General Temperature Guidelines for Winter Dormancy

Trebor

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Hello all,
I have a small maple and chinese elm that are dormant and I'm having trouble finding good general info on the best location for them over the winter. I've got the bonsai pots in a cardboard box filled with soil which I place on a plastic tub (so it doesn't freeze to the ground). My only real options are the top of a stairwell in my condo building (which has temperatures in the low 40s) or outside on a 3rd floor deck (which generally is in the 20s/30s in the winter, occasionally below).

Is 40F too warm to keep the plants dormant? Are temperatures over 20 nothing to worry about (i.e. I could bring them to the stairwell if it gets really cold)?

As always, appreciate any info/help!
 

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HorseloverFat

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They (Temperate plants) start “revvin’ their engines at 40-41 degrees air-temperature...
So dancing around 40 is most likely not the greatest idea.
Temperates require 40(ish) days of temperatures between 32 -40F... these are usually accomplished outdoors during the transition TO... and transition FROM “Winter”.. because days BELOW 32F 0C... “don’t count” in the tally.

🤓

Your deck sounds like your best bet.

20-30 degrees with a tad of “protection” is NO problem for the Maple... I am unfamiliar with Ulmus Parvifolias specific needs.. but someone else should chime in soon.
 

PA_Penjing

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I've left both of these species outside all winter with no protection and never have had trouble. My pots were similar sizes BUT they were on the ground, which is helpful. It looks to me like your set up is prefect for your area. If I were you, my goal would be to keep that box as cold as possible and not let sunlight touch it more than it has to.
 

leatherback

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Yeah, I would third that. 20f is no issue for that plants you have, assuming they are not stuck in sun and wind at those temps/
 

PA_Penjing

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@Trebor I was reading an article by Brent Walston that answered your question. Before I give the numbers remember that each tree is unique but this will hold roughly true for most trees.

water in soil freezes from 32 to 23 degrees. That is totally harmless

inter-cellular freezing happens from 23 down to 14 degrees. The "plant appears" to be frozen solid but is not, this is also harmless for healthy trees but now we're getting close.

intra-cellular freezing happens below 14 degrees F and beyond. This is fatal and your tree will lose the tree or at least the parts that got that cold inside the tissue.

I have heard it quoted before that trees shouldn't be left unprotected below 14F or -10C, so that checks out. There you have it, keep em above 14 degrees. But remember that 14 degrees outside doesn't mean 14 degrees in your protected bin, you've bought yourself a couple extra degrees. And I remind you that having a tree wake up from dormancy too early puts it at a huge risk, so dont baby them by keeping it too warm.
 

Trebor

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@Trebor I was reading an article by Brent Walston that answered your question. Before I give the numbers remember that each tree is unique but this will hold roughly true for most trees.

water in soil freezes from 32 to 23 degrees. That is totally harmless

inter-cellular freezing happens from 23 down to 14 degrees. The "plant appears" to be frozen solid but is not, this is also harmless for healthy trees but now we're getting close.

intra-cellular freezing happens below 14 degrees F and beyond. This is fatal and your tree will lose the tree or at least the parts that got that cold inside the tissue.

I have heard it quoted before that trees shouldn't be left unprotected below 14F or -10C, so that checks out. There you have it, keep em above 14 degrees. But remember that 14 degrees outside doesn't mean 14 degrees in your protected bin, you've bought yourself a couple extra degrees. And I remind you that having a tree wake up from dormancy too early puts it at a huge risk, so dont baby them by keeping it too warm.
I come back your response every winter. Thanks again!
 

Deep Sea Diver

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Glad to hear things are working for you.

There are many other types of freeze damage (see resource) that can occur earlier then mentioned above. Some kill the tree, some only slow down it’s growth in springtime

That said, Intracellular freezing, granted is one very fast road to tree death!

cheers
DSD sends
 

NaoTK

Chumono
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Could someone once and for all make a chart like below for trees in pots? Then we can just point to the chart
1667241038894.png
 
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