Plant cells will not usually freeze at freezing temperature because of the presence of sugars and solutes. It would be below the freezing temperature for them to freeze over. That is why drought tolerant plants are often very cold hardy because it is based on the same principles.

Now if you have other bacteria or pathogens, those could serve as freezing nucleus and it could freeze up above the freezing point of water, but if your plant is healthy, the freezing point of the plant cells would be well below that of water depending on age, concentration of solutes, the bark insulation, and the general overall energy balance of a particular spot in your plant.

Snickles
You have given me a lot to digest and I am not sure if I completely understand all of it. The temperature did fall below freezing by 10 PM but did not reach 26 degrees until a couple of hours before dawn. The system was turned on several hours before the temperature dropped below freezing and allowed to run a couple of hours to saturate the soil with moisture. I understand that that wet soil helps to transfer heat to the surface.
If I understand you, the icing over of the entire canopy is an option used only to protect fruit and that this option could backfire if temps dropped too low. Too much ice from continuous spray would break limbs and destroy trees, but I don’t think that this is what you are saying. My concern is with the tree and not the fruit. There will always be another crop if I can save the tree
The facts about freeze protection to me are like getting politicians to give you a straight answer. I have read so many different spray volumes that I just decided to go with the upper limits.
Here is some controversy that you may find interesting. The freeze of 1999 virtually wiped out the citrus in my area. The following year farmer A planted 400 Satsuma’s and he has a micro jet system that he employs approximately four times per year. All of his trees are large and very productive. He never lost one tree to a freeze. One mile away (farmer B) a friend has four trees planted the same year (owari, Hamlin and Washington Navel) and has never employed freeze protection, fertilizer or pesticides.
Paranoia? Yes probably because farmer A and I both had the water flowing last night and farmer B, well he is still doing well.
Karl

Once we cover over the canopy with water we can stop using the overhead
watering. As soon as ice forms we cut back on the water or cut out on the
water entirely. All we want is a thin covering of ice to protect the fruit and
the tree. The limbs on well developed and nicely shaped trees are rather
pliable. They don't break as easily as one might think. It is the weight of
lots of ice that can cause twig and branch damage in this case, much like
how snowfall can knock a 30 foot tall Limber Pine right to the ground but
as long as the trees trunk was not broken (cracked) we can prop the Pine
right back up and in about a year or less the Pine should be able to stand
on its own again. Where we have the most problems is with high wind and
heavy snowfall. The ice serves to protect the Citrus from the winds or the
still, freezing air.

The problem with how much water we use for freeze protection is not the
amount of water applied but how much water is left standing and how long
it remains standing. There is the issue for trees that have water sensitivity
issues in their root systems like Citrus. We generally don't kill an Azalea
by under watering it, we kill it by over watering the plant and allowing standing
water to saturate into the roots and stay there too long. We essentially rot
out the roots all by ourselves. We did not need a fungus to do the damage
for us, we did it to the plant. We can have the same effect with Citrus.

400 hundred trees compared to 4 is a huge difference. The guy with
the 400 will want to try to keep that many trees alive as he has a far
more significant investment in those trees. A hobbyist generally thinks
in terms of money lost after the plant dies, not beforehand. The grower
has absorbed a considerable out of pocket cost just to get to where he
is now. Time is money and we realize that more when we have to go
back in and replace trees that died out on us. Start the water and
hope for the best.

Jim

Jim
Ok what you described is very clear to me on saving foliage and fruit which reminds me of a new acquaintance who lives just west of me. He uses sprinklers to form ice when the temps allow the formation of ice on the foliage. It’s a late hour practice for him, but ironically he thinks that this will save the tree from fatality during a severe prolonged drop in temperature. He does this even after the fruit is harvested and I agree that it could save foliage, but would do little for trunk freeze.
With the proof that the trunks of my trees did not form ice from a gradual freeze for approximately 10 hours, I realize that there is some insulation in the fiber of the tree and it would take a much colder and prolonged freeze to kill a citrus tree. I believe this is what Joe Real was trying to express when he went over my head with freezing nucleus and concentration of solutes. Arrepentido Joe
I had forgotten a conversation with another citrus orchard grower who told me that eight hours of 27 degrees would kill a Hamlin tree. Looking at the normal curve of cooling from dusk to dawn would indicate that there is a gradual cooling culminating in a sharp drop in temperature just before dawn. Paranoia runs deep and when one sees a low of 23 degrees, the red flags fly.
Millet probably sleeps good knowing that he can controll the temps. There may be a small green house in my future.
Karl

