Advice on Nut Thinning

Monte Nesbitt 

            The first time any pecan grower shakes his or her trees in July and puts 20-30% of the crop on the ground, he or she may feel a little queasy.  Indeed it can be an alarming sight to see the ground covered with perfectly healthy pecans, but it gets easier to bear when you come to understand that your trees are producing better quality nuts with increased production in the Off-year.  Many growers today have embraced the practice of nut thinning, and worry less about how much they are shaking off than whether they’ve shaken enough off the trees.  Mechanical thinning has been practiced actively in the Southeast since 1993, and both research trials and grower experience alike testify that it is a sound management tool.

Advice

1.     Evaluate crop load by variety, tree age, and field.  In many On-years with a big crop, not all varieties may be cropping as heavily as others.  Determine what varieties in your orchard are overloaded.  This can be done by the third or fourth week in June, when most of the natural nut drop will be finished, and the real crop potential can be seen.  Nutlet abortion from low vigor, lack of pollination, self-pollination, casebearer damage, and drought usually ceases, and what the trees have set and can carry to harvest is more apparent.  A drive-through evaluation of crop load from the cab of a pickup truck will not do. It is recommended to gather some real data.  Our method of crop evaluation is to walk around a tree and observe 50-100 terminal growing points at random, developing a percentage of nut-bearing terminals for each variety that you have. A light crop is a percentage less than 30%; medium to good crop is 50-70%; and heavy or overload crop is 75-100%.

           If your counts show that most trees of a particular variety and age class are overloaded, then knowing what trees to shake is pretty straightforward.  If some trees are overloaded and some not, then some means of marking or flagging the trees should be done.  Remember that shaking can not only stabilize production from year to year, but can also make tree-to-tree production within the orchard more uniform if the right trees are shaken.

2.     Monitor nut development.  From mid July to the third week in July, nuts should be periodically monitored for kernel and shell development, so that the earliest time to thin the crop can be identified.  In crop thinning with a mechanical shaker, it is important to shake as early as possible.  Some of the early experiments on shaking in Oklahoma concluded that trees should be shaken at about 75% kernel elongation in the water stage, before shell hardening or early dough stage.  Shaking prior to the dough stage is critical for getting good return bloom next year.  After these stages of kernel development have been reached, thinning the crop down helps improve kernel development in this year’s crop, but doesn’t benefit return bloom as much.  A simple long-ways slice through the shuck from tip to base with a pocket knife will tell you how far toward the base of the shuck the water-filled “kernel sacks”  have extended. Once they have extended 75%, you should begin test-shaking a few trees to see if the nuts will break loose from the clusters without excessive force.   Excessive force is shaking that brings off leaves or lasting longer than 6 seconds.  It is ok to lightly shake each tree 2-3 times if necessary with short bursts (2-3 seconds long). 

3.     Preventing bark damage is imperative in pecan nut thinning.  Monitor the trunks for signs of shaker damage.  Under conditions of abundant rainfall, the cambium layer under the bark will slip more readily, making it easier to split or slip the bark off the tree trunk.  It may be necessary to delay a few days after a rain to minimize trunk damage.  On Savage-style or three-point hitch shakers, it is recommended to have donut pads that give better grip and less crushing of the trunk.  With these style shakers, it is important to have a firm grip on the trunk, and advisable to have someone on the ground helping position the clamps on the trunk.  Mono-boom, self-powered shakers (OMC, FMC) with cylinder pads filled with walnut hulls do a good job holding the trunk; however, over-tightening of the clamp can cause compression damage to the trunk.  The shaker manufacturer should be consulted about pressure settings, and pressure gauges should be checked for accuracy.  Use lubrication between rubber flaps that cover the pads themselves.  If rainfall has been plentiful and trees appear to be flushing growth, it is advisable to shake a small number of trees, and observe them 24-36 hours for signs of compression injury (splits and cracks in the bark in the area held by the pads).

4.     In our experience, nut thinning is best accomplished with one person on the shaker and one person on the ground watching what is happening (nuts falling, limbs rising up as weight decreases, and breaking and twisting of branches in the tops of trees).  The ground person can communicate with hand signals when to stop or when shake harder.  It is important to shake briefly, stop, evaluate how many nuts are still in the tree, and then shake again if necessary.  The ground person can walk around the tree between shakes and reevaluate crop load.  Small nut varieties like Elliott should be reduced from 90% down to 70% terminals with clusters.  The target for heavy-bearing/large nut varieties like Cape Fear or Mohawk should be thinned to 50-60% terminals with clusters.  If nuts are not developed enough, they may be too hard to shake from the tree.  If leaves are being shaken off and very few nuts are coming down, it may be desirable to allow a few days for nut development and then try again. At half shell hardening, nuts will come off very easily, but the improvement in return bloom will not be as good.  So the key, is not starting too early or waiting too late.  Remember, crop thinning pays dividends if done correctly.