Freeze Damage in Woody Plants
by Andy Walsh
This article is adapted from an Internet Bonsai Club post. In it, Andy
discusses the physical and chemical changes involved when plant stems
and roots freeze. This kind of information is crucial to constructing
cold weather protection for many areas of the country. Some slight
editing has been done to make it more readable in this context.
The Three Stages of Freezing
First off, several times here writers have stated that their trees are frozen
in the winter and survive. It's clear to me that there is great
misunderstanding around what some people think when they say a plant is
frozen. If a plant truly freezes it dies. The formation of ice within the
cells of a plant is invariably fatal. What I think many people see in winter
is the soil of their trees frozen and they equate this with the plant being
frozen. This is not the case.
From my readings, there are basically three stages of freezing that can be
observed with, and have significance to, a Bonsai:
- The freezing of the water in the Bonsai's soil.
- The freezing of "inter"-cellular water in the plant's tissues.
- The freezing of "intra"-cellular water in the plant's tissues.
Freezing of Water in the Soil
The freezing of the water in the soil occurs at the highest temperature of
the three. Water in nature rarely freezes at its physical constant of 0C
(32F) as there is always something dissolved in it that lowers the freezing
point. However, since soil is primarily comprised of particles and (usually)
not a lot of solute, water freezes at just a little below 0C. When ice forms
in soil it actually comes out of the soil solution leaving particles and
solutes behind (Ref. 2). Those of you in the colder areas who
experience frost heaves will note that the ground has ice
crystals on the soil surface. In many instances it is not the soil itself
that is frozen (it is muddy underneath). However, this only occurs where
there is room for the ice crystals to grow - namely the surface. In the
ground, and in the soil of a Bonsai, the water has nowhere to go and freezes
in place. I have seen the water in the soil of my Bonsai frozen like this for
(By the way, it has also been said here that this freezing puts serious and damaging
pressure on the roots due to the expansion from the ice. However, plant cells
have rigid cell walls and are capable of withstanding several atmospheres of
pressure on a regular basis due to their own normal internal turgor pressure.
Actually, ordinary land plants have been shown to survive hyrdostatic
pressures of over 1000 atmospheres. The resistance to pressure stress varies
seasonally and, believe it or not, has been shown to increase with cold
hardening (Alexandrov 1964, in Ref. 2).
What is important to understand at this point is that the water in the soil
is frozen and NOT the roots or the shoots of the Bonsai. Water freezing in
the soil of a Bonsai is not a problem for the tree (except for what I mention
At the freezing temperature of water in the soil, the tree has little
worries; but as the temperature falls further, the roots and shoots of the
tree are placed under greater risk. As the temperature gets lower the plant
is in danger of freezing itself which as I said before is invariably fatal to
the plant. Plants have several mechanisms by which they avoid freezing which
are not completely understood yet.
One mechanism is the storage of solutes (sugars, sugar alcohols, proteins,
etc.) within the cell protoplasm. The presence of solutes lowers the freezing
point of water and keeps the plant from freezing if the temperature falls
below 0C. In some cases this mechanism provides great protection. Japanese
researchers (Ichiki and Yamaya in Ref. 2) found that Apples (Malus sp.)
increase the level of sorbitol to over 30 times its pre-winter levels and
correlated the degree of winter hardiness to the level of sorbitol in the
tracheal sap. At peak sorbitol levels, hardiness was experimentally measured
to as low as -25C. Interestingly, sorbitol levels varied throughout the
season depending on the external temperature, that is, as the temperature
fell the sorbitol levels increased and vice versa. This is true of many other
species as well - but not all. (by the way, hardiness is not something that can be
built into such trees ahead of time by special fertilizing routines. A quick
cold snap following a warm spell can kill such a tree at temperatures well
above that plant's lowest known hardiness level).
Freezing of Inter-Cellular Water
Another mechanism that plants use is the expulsion of water from the cell
protoplasm into the inter-cellular spaces. Changes in cell membrane
permeability allow water to leave the cell and enter the spaces between the
cells and the water then freezes there instead of within the cell. This is
the second stage of freezing that I mentioned above that can be observed by a
Bonsai grower on his/her trees. In some trees when the temperature falls low
enough (quite low) this phenomenon occurs and in the case of Pines can be
seen by the grower. The needles of a Pine can seem like they are frozen as
they are stiff and will crack when bent. The cells of the Pine needle have
allowed water to "escape" into the intercellular spaces and it freezes there
giving the needles the appearance that they are frozen. AGAIN, this is an
appearance and the plant itself (living tissue) has not frozen. This
mechanism also allows the "intra"-cellular water to avoid being frozen.
