Does Sub-15 Degrees at the Roots Kill Trees?

HardBall

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@DrTolhur mentions that the trees were overwintered in nursery soil in the garage with no problems. Given there was overwintering issues or damage last year it’s highly likely there won’t be damage this year. An issue might be a greater propensity to dissecation of the bonsai media.

As one who uses a garage to grow out young satsuki cuttings into whips during the winter, It might be a good idea for OP to place a decent thermometer in the garage to monitor the temperatures. @PA_Penjing points out that there is also a risk of breaking dormancy early. This situation might be resolved by opening the garage door when temperatures get too high.

To add to @rockm ‘s explanation about mulch, wood also has the attribute of being an insulator. So the wood chips, if the layer is thick enough, add insulation when placed atop a pot. If the wood chips, I use sifted medium bark nuggets, are placed under a pot that is dig into the ground too these insulate and assure drainage. Similarly if placed on the side of the dug in pots…

I’m always amazed when people come up with a commonsense, yet novel idea, like @HardBall did using temperature probes in the media of wintering over bonsai. @HardBall Can you share the equipment you used to monitor the soil temperatures?

cheers
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Thanks DSD for the kind words. It was nothing fancy, just and add on to a set I have. I keep the sensor in a quart yogurt container with a hole in the bottom for the probe (redneck weatherproofing) and place the probe an inch or so deep in a pot. Seems to stay about 35F most of the winter. That cold snap we had in February down to -15, pot temp went to about 32.5.

https://www.amazon.com/Ambient-Weather-WH31P-Water-Sensor/dp/B08CQ249TR/ref=mp_s_a_1_17?crid=3OKRRRZODPC3C&dchild=1&keywords=ambient+remote+temperature+sensor&pscroll=1&qid=1635386927&sprefix=ambient+remote+temperature+sensor,aps,158&sr=8-17&wIndexMainSlot=35
 
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penumbra

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Not compost, too fine and you risk constantly soggy conditions, which isn't ideal. I use shredded hardwood mulch. Also if you're mulching on the ground, insure there is an air pocket underneath the bottom of the pot to insure drainage. I put my bonsai pot's feet on paving bricks placed on the surface of the soil, THEN I mulch everything up and over the trunk of the tree to about five inches. I typically do this AFTER a few frosts and shallow freezes. Exposure to that initial cold insures the plant is dormant and helps "harden off" the roots for winter.
Could still be awhile coming.
 

HardBall

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Forgot...I also keep a regular air temp sensor in there so I see both both air temp and pot temp side by side from a monitor inside the house.
 
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Ok, thanks.

Looks like I’m gonna be up to do some really fun experimenting this winter!

Best
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Potawatomi13

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It depends🤣. Bonsai Heresy has chart showing root kill temps for many Bonsai subject trees. Sustained cold temps on/in ground no different than on bench.
 

Carol 83

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Yeah, no frost in October is weird.
It's fine by me. Most of the trop's are still outside, but will be coming in this weekend. Last year we had a frost the first week of October.
 

penumbra

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It's fine by me. Most of the trop's are still outside, but will be coming in this weekend. Last year we had a frost the first week of October.
You must keep a weather calendar like my wife does. She goes back 30 years I believe. Not age, but weather calendar. 🤣
I actually want a good hard frost to come along soon.
 

Carol 83

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You must keep a weather calendar like my wife does.
I just remember the unusual weather as it pertains to my trees. Early frost last year and snow on April 20th!
 

rockm

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You must keep a weather calendar like my wife does. She goes back 30 years I believe. Not age, but weather calendar. 🤣
I actually want a good hard frost to come along soon.
I am cautious that this year might be like a few years ago--no frost until a mid-November plunge into the teens. A recipe for disaster for trees not gradually exposed to colder and colder temps.

I would be more confident with a few frosts and a couple of nights in the low 30's between now and mid-November.
 

Leo in N E Illinois

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@DrTolhur

I did not read the full thread. I'm living north of Chicago, you are in Midland, Michigan. Obviously any trees native to our immediate areas, and introduced species that were planted before 1980, these have survived the brutal Winters of 1980-1, 1981-2, and 1982-1983, where we experienced week long sub-zero daytime highs (that's daytime highs of -18 C) and night time lows of -20F or -28 C. The snow cover was not deep at this time. The ground froze cold and deep. Obviously trees native to our area (zone 5) or further north can survive much, much colder than +15 F at the roots. There's other mechanisms that allows native to cold area species to survive root temps below 15 F. I don't recall details but there are at least 3 different cellular mechanisms to survive extreme cold that kick in as temps drop.

