Does Sub-15 Degrees at the Roots Kill Trees?

DrTolhur

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I know about all the discussions around over-wintering with regard to air temperature vs. soil temperature, indoor protection, and all that, but there's one piece I feel like I'm not able to reconcile. They say that root/soil temperatures below 15ºF will kill roots, but then they also say that keeping trees in an unheated garage/shed is sufficient in regions that get that cold. While that makes sense for temperatures around 10-15ºF, it doesn't make as much sense when you get days in a row of near-0 days, which happens here. I've got to think that after 3 solid days around 0ºF, anything in my garage is going to be below 15ºF, including bonsai soil. So it seems there are two options:
1) My assumption is wrong, and my garage does not get cold enough to drop soil temperatures below 15ºF.
2) Soil temperatures below 15ºF aren't a death sentence for trees, and there are other factors at play.

I started bonsai last spring, so I've been through one over-wintering season, but I didn't have any plants in bonsai soil in the garage. (It seems like the potting soil I was using might be more thermally insulting than bonsai soil.) As this year I have trees I care about more and they're in bonsai soil, I'm giving it more consideration. So what am I missing?

For context, the trees I'll be over-wintering in the garage are:
- Chinese elm
- Boxwood
- Japanese hornbeam
- Amur maple
- Japanese maple
 

ShadyStump

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There are no firm rules except for the ones you find work for you and your trees.
Some trees can manage freezing roots alright, and others root so quickly that they can replace the ones lost over winter before you notice it happened.

Depending on the location and usage of your garage- attached vs detached, sunlight, vehicles in and out, etc- it could very easily be staying above 15* F even after a week of 0* weather outside. It also makes a difference to water them right before a hard cold snap. The water, though it can freeze, builds up mass in the pot making your pot hold it's temperature much more consistently. It will be a freezing temperature, but not likely a killing temp.
The old standard of mulching around the trees to insulate them (maybe have your trees sitting in a box or something for this one).
 

Dav4

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Many cold hardy trees can and will survive root temperatures at 15 F but that's cutting it close to concerning the possibility of injury or worse, so keeping the roots well above that range of cold is warranted. Concerning winter storage of trees in an unheated outbuilding, being kept on the ground and having the pots mulched is key. Anything on the ground will benefit greatly from the thermal sink of the earth... even as the water in the soil freezes, the temperature doesn't fall below 32 F for a lonnnnggggg time. Throw some mulch around the pots and the root zone will never get remotely that cold.
 

rockm

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I know about all the discussions around over-wintering with regard to air temperature vs. soil temperature, indoor protection, and all that, but there's one piece I feel like I'm not able to reconcile. They say that root/soil temperatures below 15ºF will kill roots, but then they also say that keeping trees in an unheated garage/shed is sufficient in regions that get that cold. While that makes sense for temperatures around 10-15ºF, it doesn't make as much sense when you get days in a row of near-0 days, which happens here. I've got to think that after 3 solid days around 0ºF, anything in my garage is going to be below 15ºF, including bonsai soil. So it seems there are two options:
1) My assumption is wrong, and my garage does not get cold enough to drop soil temperatures below 15ºF.
2) Soil temperatures below 15ºF aren't a death sentence for trees, and there are other factors at play.

I started bonsai last spring, so I've been through one over-wintering season, but I didn't have any plants in bonsai soil in the garage. (It seems like the potting soil I was using might be more thermally insulting than bonsai soil.) As this year I have trees I care about more and they're in bonsai soil, I'm giving it more consideration. So what am I missing?

For context, the trees I'll be over-wintering in the garage are:
- Chinese elm
- Boxwood
- Japanese hornbeam
- Amur maple
- Japanese maple
Root hardiness depends on species. Sub 15 degrees allows intracellular water to freeze, as opposed to freezing between the roots. When water freezes inside a cell, it acts much like freezing water in an aluminum can--freezing can destroy cell wall as water expands inside the cell.

Sustained low temps can and will kill some species. Amur maple (since it is native to Siberia --Amur) can take a lot of cold. Others on your list, not so much--Chinese elm and Japanse maple will probably have some damage. A garage can provide protection, but you have to keep temps above 25 or so...
 