Freezing of Intra-Cellular Water
The third freezing stage is when the intra-cellular water freezes. This
causes the death of the cells that do freeze and is shown to the Bonsai
grower through the loss of branches or the tree itself come springtime. One
freezing avoidance mechanism that is not clearly understood is what is called
"supercooling". By some means, the cellular "sap" remains liquid at
temperatures well below the known freezing point for that "sap". There is
some physical relationship to the diameter of the vessels that this "sap" is
in, since in larger vessels the "sap" does freeze at higher temperatures.
At this point I must say, [about the discussion of] "round" vs
"sharp" ice crystals forming when there are sugars in solution, that I don't
know that any such thing occurs. But it really is a moot point since
intra-cellular ice formation, whether from "round" or "sharp" ice crystals,
will be fatal to the cell. It matters not, what shape the ice is in. This is
a cold hard fact. I will add though, that it is specifically ice [crystal] formation
and not freezing that is believed to do the damage, as animal and plant cells
can be quickly frozen in liquid nitrogen and have recoveries of in the greater than 90%
range on thawing. This rapid freezing causes the intra-cellular water to
freeze in-place without the formation of ice within the cells. This is
successful only if the freezing process is taking place at a rate of =/>
1degree C/minute. However, plant cells were the least (by far) affected by
the rate of the freezing process. (I was involved with the validation of
several liquid nitrogen cell bank freezers a few years back, so I have some
personal experience with this one).
Temperature Ranges of the Three Stages
Now, each one of these 3 freezing events takes place in a specific
temperature range. Soil freezes first, "inter"-cellular water next, and
"intra"-cellular water last. In the case of plant shoots (remember I said
shoots; very important word) these temperature ranges could be something
- 0 to -5C (32 to 23F) for soil water
- -5 to -10C (23 to 14F) for "inter"-cellular water
- -20 to -40C (-4 to -40F) for "intra"-cellular water.
These are arbitrary ranges for the sake of example and not specific to any
particular species. However, such ranges would probably apply to many species
in many temperate zones. Many of you will note that the winter temperatures
for your area would not meet the "criteria" for "intra"-cellular freezing and
hence your trees should be pretty safe in your area. That is indeed the case.
What gives then with all this talk of winter protection for Bonsai?
What gives is that the hardiness values for various species that have been
bandied about here of late ONLY apply to the above ground tissues. The root
systems of plants do not undergo the same degree of hardening by any stretch
of the imagination. This is where trees as Bonsai, and trees in nature part
company. In nature, a tree's roots are below ground and are not subjected to
anywhere near the widely varying and deeply cold temperatures that the above
ground shoots are. They do cold harden somewhat; but not to a great degree.
(by the way, if the trunk of a tree is buried under soil prior to the onset of the
fall temperature drop, the tissues of the trunk under the soil will not cold
harden like the rest of the above ground parts and will be easily damaged if
exposed). When a tree is removed from the ground and placed in a pot, it's
life can be placed in jeopardy since its roots can now see temperatures they
never saw in nature. This is the very reason why winter protection of Bonsai
is necessary. (Indeed if Bonsai were hardy to -40 etc. why would most of us
even bother with winter storage?) Nothing in particular needs to be done to
help the above ground parts make it through the winter; it is the below
ground parts that need the help. I know of no special tricks other than
avoidance of low temperatures to prevent freezing damage to roots.
Methods of Protection
Such protection is typically accomplished by keeping the Bonsai in a
temperature controlled greenhouse, by keeping the Bonsai in a coldframe, by
placing the tree back in the ground, or by mulching around the pot (in order
of decreasing effectiveness). All these actions can help prevent the root
systems from reaching their "killing temperatures" and prevent root damage or
death. When Bonsai people go to any of these various lengths in winter they
are really protecting the roots of their trees from reaching their "killing
temperatures". But many growers don't seem to know or appreciate that.
