The Doucet Theory on Pumpkin’s with SplitPersonalities

I have a theory on a leading cause of sag lines (Dill Rings), blowouts
and splits in large pumpkins that occur in maturing fruit mid to late in
the season.  These problems usually lead to catastrophic failure of the
fruit.  Since we are now upon the season that these types of problems
occur, it is a good time to throw my theory on the table for discussion.

First a little background.   When a pumpkin is small, it is solid
pumpkin meat.  As it grows, a large cavity forms in the interior of the
fruit.  This cavity is filled with air, seeds and pumpkin guts.  By the
time the pumpkin is mature, air comprises a significant percentage of
the interior cavity.

Did you ever wonder how air gets inside the pumpkin?  There are no holes
in the fruit to let the air in as the pumpkin grows and the interior
cavity expands in volume.  The answer is that air must permeate the
shell at some measurable rate and fill the cavity as the pumpkin grows.

How quickly air can permeates through the pumpkin shell probably varies
somewhat from pumpkin to pumpkin depending on the thickness and density
of the shell. As the pumpkin grows, the rate air can permeate probably
increases as the total surface area of the pumpkin shell increases.  The
rate that air permeates probably also increases in a proportional
fashion to increases in the pressure differential between the outside
and the interior of the pumpkin.  (Are you following all this?)

A pressure differential between the interior and the exterior of the
pumpkin will occur if the fruit has a sudden growth spurt and the cavity
expands faster than air can permeate through the shell into the pumpkin.
A pressure differential would also result from rapid changes in the air
temperature within the interior of the pumpkin or to a lesser degree
from abrupt changes in atmospheric pressure.

All this discussion on pressure differentials is leading to my theory so
please bear with me.  When I was in high school, we did an experiment in
science class where we heated a gallon sized metal container and then
tightened the cap on so that it was airtight.  We then let the container
cool.  As the air inside the can cooled, it contracted.  The container
slowly collapsed in upon itself as if an invisible hand was crushing it.

As the air contracted, it created a pressure differential between the
inside and outside of the can creating a crushing force on the can.  A
pressure differential of just one pound per square inch (psi) could
create a crushing force of two hundred pounds on the can because the
surface area of the can was about two hundred square inches. (Multiply
the pressure differential in psi times the surface area of the container
in square inches to obtain the total crushing force.)

If we now talk about pumpkins and think of them as large sealed
containers filled with air like the containers in my old experiment; a
similar process is at work on a daily basis. The air pressure on the
outside of the pumpkin is atmospheric pressure, which is relatively
constant regardless of temperature.  The air inside the pumpkin is
heated during a hot day expanding the air inside the pumpkin and
increasing the internal pressure on the shell walls. If the air expands
enough, the pressure in the pumpkin will exceed the outside pressure and
air will start permeating slowly out of the pumpkin cavity.  At night,
as the air in the pumpkin cools, it contracts creating a negative
pressure differential between the inside and the outside of the pumpkin
and a crushing force on the shell walls.  The pumpkin also grows during
the night, increasing the cavity size and magnifying the crushing force
on the shell walls.  Air slowly permeates into the pumpkin to relieve
this pressure but remember this is a slow process so the pumpkin acts
like a sealed container with a very slow leak in it.

A pressure differential of just a pound per square inch in a large
pumpkin could result in a crushing force of over a thousand pounds being
exerted due to the large surface area of the pumpkin shell.  Just think
of five big guys sitting on your pumpkin drinking beer!

This brings me to my Theory.

The Doucet Theory on Pumpkins with Split Personalities – Expansion and
contraction of the sealed pocket of air inside the pumpkin is a major
culprit in shell integrity disorders.

This theory leads to a host of possible failure modes:

The constant mechanical cycling of the shell, as the air inside the
pumpkin warms and cools on a day to day basis, causes weak areas in the
shell structure to wear down and eventually fail.  Splits along a rib
seam where the shell is the thinnest or a blossom end split would be
possible examples of this type of failure.

A large negative pressure differential caused by rapid cooling of the
internal air and/or a growth spurt would cause a significant crushing
force on the gourd as discussed above.   If the crushing force becomes
too great, it will cause the shell wall to collapse catastrophically and
split internally.  An internal split perpendicular to the ribs would
cause a sag line (Dill Ring).  An internal split along a rib seam might
manifest itself as a weak area and become a hole if it reaches the
surface of the shell.

A large positive pressure differential caused by rapid heating of the
internal air will blow the pumpkin up like a balloon and could cause a
blow hole in a thin area of the shell or external split in a weak area.

I’m sure that other factors cause the pumpkin shell walls to fail.  Just
the weight of the pumpkin shell itself and how it is supported by the
ground and stressed by the stem could cause the shell to fail
mechanically and split.  Also uneven growth in the shell walls could
cause internal stresses that eventually result in structural failure.
But, I believe expansion and contraction of the sealed pocket of air
inside the pumpkin is a major culprit in shell integrity disorders.

This theory suggests that steps to minimize the rapid heating and
cooling of the pumpkin should be taken.  Shading should be provided
during the day and the pumpkin should be covered with a blanket at
night.  It is also important to avoid growth spurts caused by the uneven
application of water or fertilizer.

Since I have never seen this idea posted or written about before, I’m
seizing the opportunity to take credit for it.  I would like it if
anyone of you out there tried to poke holes in my theory or had any
comments.  Until you prove me wrong, don’t temperature cycle your
gourds.

diana

3rd year grower
best 587 est. 1999
zone 9, SF Bay Area, CA

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