Freeze Damage in Woody Plants

by Andy Walsh

Introduction

The Three Stages of Freezing

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

(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 later).

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

Freezing of Intra-Cellular Water

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

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

Killing Temperatures for Roots Vary by Species

Stem Tissue Dehydration

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.

In Summary

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 resume growth. 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 winter protection.

And finally

I hope this was helpful (and readable).

Andy

References:

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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