limited by nut production. The photo at left shows a typical lower-canopy branch. Last year's growth is highlighted by the yellow line and extends about 5 inches before terminating in a peduncle or the former fruit bearing stalk. Note that last year's shoot growth is lighter in color and displays large, prominent buds. However, this shoot does not make for very good scionwood. The growth is short, the buds are close together, and the stem is crooked. 

    Walking around to the top of the tree, I found really long, healthy shoots that would make great scions (photo at right). What was once 40 feet up at the very top of the tree, was now easily accessible from the ground.

   I cut dozens of one-year-old shoots that measured nearly 3 feet in length (photo at right). This is the perfect type of wood for cutting into scions. 

    Taking the 34 inch shoot pictured above, I cut the wood into pieces. In the photo at left, I've arranged the wood from the lowest portion of the stem to the terminal. In looking over these pieces of wood, notice how bud size increases and buds become closer together as you get closer to the terminal. For great scionwood, you want prominent, healthy buds, but those buds should be widely spaced to make grafting cuts easier.
    I always discard the terminal portion of the shoot when collecting scionwood. The terminal is crooked, has too many buds, and is nearly impossible to carve when grafting.
  On the other end of the shoot, the basal piece of wood has very small buds which are prone to falling off the stem (yellow arrow points to aborted bud scar). I'll save this piece of wood for grafting because the secondary buds can grow into new shoots. However, when I'm out grafting and have a pile of scions to choose from, I'll choose this type of wood only after I've run out of scions with more robust buds.

Thinning trees from a Kanza block

    In keeping with our plan to gradually thin our block of Kanza trees, I cut down nine trees yesterday (photo at right).  We started thinning this block of 144 trees back in 2012, removing 5 to 9 trees each year since that time. So far we have removed a total of 34 trees in 5 years time.

    The map at left shows the position of the trees we removed in 2016. Each green circle represents a tree with the diameter of the circle proportional the the diameter of the tree. The black circles represent trees removed this week.
    So far I've been pleased this this progressive approach to orchard thinning. By removing just a few trees each year we have been able to prevent orchard overcrowding while maintaining total orchard yield. You can review the yield and tree removal history of this block of Kanza trees in this post:

Kanza nut yield for 2015

Making pecan breeding plot selections

     To many pecan growers, thinning a pecan orchard is the hardest job they will ever have to accomplish. Cutting down trees that required so many years of work to grow seems heartbreaking. But over the weekend, I started thinning trees in our pecan breeding plot (photo above). This is more than just a simple tree thinning operation. I was making tree by tree decisions of which trees were worth saving and which would be eliminated from the breeding project.

    Many trees in the breeding plot have now produced nuts for three years giving us a good idea about nut size and percent kernel for each tree. As you would expect, the nuts produced by a field of seedling trees varies widely. The photo above shows nuts produced by three trees in our breeding plot. The nut at the far left weighed 8.17g and yielded 58.61% kernel. This is one tree I definitely needed to save. The football shaped nut in the center produced a very attractive but small kernel. However, at only 4.54g and 47.29% kernel, I choose to cut down this tree. At the far right, this small 4.99g nut had a thin shell (53.67% kernel) but the nut was way to small to remain in the trial.

    Not all tree removal decisions were based solely on nut size and percent kernel. I choose to remove trees that produced kernels with serious defects and trees that exhibited severe susceptibility to pecan scab. The photo above illustrates one kind of kernel defect that warrants tree removal. The nut on the right is marked with very unattractive dark brown mottles. Compared to the bright straw-colored kernel at left, it is easy to see which nut would be attractive to consumers (left nut) and which tree needed to be removed (right nut).

Pecan cultivars and tree size

    In setting out a new pecan cultivar trial, I anxiously wait for the trees to start bearing so I can record nut production data. Nut size, percent kernel, flowering type, maturity date, yield, and disease resistance are all among the key cultivar attributes that can define cultivar success. However, not all cultivar characteristics are revealed within the first decade of nut bearing.
    I was walking through the oldest cultivar trial on the research station when  I noticed obvious cultivar differences in over-all tree size. In the photo above, you can see that Major produced a much larger tree than Dooley. Both cultivars were grafted onto Giles seedling rootstock planted in the field back in 1981. The only difference between these two trees is the scion cultivar. 

    Looking around this 35-year-old trial, I found that trunk diameter was largely a function of cultivar. To create a visual record of cultivar differences in trunk diameter, I photographed the trunks of 6 pecan cultivars placing a one-foot-long ruler across the truck for scale (photo above ). Major and Peruque grew trees so large they seem to dwarf all others in comparision. Colby and Posey grew moderate sized trees while trees grafted to Giles or Dooley seemed small in comparison.
   I would never consider tree size a very important tree characteristic in making cultivar decisions. However, tree growth rate becomes important when planning initial orchard spacing and making subsequent thinning decisions. Large growing trees, like Major or Peruque, should be established at a wider spacing--let say, 40 feet rather than the standard 35 feet. Tree growth rate will also influence tree thinning decisions. The larger a tree grows, the sooner you will need to thin out adjacent trees to make room for future growth.
    It takes at least 30 years of cultivar testing to make accurate tree size determinations. In that time, many cultivars will prove unworthy of future propagation. Of the six cultivars pictured above, I would consider grafting only Major because of its scab resistance and regular nut production. But now, when I graft new Major trees, I'll also be sure to allow more space for these trees to grow.