Key is the trees need time to adapt. It takes 3 months of slowly declining night temperatures, for a blueberry bush to be fully Hardy, flower buds to -18 F and vegetative buds to -23 F. I used to own a blueberry farm. 48 hours of 60F can reduce the acquired hardiness so that a +10 F night immediately following the 60 F warm, did cause flower bud death and ruin about 50% of the 2014 blueberry crop for Michigan.

So yes, the supposed +15 F limit for roots does NOT apply to species native to zone 5 and colder. There's multiple metabolic strategies that cold climate trees utilize to withstand extreme cold.

For the zone 5 and colder tolerant species, in winter, I simply set their pots on the ground, otherwise fully exposed for the winter. I have had no losses with Buxus sempervirens, European boxwood, junipers, including shimpaku, stewartia, Amelancheir, Malus, mugo pine, sylvestris pine, pinus nigra, Pinus strobus, Colorado spruce, Picea orientalis, Engelmann spruce. Tsuga canadensis, larch, bur oak, carpinus native and Korean, and probably others. Point is, if they can be grown in zone 5 or colder, just set the pots on the ground and forget them for the winter, it's that easy.

I do have a underground well house for satsuki azaleas, and other zone 6 & 7 trees. The well house hovers around 40 F, most winters.
 

SC1989

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I wasn't worried when I thought you meant -15F.

I leave mine huddled on the ground with a slight wind block. That's it. Snow cover helps.

We hit at least -12F for a couple few nights a few times.

Still haven't lost a tree to winter alone.

Repotting in spring kills em but Summer Repotting doesn't.

Winter never killed anything when I left em all out on the third story windowsills either. Iced to eff.

Don't fear winter at all.

Enjoy the time off.

Sorce
-38 here the other day. After tree keeping for a few years, I wager I'm the tree killer for the most part. Non native trees go in the eaves and everything else goes in unheated garage
 

0soyoung

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@osoyoung That is so coincidental. I was just reading though this article last week while penning a chapter on wintering over azalea (and other) bonsai. IThat's an older reference, but right on point. (Michael Hagadorn refers to studies by Iseli nurseries on root kill temps I've never been able to find. Gotta call him about this.)

Here's the best and most recent scientific review (Jan 2020) on the topic of roots and cold hardiness. It gets down to the nitty gritty on all the up to date research findings. Its a really good read.

The Roots of Plant Frost Hardiness and Tolerance

https://academic.oup.com/pcp/article/61/1/3/5593656

The upshot is that roots never evolved much to withstand cold temperatures as they've been protected by soil, yet the buds, branches and trunks have.

Second, the key parts of the roots, the fine growing tips, are most susceptible to cold and frost damage. Bonsai tree roots in pots are most at risk. If damaged, but the plant is still viable, these fine roots will be regrown first - at the expense of the rest of the tree.

No pointing fingers here, just a thought..... Hobbyists 'pushing the curve' on frost protection wouldn't know this is happening each year, as "My trees push about the same time as always" would be the observation... perhaps because the fine roots were always damaged? What would happen if the roots were'nt always damaged.... Its sort of like the its always worked for me stance.

It makes one think twice about folks talking about over protecting bonsai.

My thought vis a vis cold protection is that overprotection is better then underprotection. It won't hurt and can sure help the tree's long term growth and development.

Respectfully,
DSD sends
 

0soyoung

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"My trees push about the same time as always" would be the observation... perhaps because the fine roots were always damaged? What would happen if the roots were'nt always damaged....
Bud break is independent of roots.

Simplistically, buds won't break until they have had enough chilling hours. Once this is done, they break when the temperature rises above 40F (or thereabouts). The reality is a bit more complex as buds can be forced to open with enough heat after less than the full bud chill time. Regardless, buds will open and start to extend growth, but will stall if the roots aren't warm enough. One can observe that cuttings of deciduous species will break bud and present (most likely drooping) new growth. As another example, a trick to delay azalea blooming (for a show, say) is to pack the roots in ice, if one lacks a florists refrigerator.

But back to your point about overwinter root damage, I agree, but imagine what would happen if we didn't root prune when we repotted --> that is quite a bit of root damage. It isn't clear to me what the criteria is for root damage in these studies beyond the tree wasn't necessarily dead as a result. Nevertheless, it is clear that letting roots get any colder is asking for trouble. Conversely, this should shake some people's beliefs about winter soil temperatures. I must admit that I've never stuck a meat thermometer probe into substrate/earth before it froze so that I could see directly what the substrate/soil temperature is in fact.
 