PA_Penjing

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it depends on the indoor location. my garage gets too hot for most of the winter because it's south facing and has windows. If it were on the north side of my home it would much colder and probably perfect. If your garage is attached to your house it will be much warmer than one that is stand alone. If it has leaky, drafty windows or doors it will be colder than one that doesn't. Mine is so warm I actually open the windows a bit, where as my friends shed (a mile away) is so cold his trees are only buffered by a few degrees. The only way to know is to use a thermometer
 

sorce

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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
 
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If your garage worked for wintering over your trees last year, it’s highly likely they will be just fine this year in bonsai media.

The main issue I’d be concerned about not knowing your winter horticulture is checking the moisture periodically. They may dry out more, depending on the mix.

Also dang @sorce ! At one time back in the day I lived just east of Oak Park in the City limits and would never of thought of leaving potted trees out in the open on a balcony. I still can remember those week+ periods every year when the temperatures were -20F minimum, not to mention the awesome massive snowstorms every couple years. Either the weather has radically changed or you are one Steely Eyed Missileman!

Cheers
DSD sends
 

DrTolhur

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To fill in some of the missing details:
- My garage is attached, one small window that gets no direct sunlight.
- We work from home, so the garage door isn't opened every day.
- I'm planning to keep the trees on shelves next to an interior wall, but not too far from an exterior wall.
- Mulch and floor storage aren't impossible, but they'd be very inconvenient.

If sorce is to be believed, then none of it really matters anyway. Not sure I'm game for leaving them all outside, but that bodes well for my garage situation.
 

Eckhoffw

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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
Don’t ever change man. 😊
 

rockm

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To fill in some of the missing details:
- My garage is attached, one small window that gets no direct sunlight.
- We work from home, so the garage door isn't opened every day.
- I'm planning to keep the trees on shelves next to an interior wall, but not too far from an exterior wall.
- Mulch and floor storage aren't impossible, but they'd be very inconvenient.

If sorce is to be believed, then none of it really matters anyway. Not sure I'm game for leaving them all outside, but that bodes well for my garage situation.
Light during winter dormancy doesn't matter for deciduous trees. They can be stored in complete darkness...That low light Might be an issue for the boxwood, however. As some have said, the biggest issues for garage storage is maintaining the temp between about 25 and the high 30's. Too low and, yeah, there is danger of some damage (source's experience notwithstanding), too high and you get new leaves in Feb.

There are many things at play with winter and bonsai--species, size of the container and soil volume, wind exposure, length of deep freezes to name a few. And yes, you can dodge bullets, but protective measures (garage storage, deep mulch beds, etc) are mostly meant to shield trees from the worst. I've had sub zero F stretches here in Va. and I have Southern U.S. collected trees-Bald Cypress, cedar elm and live oak. I store all my trees outdoors under deep mulch from Thanksgiving to (hopefully) most of March. A couple of years ago I had winter kill a very nice BC because of late Feb. deep freezes that caught it in the early stages of growth. I've had BC survive sub-zero temps when completely dormant. Believe me, losing a tree to winter IS ENTIRELY possible and it happens.

I used to keep my big live oak under mulch in the backyard. It showed signs of slow growth, sluggish spring activity after a couple of years of that. I decided to pay to have it stored in a cold greenhouse (where temps stay in the high 20's, low 30's). It is now one of the strongest growing trees I have.

You have probably watched this, but it's worth listening to closely.
 

A. Gorilla

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lol 15F

Only trees who are losers and feebs, maybe.

"WhAt aBoUT JBP???"

You heard me.

A winter-averse pine....

Pathetic. A cosmopolitan gate keeping species for half-wits who cower in fear from true alpha weather. 😤😤
 
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Forsoothe!

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Species matter. Fleshy roots contain a lot of water that freezes and damages cells. Fiberous roots are drier and so less freezing water damage. So Pines will stand lower temps than Magnolia, etc. The density of sap matters. SPF sap/pitch is not really water soluble, verses Maple syrup which is miscible, maybe especially so in spring when JM with bark exposed to direct sun is subject to splitting when nighttime freezing follows warm days enabling rising sap. I imagine it's a sliding scale, so knowing your species matters.

