Pistillate flower strength and pecan nut set

  I was checking on the development of the 2017 pecan crop last week and made some interesting observations about pecan flowering and nut set.  Whenever I look at pistillate flower clusters that are fully receptive and ready to be pollinated, I become extremely optimistic about the current season's nut crop. A cluster of five nutlets with bright red stigmas just looks so impressive (photo above, right).

   However, rarely do all the pistillate flowers in a cluster turn into harvestable pecans. When a pecan tree creates an pistillate flower cluster, the flowers at the end of the pedicel are often smaller or ill-formed (photo at left). Its like the tree just runs out of gas at the end of the flowering stalk.  

     Weak or poorly formed flowers may capture pollen but these flowers don't have the strength to produce a fertilized nut. This is why you might find that terminal flowers turn brown and eventually fall off the tree (photo at right). What the casual observer might blame on poor pollination, is actually weakly formed pistillate flowers being shed by the tree. 

    Pecan tree will only hold on to nuts that have become fertilized. The developing seed produces a plant hormone that sends a signal to the rest of the plant that basically says-- "hold me and feed me". The photo at left illustrates how a pecan tree shed unwanted unfertilized pistillate flowers. At first the weak flower turns brown (labeled A). The tree responds by forming an abscission layer between the flower and the pedicel. The location marked B once held a pistillate flower that has since aborted. If the pedicel does not contain a single fertilized nut, the entire pedicel will be removed from the tree. You can see an abscission layer forming at the base of the pedicel (marked C). 
     Without an actively growing nut cluster, you will note that the buds just below the pedicel are starting to swell. A second flush of vegetative growth  will soon appear on this branch terminal.

Checking on pecan pollination

    Yesterday, I wandered through our pecan cultivar trials to check on the progress of pecan pollination.  At this point, we are half way through pollination. All of the protogynous cultivars are releasing pollen and the protandrous cultivars have receptive pistillate flowers. Pawnee is a protandrous cultivar that is now displaying large, red stigmas on the ends of pistillate flowers (photo above).  The catkins on Pawnee shed their pollen a while back and have now dropped to the ground.

     The pistillate flowers of Kanza are fully pollinated (photo at left). Stigmas of female flowers dry up and turn black once they become pollinated.

    Kanza, being a protogynous cultivar, has late pollen release. Yesterday, Kanza catkins were releasing millions of pollen grains into the warm springtime winds.

    One thing I noticed about pecan flowers is that pistillate flowers of protogynous cultivars are smaller than pistillate flowers of protandrous cultivars.  In the photo above, you can see this size difference. Posey is protogynous while Pawnee is protandrous. By the end of the growing season the nuts of these two cultivars will be roughly the same size.

    While inspecting pecan flowers, I was also reminded that the color of receptive stigmas can vary from green, to orange, to bright red. The photo above illustrates some of this color variation. Both Major and Waccamaw had fully receptive pistillate flowers yesterday. Major displayed green stigmas while the Waccamaw stigmas are bright red. Since pecan trees are wind pollinated, stigma color has zero impact on pollination success. However, stigma color can sometimes be used to help identify certain cultivars. 

Finishing up the grafting season

   Last weekend, I finally finished grafting pecan trees in my orchard. After the flood receded, it seemed like I spent a week in rubber boots wading in the mud to finish up making bark, arrowhead, and 3-flap grafts. I've been grafting pecan tree since late April this year and those first grafts I made are starting to break bud (photo at right). Now it was time to finish up.

   Not every tree is cooperative with the grafting process and the last tree that I needed to graft this year turned out to be a graft failure from 2016 (photo at left).  Its a good thing that I had strong trunk sprouts grow up last summer to provide a perfect spot for grafting this Spring. My first step was to remove the failed graft and trim the tree down to a single trunk.

    I choose to keep the larger of the two sprouts for my new central leader. I trimmed off the old failed graft and the smaller trunk sprout with one cut using a chainsaw. I made the cut at about a 45 degree angle to aid in rapid wound healing.

    I cut the remaining portion of the tree at about 2 feet above ground level. At this point, the tree was about one inch in diameter--somewhat small for a bark graft. I selected a small diameter scion from my cooler and held it up to the stock. This is how I search for a flat spot on the stock wide enough to suit my scion (photo above right).

