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Proper timing of fruit thinning is the key to achieving the desired results! In most
years, there is a window of only 10 – 14 days in which fruit thinning can be done
successfully. The calendar time varies with cultivar and orchard location. The time
is usually August for most pecan orchards in Arkansas.
Through the process of photosynthesis, leaves convert water and carbon dioxide into
carbohydrates needed by pecan trees for growth, flowering, and nut development. The
more leaves the tree has for each nut, the greater the chance that adequate carbohydrates
will be available for both tree growth and nut production. Research shows that at
least 8 to 10 functional leaves are needed to adequately fill one nut. Large-fruited
cultivars will require even more leaves per nut. Increased problems with over production
and poor fruit quality as trees mature are related to a decrease in the number of
leaves per fruit, especially in heavy crop years.
The apple and peach industries have long recognized the need to remove part of the crop to assure larger, higher quality fruit. The same principle applies to pecan.
Research at Oklahoma State University and at Kansas State University demonstrated
the benefits of fruit thinning in pecan. Thinning fruit from overloaded Mohawk and
Shoshoni pecan trees increased nut size and percent kernel, plus improved kernel grade.
Return bloom (flower production for next year’s crop) was increased in Shoshoni but
Fruit thinning decreases total yield per tree for the current year, but in some cases
increases marketable yield. The benefits of nut thinning are increased nut quality
in terms of higher kernel percentage and kernel grade, as well as more stable yield
and cash flow from year to year.
After the flower is pollinated and fertilized, the ovule (the tissue that becomes
the kernel) begins to expand and lengthen from the nut tip toward the stem.
During June and July, the nut enlarges and reaches its final size as the ovule grows.
As the ovule expands, the space is filled with liquid endosperm (a watery substance)
until the ovule extends to the stem end of the nut. The nut shell begins to harden
from the tip shortly after the nut reaches full size. After 100% ovule expansion
is reached, the kernel begins to fill (kernel deposition) and nuts pass from the water
stage to the dough stage.
The process of kernel deposition requires a large amount of readily available carbohydrates.
During this process, each nut competes with other nuts for the carbohydrates manufactured
by surrounding leaves or held in the wood of supporting branches.
Removing a portion of the crop (nut thinning) before nuts begin to actively compete
for a fixed amount of carbohydrate reserve provides each remaining nut with a greater
supply of carbohydrates from which to draw. It also provides the tree with enough
reserves to support a flower crop for the following season.
In most years, there is a window of only 10 – 14 days in which fruit thinning can
be done successfully. Research has shown that nuts should be removed when the ovule
is 50% to 100% expanded, but before the kernel enters the dough stage.
The time is now in most of our Arkansas orchards. Thinning the nuts earlier, while
they are too small, requires force that can damage the tree. Thinning too late, (after
the nuts enter the dough stage) reduces thinning benefits on kernel quality, return
bloom, and cold hardiness.
Determination of nut development is done by cutting nuts to expose the ovule. When
a nut is pulled from the tree an oval shaped scar is left on the shuck. To obtain
the proper view of the ovule, the nut should be cut lengthwise at a right angle to
the long axis of the oval scar.
A straight cut at the length of the nut will expose the expanding ovule as well as
the liquid endosperm within the nut. Thinning should be done when the ovule or kernel
has extended more than 50% of the distance to the stem end of the nut but before the
nut reaches the dough stage (Figure 1). Cultivars with large nuts should be thinned
when 50% ovule expansion has occurred. Thinning can be delayed to near 100% ovule
expansion on cultivars with small nuts.
Learn more about the growth stages of pecan.
Nut thinning can be accomplished with a conventional tree shaker equipped with donut
pads. Donut pads are essential in preventing injury to the tree trunk. As an additional
precaution, apply a coating of grease between the rubber flap and the donut pad.
This allows movement at that point and prevents movement of the bark during tree shaking.
The quantity of nuts to remove varies with a variety of factors, including nut size,
cultivar, crop load, and environmental factors. Judging the tree’s crop load is another
important factor in making the determination about whether or not a crop needs thinning.
This is a practice that takes experience. The percentage of nut-bearing terminals
can be developed by observing the number of fruiting terminals counted from an observation
of 50-100 random terminals on a tree.
Trees with almost 100% of the shoots bearing fruit and a cluster size greater than
three are overloaded and should benefit from thinning. Optimum crop load varies with
cultivar and may range from 50-70% fruiting shoots. Varieties with small nut size
can be thinned more lightly than those with large nut size. For example, optimum crop
load on cultivars greater than 70 nuts per pound may be 60-70% fruiting shoots. Nuts
in the size range of 50-70 nuts per pound, like Cape Fear or Stuart, have an optimum
crop load of 50-60% fruiting shoots. While varieties with large nut size of less than
50 nuts per lb may have an optimum crop load of only 45-50% fruiting shoots.
The major mistake most growers make when attempting to fruit thin for the first time
is that not enough nuts are removed. When shaking begins, don’t be alarmed by the number of nuts that fall. Trees should
be shaken two to three seconds at a time, evaluated, and then shaken again if needed.
This process should be repeated until the operator has a feel for how hard to shake
to achieve the desired results.
Trees should be shaken in 2 to 3 second spurts until thinning is complete. The distribution
of nut removal throughout the tree may be enhanced by shaking from two directions:
a north/south orientation and an east/west orientation. This compensates for branch
structure that can relay the force of shaking to various parts of the tree canopy.
Shaking orientation may vary with the type of shaker used. Growers should try variations
to determine the method that works best in each situation.
Proper management of crop load requires the grower to be thoroughly familiar with
the orchard. Not all cultivars or trees within a cultivar will require nut thinning
in a single year.
Cultivars that have shown increased profitability potential in response to fruit thinning
Elliot and Stuart showed no positive response to fruit thinning.
For those with no mechanical shakers, a PVC pipe hitting the limbs to remove nutlets
or clusters has proven to be beneficial.
Thinning the pecan nut load in heavy crop years is beneficial. It increases nut quality
as indicated by increased percentage of kernel, kernel grade, and nut weight. Increased
quality is presumably reflected in greater value. Nut thinning improves return bloom
of some cultivars, thus reducing crop variation from year to year, and has been shown
to reduce the tree’s susceptibility to cold injury. Nut thinning can be accomplished
with a conventional pecan shaker equipped with donut pads.
To make informed management decisions, the grower must be familiar with the concept
and the technique. We encourage our homeowners or growers to test nut thinning in
a portion of their orchards and personally evaluate the results.
*Information adapted from the University of Georgia and Oklahoma State University.
For more information reach out any member of the Tri-County Pecan Production team!
Prairie County- Danielle Dickson
or reach out to your local county extension agent.