Freeze Protecton

An important thread on Freeze Protection today on BonsaiTalk. HERE is the whole thread. Somehow, their program ate the first paragraph of my reply, so I am reproducing it below. When will I ever learn to periodically protect my posts by copying them to something OFF the internet?

The short answer is that bonsai artists are not necessarily good physicists. For any overwintering scheme you have to consider all the factors, your minimum low winter temperatures, the size and nature of your structure, and the species and physiological state of your trees. Below I have analyzed the typical situations most of us encounter.

The parameters of the situation: Indeed, in most cold weather areas, the near surface earth is much warmer than the ambient air, at least at night. Concrete is a good conductor. Cold air sinks. Moving air stirs the layering. Heat is lost through radiation as well as conduction. These are the things with which you have to work. Measuring the effects of each of these will give you the answers to your questions.

The inside of your garage will always be warmer than the ambient air temperature AT THE NORMAL LOW OF THE 24 HR PERIOD. Why is this? The earth is warmer than the ambient air, typically 60F a few feet down except for the coldest areas where the frost line can be even lower, but it will still be warmer than the typical ambient air low temperature of the day. Radiation is limited by the walls and the roof and thus bounced back into the space. Conduction is limited by preventing contact of the inside air with the outside air. Even given these factors, the air in the room will eventually cool, and upon cooling it will stratify, warm air rising to the roof, cool air sinking to the floor. So, after a few hours of cooling, the coldest air in the garage will be at floor level IF the pots are exposed, that is, just sitting on the floor. It won't make much or any difference if the pots are sitting on the concrete, or sitting on a piece of foam preventing contact but keeping them essentially at the same level. The concrete/earth is the source of the heat, the ambient outdoor air is the 'magnet' or sink drawing the heat out of the garage. The cold air sinking in the garage is what will chill the pots and the floor, and this will be essentially the same whether they are on the floor or sitting on foam. There may be a slight difference due to the conductance of the concrete, but it won't be significant since the surface of the entire slab is also being chilled by the cooling air.

The cool air layering is significant, so the bench level layer should be warmer than the floor after a few hours of cooling to equalize the temperature of the objects in the garage to the air temperature in the garage. By significant, I mean about 2 to 5F. The only time this is going to matter is when approaching the critical low temperature of freeze damage to the roots. Too much to explain here, see http://www.evergreengardenworks.com/frzekill.htm.

The critical low temperature for freeze damage to ROOTS varies by both species and by physiological state of the roots. It ranges from about 31F (Fuchsia, other very tender tropicals) to probably about -40F for most of the far north species. This isn't very helpful. What I have discovered through many years of trying to prevent freeze damage is that MOST temperate climate woody plant species will not suffer root freeze damage (or hardened top tissue damage) down to about 15F, even properly hardened off larger Acer palmatum and Acer buergerianum. So from a bonsai point of view, if you grow a number of traditional species, this is your magic number, and you don't want to go below it for more than about 2 hours. Maximum/minimum thermometers are quite inexpensive and the new digital thermometers frequently come with max/min programs, so you can even check remotely (about $20 to $40).

The exceptions: Smaller Japanese maples, trident maples and many subtropicals should stay above 20F to be on the safe side. Recently repotted plants of these species should not fall below 26F because of the tender fleshy nature of new roots. New cuttings of these species fall in this category. Mediterranean climate species fall into this category. Ulmus parvifolia, although considered a subtropical, is fine down to 20F and even to lower temperatures, perhaps 15F for larger plants that are fully hardened off. My trees have often seen 14 to 15F with no apparent damage, but the smaller plants are kept 25F and above.

