Thursday, June 2, 2016

Pecan crop load and pistillate flower strength

   Yesterday, I spent some time scouting the orchard to get a feel for the 2016 pecan crop. All of the pistillate flowers have been pollinated and I was encouraged when I found a cluster of five nuts on a Kanza tree (photo at right)
    However, most pecan nut clusters have 3 to 4 nuts at this time of year.  Look closely at the nut cluster pictured at left. Above the 4 nuts in this cluster you can see an empty flower stalk (red arrow). This portion of the pistillate flower inflorescence had also produced additional female flowers that have since dropped off the stem. Look closely enough and you will see a nut attachment scar on the stem that indicates the position of the now aborted flower. Now look at the very top of the stem. You should see four small sepals and a rudimentary pistillate flower stigma. This  is an example of a small, poorly-formed flower that would never have the strength to become fertilized or grow into a pecan.
   This single photo can help explain how pecan trees create flower clusters and why some flower clusters are larger than others. Pistillate flower stalks are formed at the end of the current seasons new grow. The number of flowers produced and the ability of those flowers to remain viable is determined by the energy stored in last year's wood. Given enough plant energy, a current season's terminal will start creating pistillate flowers and continue producing new flowers up the stem until the shoot basically "runs out of gas".  Flowers near the base of the flowering stalk are the strongest and are the most likely to become fertilized and produce a viable pecan. As the tree produces additional flowers, the tree gets to a point where it only has enough energy left to produce small or poorly formed pistillate flowers. These poorly formed flowers drop from the tree during the final stages of the pollination season.
    The amount of energy a shoot has available to create female flowers is largely determined by the general health of the tree and by the previous season's crop load. A heavy crop load can drain the tree's energy reserves to a point where a tree finds it difficult to produce viable female flowers or even initiate a pistillate inflorescence during the spring following a heavy crop the previous fall. The photo at right shows a flowering stalk that has dried up and all pistillate flowers have aborted (red arrow). Although this shoot had enough energy to initiate flowing, the size of this flower stalk indicates that the flowers were all small and weak.
    Many might think the lack of nut set on this shoot was due to a lack of pollination. However, pollination has nothing to do with the abortion of the flowers on this terminal. Weak and poorly formed flowers will never produce nuts regardless of the amount of pollen flying through the air.

   This year, I've also seen some trees that never bothered to try to create a pistillate flower clusters. Totally vegetative shoots are easy to spot at this time of year because they are still elongating and setting on new leaves (photo at left).  Trees covered with vegetative shoots this year are setting themselves up for a massive nut crop next year.