PECAN TREES AND WHY LIMBS BREAK
Larry Stein, Professor and Extension Horticulturist
Texas Cooperative Extension
The dawn of a new pecan harvest is upon us once again. Some
are rejoicing after battling the 2005 vicious summer elements
of heat and drought for the last four months, whereas others are
wondering how it could already be that time. Others are worried
about the high price of fuel and what that is going to do to their
harvest costs while some are pondering the potential price.
Regardless, most are in the process of getting ready to shake
their first tree which means that not only must equipment be readied,
but also the native bottom floor must be prepared. It is probably
too late to do any serious leveling but armadillo holes ought
to be filled at the very least. Not only do the holes capture
nuts, but they can bounce tractor operators right off the seat.
As everyone knows a firm "floor" is needed to mechanically
One of the pressing issues at hand is the removal of broken
limbs and branches and one of the nagging questions that remains
is "Why do these limbs fall?" Everyone has heard the
expression, "so and so fell and broke their hip." But
in reality in almost all cases the person's hip broke and then
they fell. In the case of trees, there is no question what breaks
first, but the reason is far from being clear and in most cases,
there can be numerous reasons and more often than not is a combination
event. Potential reasons for limb fall include: crop load, "breach"
of the bark, wind, maybe drought, trapped bark, "shading
out" and age.
Even though it is common for humans to lose strength as they
age, age itself does not necessarily make wood more prone to breakage.
But one can liken a mature tree to an aging house; as the house
ages there is potential for the roof to leak, the plumbing to
break, and various things to wear out. Still the strength of the
wood is the same, but the stress and rigor from various outside
forces are at play on the tree. As a young tree, limbs are vigorous
and strong. It is nothing for the tree to carry 50 to 60 pounds
of pecans. As the tree ages it sets and carries more pecans until
one day the limit of some of the branches is exceeded and a branch
here and there snaps in two which successfully "breaches"
the tree's protective mechanism of the outer bark. The most ideal
thing to do when a limb breaks is to saw the rough, jagged broke
area smooth and in such a way that water is shed and cannot accumulate
in the wound. Still who in their right mind is going to climb
a 120 foot native tree with no safety net to correct one of these
wounded areas. Trees will wall off the area and they do a good
job; it is just that it is almost impossible for a tree to completely
cover a rough area. So over time water accumulates and before
long the heart wood (non-living portion of the tree has begun
to rot). Over time this rotted area grows bigger and eventually
another limb falls.
Realize we are talking tens of years and more, but it happens
just the same. In some cases the initial breach may have occurred
30 years ago.
Pecan has also been accused of having "brittle wood,"
especially as it gets older. I don't subscribe to this theory
and in reality where the brittle wood theory comes from is probably
due to "shading out."
Pecan is a sun loving plant and leaves on branches must receive
full sunlight to remain healthy. As trees get larger and branches
begin to touch, the lower limbs receive less and less light and
over time they die! As a result, it is not uncommon for limbs
to come "raining out" of these older native trees. So
is the wood brittle? Yes, but only because the limbs were already
dead. If all pecan wood got brittle with age, none of the famous
human pecan threshers would have survived.
Trapped bark can be a problem when limbs form "V"
Over time the bark accumulates in the "V" causing the
areas to be weak.
A good crop of nuts, leaves or a wind can bring this limb to the
ground again breaching the bark.
Drought has been rumored by some to be the culprit of limb breakage.
There is no question that pecans are large users of water.
Most of the water is simply lost as water vapor via transpiration.
Pecans are not one to rapidly close their stomates due to fluctuations
in light, hence water loss continues as long as the tree has access
to water. As water becomes limiting the stomates close, and eventually
leaves will scorch due to the lack of water. In severe cases the
tree dies back to what it can support. Many of the fallen limbs
which were blamed on drought have a very healthy crop of leaves
meaning water was not limiting. So in my opinion, drought is not
causing the limbs to weaken and break. What is really interesting
about these limbs is that usually they break when it is dead still.
One would think it happens in a wind storm, but this is generally
not the case. In some cases crop load again is the cause, but
sometimes the limb is barren. More than likely the answer lies
in a breach of the bark.
Realize that breaches in the bark can occur for many reasons.
Hail can be especially hard on trees. Most times the wounds heal,
but a small hole may remain. Eventually water can make the hole
larger, or it can be gnawed around on by a squirrel. Woodpeckers
also do their share of breaching the bark. So what was a small
hole gradually increases in size. Water accumulates in the area
and the wood rotting mechanisms are set in place. Over time squirrels
hollow out the area and it grows.
Growers take care of their trees with fertilizer and zinc and
not only do the trees put on a heavy crop of leaves but also a
good nut crop. As the nuts begin to fill in August, it is simply
too much for the limb to carry and the branches come tumbling
down. So it is an on going process typically started by a simple
event. The good news is these trees have been around for hundreds
of years, more than 300 in many cases, and guess what, they are
going be here long after us. They are resilient in nature and
will recover. So don't dwell on the why the limb broke and the
amount of pecans you lost, simply move on; clean the limb up because
just as soon as you think you are done, another will be down to
take its place.
Some limb loss you can blame directly on crop load; there is
simply a limit as to what a branch can hold. Other losses are
from a combination of factors including the wind, but in the end
it is those small, seemingly insignificant breaches in the bark
which leads to more limb loss. Many times such limb loss can be
used as an opportunity to remove trees as tree crowding is one
of the greatest challenges which
native growers must continually address.