The ground only gets to the high twenties for the most part here in the city where buildings break up winds, and only to ~30°F under a mulch in the same situation. The more dry air spaces in the mulch, the better. Drier mulch under a wetter surface layer works wonders because the outside air can only touch the surface which doesn't conduct well. I suppose it varies a lot depending upon where where is. Farmland is entirely exposed all day and night to direct deep cold winds so that's different. The air gets down to some temp, but the air only has access to the surface of the ground and once frozen is tighter because the moisture expands and seals off air passage. Snow is a good insulator and only gets down to 32 and then resists getting colder because it doesn't conduct the lower air temps very well. People who are lost or otherwise find themselves out in the hinterlands are advised to make a snow cave to get out of the weather. It ain't warm, but you are protected from the deeper lows of the open air/wind. The air you exhale and your body provide some heat verses being immediately lost out in the open.
 

PA_Penjing

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To me (for whatever it's worth) your winter storage situation sounds downright warm. I'd be very careful to only use it during the coldest nights. If your Japanese maple starts growing in February you're going to have a very hard problem to solve.
 

PA_Penjing

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I guess I don't need to echo the sentiment that chinese elm are from sub tropical china, Amur maple from the russian taiga and japanese maple from a volcanic island. So there isn't a blanket answer. I always err on the side of too cold and move the most sensitive to protection when it's warranted
 

August44

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I am not understanding something here...if I have a conifer in a grow pot on the ground that has been watered and is in bonsai soil in a place that is protected from the wind and it is "0" degrees F for 1 week, what is the temp of the soil? I was thinking that it would be "0" degrees F, but maybe wrong according to what I am reading. Also, if I mulch the same tree, what would be the temp difference between the two settings?
 

rockm

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I am not understanding something here...if I have a conifer in a grow pot on the ground that has been watered and is in bonsai soil in a place that is protected from the wind and it is "0" degrees F for 1 week, what is the temp of the soil? I was thinking that it would be "0" degrees F, but maybe wrong according to what I am reading. Also, if I mulch the same tree, what would be the temp difference between the two settings?
Yeah, if the pot is exposed on all sides to that temperature and the pot is relatively small, it's the same temp as the surrounding air. Larger pots hold more soil. the more soil, the larger mass that has to cool. If the pot is under and surrounded by say six inches of mulch and well watered, that exposure is minimized--roads on bridges freeze before surface roads because the bridge road is exposed to more air. Mulch "traps" ambient warmth (and warm is a relative thing) from the ground. In ground trees' roots are protected from the worst cold (up to a certain extent) because of the vast volume of earth surrounding them. It takes a while for that cold to creep down into the ground. The mass of soil can take weeks to freeze a foot down. Mulch taps some of that capacity. Mulch will freeze through, particularly during deep cold spells. However, that cold is blunted because of the ambient warmth and additional mass of the mulch and ground. The mulch "lags" the worst of the cold.
 

HardBall

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Many cold hardy trees can and will survive root temperatures at 15 F but that's cutting it close to concerning the possibility of injury or worse, so keeping the roots well above that range of cold is warranted. Concerning winter storage of trees in an unheated outbuilding, being kept on the ground and having the pots mulched is key. Anything on the ground will benefit greatly from the thermal sink of the earth... even as the water in the soil freezes, the temperature doesn't fall below 32 F for a lonnnnggggg time. Throw some mulch around the pots and the root zone will never get remotely that cold.
This is the part that doesn't get said enough for newbs to hear and understand...took me a while too until I started putting temperature probes in with my mulched in trees. It's really not as scary as you think it would be.

My experience in 6a... Mulched in pots stay above freezing (32F) all winter long even with drops down to -15F.
 
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August44

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And what is the best sized mulch to use? I can go from compost to large bark.
 
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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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rockm

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And what is the best sized mulch to use? I can go from compost to large bark.
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.
 

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