   In choosing a flat spot to insert the scion, I also took into account the locations of buds on the stock. Each bud has a bud trace or small branch of wood that grows up into the bud. If you try to insert a scion right under a bud on the stock, the scion might get hung up on the bud trace. I found two buds on the stock tree that I needed to avoid. In the photo above, the red arrow points to a bud at the very top of the stock. This location looks like a nice flat spot, but the bud trace could create a problem for inserting the scion.  I found a second bud in the grafting zone (yellow arrow) and I chose to avoid this bud also. Ultimately, I decided to place the graft in location of clear bark that seemed just wide enough for the scion (the scion in the photo hovers above that location).
  

    The way I carve my scion for a bark graft means that I always place the scion on the left side of the split in the stock's bark. Keeping that in mind, I made the downward slice in the stock's bark on the very right side of the area I had identified for scion placement. I proceeded to carve the scion and staple it in place (photo at left).

    This year's grafting season for me ended up the same way as always--covering a graft with aluminum foil and plastic bag, then attaching a bird perch. For me the end of the grafting season means the beginning of the directive pruning season, especially on trees grafted in 2016.

Cages protect young pecan trees from deer browse

    A picture may be worth a thousand words but the two photos above teach a valuable lesson. The photo on the left shows an unprotected seedling pecan trees that has been heavily browsed by deer. The tree on the right is enclosed by a cage constructed of welded wire fencing. This tree has a full canopy of leaves and has already made over ten inches of new shoot growth.

    Although browsing doesn't kill the tree, the removal of new growth ultimately stunts the growth of the tree.  The photo at right shows the terminal of the tree that was browsed. The yellow arrow points to the stub of this year's new growth left behind after a deer bit off the top of the tree. You can tell it this stump  is from the spring flush of new growth because of its green color.  Once the deer removed the first flush of new growth, its taken the secondary buds a couple of weeks to start breaking.
    The lesson I've learned is that tree cages are now necessary for every young tree on my farm.  I use 2 x 4 inch welded wire to build the cage. I've used both 4 foot and 5 foot tall wire but find that 4 tall works just as well in curtailing deer browse. I've used both light gauge and heavy gauge wire but find that the greater expense of the heavy gauge wire is offset by the cage's greater durability. As the trees grow above browse height, I'll remove the welded wire cage then re-use the cage to protect newly planted trees.

Training last year's bark graft

    When grafting a vigorously growing tree with a bark graft I often produce a tree with 5 to 7 feet of new growth on the scion during the first summer. I achieve that kind of growth by meticulously  pruning to preserve a strong central leader. But by the end of the growing season, I usually find that I've created nothing but a tall, branchless tree. When the one-year-old graft breaks bud the next spring it seems like all the new growth is confined to the very top of the tree (photo above, right). How am I going to promote lateral branch formation with a tree like this?  All it takes is some careful directive pruning.

   In pruning this tree I start at the very top then work my way down.The photo at left shows the cluster of new shoots that have developed at the very top of the tree. If I were to leave all these shoots in place, I would quicky lose my central leader and the tree would be topped by a sprawling assembly of branches pointing in every direction but straight up. In addition, allowing the tree to grow freely at the very top of the tree will create a top-heavy tree that causes the tree to bend over under the weight of the foliage.

     My first pruning cuts were made to encourage the growth of a single new leader. Here's where I use the 2-foot rule in tree training. In the photo at right, you can see that I removed all the the new shoots in the area of the trunk two feet down from the apex of the shoot that I have choosen to become the new central leader.

   When pruning off all the lateral shoots that are directly competing with the new central leader, I was careful to leave all secondary buds in place (photo at left). A few of the secondary buds had started to push and that's OK. The just-emerging shoots from secondary buds won't be able to catch up with the strongly growing central leader. In addition, shoots that develop from secondary buds form lateral branches with wide crotch angles (a good thing). 

    After pruning the top of the tree using the two-foot rule, the top of my tree has a single central leader and has lost its bushing appearance (photo at right). My next pruning task was to work my way down the stem and thin out the dense array of lateral branches that I found growing there (photo at right). There is no way the tree could support that many lateral branches all within about 18 inches of trunk.

    I removed more than one-half of the lateral branches that had formed on this portion of the trunk. In pruning lateral branches, I was careful space out the remaining branches both up and down  and around the trunk (photo at left).  After pruning, the entire tree no longer appeared so top heavy with foliage
    Thinning out lateral branches near the top of the tree has an additional advantage. Buds lower down on the trunk will be stimulated to grow and form branches along the entire length of the last year's scion growth.