Different cultivars would have different concentration of sugars and solutes in their cells that is why there are differences in the cold and freeze hardiness. There are many other mechanisms used by plants to survive this. One of the common ones also is to become dormant, which we often observe. New growth flushes for example, are easily damaged compared to the more dormant leaves during a sudden cold spell. Combinations of various cold hardiness mechanisms would spell out the specific cultivar's survival.

Both the solute manipulation and dormancy are also used by most drought tolerant plants. And you may have also guessed that cold hardier citruses are often more drought tolerant than those that are not cold hardy.

Ice indeed will be a very good thermal insulator with very low heat transfer coefficients, so it can keep the trap the heat inside of it, maintainig the citrus tree's cells below freezing point of ice, but preventing it from dipping below the point of death cell freezing temperatures. Thus the microsprays can be used properly for additional protection.

Karl, in many cases we have to think in terms of how long it was cold.
Gradual declines in temperatures give us a little maneuverability but
sharp declines from 32 degrees down 20 degrees in a matter of a
couple of hours just messes us up. Most growers around here will
get nervous when the temperatures get right at 30. As soon as it
seems like the temperatures are going to dip below 28 is when they
will start their frost protection engines.

As a rule of thumb many in ground Citrus will have damage to them
at 27 degrees for 4-8 hours, the longer the period of cold, the more
damage we can expect. Citrus can show less damage at 23 degrees
than 27 degrees depending on how long the low temperatures were.
In some years we can be at 23 for a matter of minutes and in about
4 hours be back at 32. In some years we may be at 26 for 8 hours
with a slight gradual increase in temperatures back to 32 by noon.
There are several factors that can determine how low and how long
a tree can endure cold temperatures, such as the soil type, the amount
of organic matter in the soil, the soil temperature, soil moisture, salt
content in the soil. Then we can add to it dealing with the plant itself
such as what is the rootstock our Hamlin is budded or grafted onto,
is the Hamlin on its own roots and how long has the tree been in the
ground. Another factor that can lead to greater cold damage is the
overall health of the tree, does it have signs of scale damage such
as Citricola scale or worse yet red, yellow or black scale? I've seen
evidence of trees that endured the cold but were chewed up more
by insect damage we did not see before the cold that became even
more pronounced after we saw signs of the cold damage. The overall
health is a key factor for me in how long the tree has been planted in
the ground and how long it has been exposed to such cold before as
each year the tree has dealt with such cold it should show less damage
for the most part. A neighbors Lisbon Lemon is a case in point. In
1990-1991 I had to prune that tree way back, so much so the tree after
I was done with it went from about 20' tall to about 5' tall due to major
cold damage, bark split open allover with open trunk splits exposing
cambium. Now when temperatures get into the 20's all we generally
see are the youngest, exposed shoot dieback, yellowing of the leaves
but what causes the die out now is from the effects of yellow scale
that we have not treated for other than hose water sprays the last few
years. The tree borders our two properties on the North side of our
home, South side of theirs.

I would not call it being paranoid, I think of it as being prudent.
We side with caution as the risk factor of us not doing anything
or worse yet realizing what we could have done and didn't do, is
a far worse feeling for us, than us trying to do what we can to
help these trees along. As long as we know the trees limitations
then it is up to us to work around them or work with them.

Jim

Thanks
We don’t get many of these really cold days down here and if nothing else it was a good exercise and learning experience from some really knowledgeable folks.
These darn Hamlins just did not quit growing even after I shut off the fertilizer back in early August. I lost a lot of newly sprouted leafs, but I don’t think that there is any limb damage.
Karl

It would be better to cut the fertilizer by June 1 or July 1 at the latest and quit watering for best chance of winter dormancy here in SE Texas/SW Louisiana. One major difference between here and California is that it could be 75F and humid one day and 19F the next morning. Many times we have prolonged periods of warm weather in between freezes that keeps citrus trees out of dormancy and easily freeze damaged.