The role of radiation. Most people don't understand heat radiation, but are painfully aware of conduction. They are not the same. If you have a roaring woodstove fire or put your hand in front of a glowing electric space heater, you can FEEL the heat. This is independent of the air temperature. Your hand is absorbing the RADIATION of the heat source, using infrared radiation. As the room air heats up, it becomes comfortably warm throughout the room. You get warmed by CONDUCTION, just as you get burned by touching a hot pot. Conduction requires physical contact, be it solid, liquid, or gas. Objects heat and cool by both methods. You can block radiation by a single thin sheet of almost any opague substance (infrared radiation is just like visible light), just as you can easily block the heat radiated from the space heater with even a sheet of paper. However, really good radiation blockers are shiny metal like aluminum foil. Conduction insulation is more complicated and mostly requires multiple gas/solid layers or barriers.The more barriers, the thicker the insulation and the better the effect. Bubble foams are great for this since you get all these multiple gas solid interfaces that slow air movement and thus conduction. But even a double barrier than encloses a single layer of air will work wonders. This is why greenhouses with an inflated double layer of plastic are so popular, as well as dual glazed windows.

Now put some of these concepts to work. If you are overwintering outside instead of the garage, you wouldn't want your plants on concrete when you could have them on earth. Why not? Concrete is a good conductor, and heat will be lost even if you put an insulated box on top of it. Most soil is a relatively poor conductor, even when wet or frozen because of all the air spaces. So if you put an insulated box, or even mulch around plants that are sitting on earth, you can benefit from the heat radiated and conducted from the earth. You keep it from being lost by blocking the flow of air (conduction) and by blocking  radiation by using an opague barrier (foil, foam, or even better- both). Now you might not think you can get much protection this way, but it is surprising. With even the most meager barrier you can get 10 to15F temperature differential. I see this all the time in my valve boxes. This are simple thick plastic boxes that cover my valves in the field to keep them from freezing. The valves are only a couple inches below soil level and the boxes are less than a foot high above soil level. The valves never freeze, not even down to 10F. Normally, at 15F the temperature inside the boxes is right around freezing. So, you can see how you  can get a tremendous amount of protection basically for free (no added heat input) by building an insulated structure on the ground or near the ground.

Off the ground: Now, unlike in the garage, when you put plants on benches outside well off the ground, you have a whole new ball game. You have effectively lost ALL the conductance and radiation of heat from the earth. I don't care how much insulation you put around them, in just a few hours the temperature will approach the ambient air temperature (assuming air is allowed to flow under the bench). To get the benefit of the heat of the earth, the enclosure must extend down to and include the earth.

Freezing Water: I keep my bonsai on benches because I have a lot of them and don't want to move them around. We frequently get down to around 15F, so I am on the ragged edge. I solve the problem by automatically turning on the overhead irrigation (drip spray) at 27F. I have drain downs so there is no water in the risers to freeze. This is well water, so the temperature of the water is around 60F when it comes out, but the heat that really protects the plants is from the release of heat when the water freezes. It's hard for people to understand this process, but when a liquid turns to a solid, this is known as a phase change and the crystalline structure requires a lot less heat to maintain than the amorphous liquid. So when the water freezes into crystals it loses the heat that it took (absorbed) to melt it in the first place. Because of the nature of water, it has a very high HEAT OF CRYSTALLIZATION, or a lot of heat is released on the phase change. Now, the water or ice doesn't get 'hot' but the released heat helps the water and ice mixture stay at 32F. In fact, any slurry mixture of ice and water will always stay at 32F (how you calibrate a thermometer) until one of two things happens, one, it freezes solid and the temperature then drops, or two, all the ice melts and the liquid water begins to warm up. As long as you can keep applying liquid water, the ice/water interface will stay exactly at 32F. Yes, the pots, plants, benches and everything else will be coated with a thick layer of ice, but as long as the ice is staying 'wet' by the additional application of water, the surface of the ice will stay at 32F. This is a tremendous source of heat and this principle is used to heat whole vineyards and orchards around here with the only cost being what it takes to pump the water.

Of course, if the ambient temperature drops low enough, on practical basis 15 to 20F, the system will be overwhelmed and everything will freeze up, including the system (riser and emitters) providing the water. And, if you use a system like this, you have to maintain the plants so that the weight of the ice won't break branches or trunks. This system, of course, isn't for everyone, but for those of us in zone 8 and 9 with a lot of plants, it deserves consideration.

Cydonia oblonga covered with ice from the freeze protection system

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