Hello everyone,
I might have missed a discussion on this topic (feel free to point me to the right lecture), but as the temperature starts to drop towards freezing in my area, I thought it would be wise to seek your insights here. There seems to be a lot of differing opinions about the effects of freezing on roots. I’d love to hear what the real consensus is, if there is one.
Edit: Location: Northern Illinois Zone 6b
Thanks!
The best discussion of this topic I have seen is in chapter 13 of Michael Hagedorn’s book Bonsai Heresy which is available from Stone Lantern. The biggest take away is that root kill temperatures are typically much warmer than top kill temperatures (i.e. plant hardiness) and are sometimes as high as 25F (-4C) species that can stand temperatures of 0F (-18C) or lower in the ground. The other thing to keep in mind is that frozen roots cannot provide any water for transpiration for the top of the tree when it is above freezing.
Thanks for the reply! Does the chapter provide any information on how long a tree’s roots need to be frozen for the tree to die? For instance, if a tree’s rootball were to freeze during a cold night but then thaw with the next day’s warmth, would the tree be okay?
So I winter all deciduous and conifers outside in central Iowa. The freeze thaw cycle and desiccation from freezing winds has been the real killer in my environment. I protect the trees from north wind with cedar fencing, and reduce wind from the west with louvered fencing. For the temperature regulation of the pots and roots I try to keep fairly flat by grouping the trees together in a landscape block enclosure with fencing on two sides. To insulate the pots I surround them with chicken grit, granite or quartzite, not oyster shell or calcium. Then cover the surface with wood chip animal bedding. Another layer of pine needles over that and hope for a blanket of snow. This protocol has worked for me down to negative 20 degrees fahrenheit. Letting the plant freeze and thaw damages more roots that having them frozen for months.
I just reread the chapter in Bonsai Heresy and did not find any information on the effect of time when the pot was frozen. My interpretation of the chapter and data is that they is the roots reaching that root zone kill temperature which I presume causes ice crystals in the cells and damage. How well the trees are acclimatized to the winter temperatures (i.e. storage of sugars and starches that act as antifreeze) will have an impact and a couple of stories of early season cold snaps are listed.
One of the primary time effects is how long the pot is exposed to the low temperatures. It takes time for the heat to leave or enter a pot from the air. A tree with a root zone kill temperature of 20F may well survive an overnight low of 15F or even lower since the temperature is only below 20F for a few hours and it will take the pot time to cool off.
Michael also discusses that pots on the ground are often warmer than those on the benches. He lists an experiment with a 7F difference. I keep my greenhouse at 33-35F at bench level with a heater during the winter. One of my temperature monitors fell from the bench to the ground and read 46F due to ground heating (generally 55-57F at 1+m depth in my area). This matches up well with @Bonsai_bob discussion of how he stores his trees - good ground contact and some decent insulation about the pots.
Finally, remember that radiation heat transfer to a clear sky can also be important in cooling. I have had frost on my sleeping bag at temperatures above 40F when camping in the desert - the surface of the bag was below 32F even though the air temperature was warmer. Always a good discussion when I taught my heat transfer class.
Awesome information thanks for the reply Marty!
I am further north near MPLSSTPAUL area in zone 3. I say it depends on the tree. Some trees, particularly the small ones, cannot be left out when it freezes at night and the temps stay cold during the day. Others like ponderosa pines, spruce, fir, potentilla, handle cold fine and my experience prefer to be wintered frozen. I think some freeze thaw does occur with the “new” weather but wind is worse. I store in a building that I don’t let get below 20 F. I throw snow on them during the winter so they get some water on the soil. Just scoop it on and replenish when it disappears. I haven’t lost any doing this and don’t get the wind damage I had when exposed outdoors but have lost a few pots that don’t handle cold. Wintering is an important element in bonsai care which I don’t think is completely understood.
I am in 6a. I keep my temperate trees outside all winter, on the ground, heeled in with a few inches of mulch. The roots don’t really freeze with this simple setup. I keep a temperature probe in a pot all winter. The coolest it ever gets is right at 32 degrees even when air temps are minus 15.
This was my plan but in a cold frame bottom insulated with foam. 
I would not bottom insulate. The ground will provide some warmth. In most areas the ground temperature is about 55F (13C) below the frost line (0.5 - 1 m in most climates). I measured a surface temperature of 46F (8C) in my greenhouse that kept the temperature on the benches 1 m above the ground at 34F (1C). A big ground heating impact.
Marty is correct. Do not isolate your trees from the ground. The ground IS your insulation! It’s counterintuitive but you use mulch, etc. to trap the earth’s heat.
I should have noted I am on a balcony so I use the layer of insulation to mimic the ground.
On a balcony you may want to consider some supplemental heat. The insulation will slow the cooling to the ambient temperature, but it will still get down close to the lowest temperatures during a long cold snap. The simplest approach is a heat tape designed to wrap in a spiral around a pipe to keep it from freezing. They normally include a thermostat when the temperature approaches freezing. They can be looped around the pots in the mulch and keep them from reaching the lowest ambient temperatures even if the pots freeze when it gets really cold.
That’s actually a great idea. Do you think the tape is more optimal solution that a heating pad?
Heating pads tend to deliver a constant amount of heat regardless of the temperature. They can be connected to a separate temperature controller (I like the Inkbird ones) so they don’t overheat on a warm day. They are great for driving root growth when set into the 80F (27C) range. I suggested the heat tape as a low cost approach to mitigate the very cold temperatures on a balcony to keep the roots above the root zone kill temperature.
Thanks Marty, I came across a video in the lectures where they use heat tape and wrap it around chicken wire… should I use the same approach or wrap the wire closer to the pots? Here’s my idea.
Layers Inside Cold Frame [Bottom - Up]:
Appreciate your advice.
Edit: Attaching a photo of my balcony setup –
I think your approach will work very well. I have not seen the video with the heat tape snaked in the chicken wire, but I am thinking that is to spread the heat a bit more like a heating pad. It sounds like a lot of work for minimal heat spreading since the wires in chicken wire are thin and a very low fraction of the area. If I was going to take that approach, I would probably use a couple of layers of wire mesh (hardware cloth) laid over the top of the heat tape to spread the heat since it has thicker, more closely spaced wires.