Killing Temperatures for Roots Vary by Species
The temperatures at which roots are killed, while directionally higher than
those for shoots, also show wide variability from species to species. For
example the "killing temperature" of mature roots for Magnolia stellata is
only -5C (23F) whereas for Juniperus conferta it is -23C (-9.4F) (Ref. 5). In
my experience, Trident Maples are very susceptible to moderately low
temperatures while many other species, for instance Azaleas, are not. I have
wintered cuttings of Azaleas outdoors on the ground with no mulching for many
years and they have survived with no problems. Reiner [Goebel] wrote here that his
collected Eastern White Cedars are not provided with any winter protection at
all (keeps them on their benches in Canada) and do just fine. Both Azaleas
and Eastern White Cedars have dense, fibrous and shallow roots systems and it
is possible that their roots are normally exposed to colder temperatures and
have greater resistance. I don't know whether their greater resistance is due
to the roots surviving at lower temperatures or their greater ability to
regrow in the spring.
Stem Tissue Dehydration
At this point it's important to return to the one other winter issue related
to the freezing of soil water that I mentioned in the beginning of this post.
I mentioned that the freezing of soil water is not a problem for the tree. It
isn't, except under certain circumstances. If the temperature of above ground
tissues rises substantially and the soil water remains frozen (as can occur
in many greenhouses), transpiration of water can occur from the shoots. If
the roots cannot obtain any water from the soil they cannot replace any that
is lost from the shoots and the shoots can dessicate. This situation can be
greatly aggravated by any wind which will help to drive transpiration losses.
This can result in the "winter dieback" of shoots and branches.
Obviously this situation is very undesirable. For Bonsai growers this is best
avoided through some form of windbreak. Greenhouses and coldframes (or
porches and garages) naturally supply this but trees planted in the ground or
mulched may need (depending on the area) some form of windbreak placed around
them for the greatest protection from this potential threat.
Now, let me try to pull all this "stuff" together into some useful summary
that anyone, anywhere, can refer to and have the necessary information in
hand to make informed and confident decisions on what they need to do for
their Bonsai in winter.
- Bonsai from temperate plants will enter a period of dormancy in fall which
is triggered by shortening days and/or falling temperatures. Exposure to
short days, low temperatures and (very importantly) frost induces the plant
to begin "cold hardening" to sustain it from the coming winter temperatures.
- Bonsai from temperate plants need to undergo a period of chilling in order
to break this dormancy and start regrowth. This period has been defined as 1
to 10C (34 to 50F) for a period of 260 to >1000 hours (Ref. 4). In many cases
these requirements are met by January (in the Northern hemisphere); however,
the return of consistently warm temperatures is necessary for the plant to
- The above ground parts (shoots, leaves, buds) of Bonsai from most
temperate plants (after "cold hardening") can withstand very low temperatures
(some as low as -70C)
- The roots of Bonsai from most temperate plants do not "cold harden" like
the above ground parts and are much more sensitive to low temperatures and
can be more easily damaged.
- Freezing of the soil in a Bonsai pot is not necessarily a threat to the
plant unless it is accompanied by prolonged periods where the shoots are at
higher temperatures and/or exposed to drying winds and dessication of the
shoots becomes an issue.
From these 5 points I believe it can probably be seen that the optimum winter
handling of Bonsai from temperate plants are:
- Allowing the plant to properly enter dormancy and "cold harden". Exposure
to the first frosts of the fall are particularly important.
- Storing the plant (remember optimally) well watered in an enclosed,
temperature controlled area where the temperature is held at 1 to 10C (34 to
50F) throughout the winter to allow the appropriate temperature and time for
dormancy to be broken, to keep the temperature from falling too low and
damaging the roots, and to keep the soil from freezing to protect the shoots
from dessication injury.
- Removing the plants from this protected area when the temperature
conditions for growth return.
Now obviously most people cannot attain step 2 easily. But attaining close to
that should be the goal. There are a number of winter storage procedures that
many people have shared here that will approximate this ideal. Coldframes,
garages, porches, mulching (with a windbreak), burying the pot (with a
windbreak), are all make-shift ways of coming close to providing the ideal
With the above ideal model in mind (and the reasons why) and knowledge of the
typical winter conditions in your area I believe most readers will be able to
confidently pick an effective winter storage scheme.
I hope this was helpful (and readable).
1) Levitt, J. (1980) "Responses of Plants to Environmental Stresses".
2) Li, P.H. and A. Sakai (1978) "Plant Cold Hardiness and Freezing Stress".
3) Long, S.P. and F.I. Woodward (1987) "Plants and Temperature".
4) Moore, T.C. (1979) "Biochemistry and Physiology of Plant Hormones".
5) Whitcomb, C. (1984) "Plant Production in Containers".
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