    Stepping back from the tree that I just pruned, I could see that the new shoots and foliage were now better distributed along the main trunk (photo at right). The ladder in the photo is 4 feet tall to give you a better idea of the size of the tree I was pruning. With lots of lateral shoots developing already this year, my single shoot graft should turn in a well bushed out tree by the end of its second growing season. Maybe by year three, I'll be producing a few nuts on this tree. 

Pecan flowering season underway

    I haven't been able to check our pecan trees at the Experiment Field since last week because the Neosho River is flooding our orchard. But yesterday, while the sun was shining, I visited several trees on my farm and noticed that pecan pollination season has begun. Pecan trees produce two types of flowers on the same tree--male and female.
    Male flowers or catkins are produced on one-year-old wood. A yellow arrow points to pecan catkins in the photo at right. If you look closely  catkins, you will notice that each catkin holds numerous pollen capsules roped along a single stem. As the catkins mature, pollen capsules split open, releasing thousands of yellow pollen grains.
    The female flowers or pistillate flowers are located at the terminals of this year's new growth. The red arrow in the photo above points to a cluster of pistillate flowers hidden among new emerging leaves. The female flowers of this pecan cultivar (Faith) are tipped by red colored stigmas which makes them a little easier to spot in the photo.
    Each pecan cultivar has a distinctive flowering habit. Some pecan cultivars are protogynous, meaning that pistillate flowers become ready to accept pollen before the catkins on that same tree releases pollen. Other tree are protandrous, meaning pollen is released from catkins before pistillate flowers become receptive. Pecan trees have this separation in the timing of of male and female flowering to ensure cross pollination among different trees and to promote hybrid vigor of the resulting progeny. 
    The timing of pecan flowering is also related to a cultivar's date of bud-break. Cultivars that start the spring flush of new growth early will also start flowering early, regardless of flowering habit (protandrous or protogynous). Lets look at some examples of the bud-break effect on flowering.

   Faith and Hark are two protandrous cultivars that have widely differing dates of bud break.  Faith is one of the earliest cultivars to start growth in the Spring while Hark breaks bud about 10 days later.  In the photo above, note that the Faith catkins are starting to turn a little yellowish indicating that pollen release is just days away. In contrast, Hark catkins are still very green and not nearing maturity. The pistillate flowers of Faith are emerging from the terminal of the new growth while it is still difficult to find Hark flower clusters even though I know the are coming (I dissected some Hark terminals and found pistillate flower primordia with a hand lens). 

    I also photographed a couple of protogynous cultivars--Kanza and Jayhawk (photo above). The bud-break dates for these cultivars are a little closer, with Jayhawk breaking bud before Kanza by just a day of two. The pistillate flowers of protogynous cultivars are small and often concealed among emerging leaves at shoot terminals. However, it looks that the stigmas on the Jayhawk tree were beginning to produce the sticky substance needed to capture pollen from the air. Kanza stigmas were still dry and not fully mature.
    With two different flowering habits and differences bud break, the pecan pollination season can usually last almost 4 weeks. Pollination is an amazing process that ushers in a new season of nut production and a time of year that I look forward to each year. 
   

Spring flood on the Neosho

   The entire Mid-west region was hammered by heavy rains over this past weekend. At the Pecan Experiment Field, we recorded almost 4.5 inches of rain for Saturday April 29 and Sunday April 30. With all this rainfall, the Neosho River has spilled over its banks and covered the pecan groves in the Chetopa, KS area. Today, I drove down the highway to get a good view of the flooding (photo above). More rain is predicted for this week and the river is forecast to stay flooded until Thursday (May 4th).
   Flooding is a natural part of the native pecan ecosystem. Pecan trees tolerate flooding for short periods of time (less that 10 days) but fibrous pecan roots will start to die if soils remain flooded for a long time (more that 3 weeks). 

Wednesday May 3rd update:

Another 2 inches of rain today, which means our flood will continue for the rest of the week. Below is a chart of the Neosho River level at Stepford bridge which is located just a couple of miles down river from Chetopa. When the river reaches 18 feet, water begins to cover the road to the Pecan Field. At 24 feet, we will start to get water in our buildings. This flood, at 22 feet, will cover all areas of the farm but should stay out of our buildings. Next week, we'll find out how much flood related debris there will be to clean up.