Brent

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I have been growing bonsai and nursery plants for over 40 years now. And being of a curious nature and having my income dependent on such things, I have learned a thing or two. If you look at my Availability List, you can see the species and cultivars that I grow, and living where I do, it has never been colder than 12F. So, I have formulated a general rule based on my experience and observations. That rule is that most temperate climate plants with their roots exposed to ambient temperatures in pots can survive to 15F. There are exceptions which I will list below. Most of my plants have no protection whatsoever. They live in the rain, wind, cold to 16 or 17 most winters, and almost daily freeze/thaw conditions, except when it rains. When it rains here, it is warm day and night. We almost never get snow, but when we do it is always between 30 and 40F and usually melts within hours.

The exceptions are significant. The first is that root freeze damage is species, and sometimes even cultivar dependent. The second is not as obvious; it is the physiological state of the roots.

The few species that I must protect include: Quercus suber or cork oak, which was a real surprise when almost my entire crop froze one year, exactly at 15F. And it was just the roots, the tops stayed alive for a few months, but eventually succumbed. The autopsy showed completely dead root balls. The ones that survived were sitting on the ground, and probably just a degree or two above the ones on the benches, which all died. They now all go in the cold greenhouse in winter. The other surprise from the same winter was Cedrus atlantica, which did the same thing. Some C. libani didn't survive, but the damage was not as complete as with the former. They also go in the cold greenhouse now. Another species that is often damaged, but not killed at some temperature below 20F and 12F is Ulmus parvifolia, the Chinese elms, although most freeze problems with them is the top and not the roots. They suffer from small branch dieback, especially in early spring freezes after a warm spell that has started the sap moving. Punica can also be a problem at almost exactly 15F, killing the top and bottom. Luma apiculata which is really a subtropical, will be killed outright at right around 20F.

The condition of the roots at the time of freezing is the other critical factor for some species near the 15F range. This involves new succulent roots from either late seedlings, late repotting and root pruning, or a too early hard freeze not preceded by gradual freezing weather and mild frosts. The species that most concerns me is Acer palmatum, and I have made meticulous observations of plants with new roots. Freeze damage begins at almost exactly 26F, which is quite a surprise too, since mature roots, and cold hardened plants can tolerate well below 15F. To some degree, Acer buergerianum, trident maple, suffers the same weakness. All my new Acer palmatum cuttings (started in spring) have very tender succulent roots and are still actively growing by the time frost arrives. They all go in the cold greenhouse and are never allowed to freeze the first winter. In fact, the cuttings of all my species receive this same freeze free protection, but are allowed to get as low as 32F most nights. A small propane heater is sufficient in our climate to keep things from freezing. Technically, most species in this state (and even most citrus) can tolerate down to 28F (at least for a few hours).

Brent
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Thanks for that great baseline information @Brent and input @Osoyoung! This is some really great information of value.

Spinning off of that information Brent, I wonder what your temperature numbers and resulting root observations would look like if instead of the night time freezes, your trees would be subjected to longer term freezes, a week, or more?

I was guilty of sloppy language earlier in that I said "My trees push about the same time as always". It was a bit facetious too.

What I really was thinking about was a way to describe the refined long term growth we look for in a bonsai that comes from creating and maintaining fibrous fine root growth as opposed to merely pushing buds and considering that as a measure of growth.…. and how cold damage to this fine type of growth would affect one’s bonsai in the whole, yet wouldn’t be noticeable to an observer in the short term…. but decidedly affect the growth of the tree as a bonsai.

Its a given that roots need to be pruned as part of the discipline. The thought is that yearly cold damaging the roots, impeding the growth as I described, would not be advantageous to bonsai.

Its kind of funny to even tell folks that I have “stuck meat thermometers” into bonsai pots every day for the past 6 weeks. The data I’m getting is giving a decent picture of what is happening in a pot…. and I’m not so sure what the researchers are studying and describing is far off of what we are discussing.

Gotta go stick a meat thermometer in some bonsai now!

cheers
DSD sends

btw I also am using 24/7 temperature loggers as one of the folks here suggested. 😉

9C45317E-4949-4BA7-86B5-E31080E713CE.jpeg
 

leatherback

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Punica can also be a problem at almost exactly 15F
I think here there are cultivar differences. One of mine was forgotten and stayed out last year full exposure in a week with -15C/5F nights. I have given up yet.. Nope. Finer branches lost but quite happy in spring.
 
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mulch everything up and over the trunk of the tree…. I typically do this AFTER a few frosts and shallow freezes. Exposure to that initial cold insures the plant is dormant and helps "harden off" the roots for winter.

That is awesome advice! Especially good for azaleas, in ground or in the pot!

Cheers
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