Direct temperature measurements and winter protection

Lars Grimm

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Hi All,

As with many of you, I obsess about winter protection this time of year. I have come to believe that many of us (myself included) go way overboard on protecting our trees. I often see people advocating to have trees placed on the ground and mulched to allow the earth to better insulate. While this intuitively makes sense to me if you have a very small and/or shallow pot, I'm not sure if it makes a difference if you have your trees in a wooden grow box, mica pot, Anderson flat, etc where there is already a sufficiently large volume of soil in place.

Is anyone aware of any references or has anyone made any temperature measurements with different overwinter protections? Let's ignore the effects of wind and desiccation as there are other ways to compensate for these (wind barrier/fence, regular watering). I am also specifically just talking about the temperature of the roots/soil media as these are the most sensitive components of bonsai to winter damage.

I am up for some controlled experiments, if no one has any direct measurements.

Sincerely,
Lars
 

coh

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One thing to keep in mind is that Durham NC is not Rochester NY is not northern Minnesota is not...

I haven't measured root temperatures under various conditions and don't feel the need to. If I take a tree in a wooden grow box or Anderson flat and leave it out exposed during one of our week long arctic blasts when temps are in the single digits at best, those roots are eventually going to get colder than I want them. If it's cold long enough they'll reach ambient air temperature (and wind will speed that process up). Do I want my trident roots to reach 0 F? No. Will they survive that? I don't know but am more than willing to let others figure it out.
 

Lars Grimm

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One thing to keep in mind is that Durham NC is not Rochester NY is not northern Minnesota is not...

I haven't measured root temperatures under various conditions and don't feel the need to. If I take a tree in a wooden grow box or Anderson flat and leave it out exposed during one of our week long arctic blasts when temps are in the single digits at best, those roots are eventually going to get colder than I want them. If it's cold long enough they'll reach ambient air temperature (and wind will speed that process up). Do I want my trident roots to reach 0 F? No. Will they survive that? I don't know but am more than willing to let others figure it out.
Agree, NC is very different than you in the colder ranges. I am fortunate to have lows in the teens but only for a few hours at most during the whole winter (it will be 70 degrees next week).

I'm just curious how much of a temperature differential you can reasonably expect from ground contact and/or mulching.
 

Dav4

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Agree, NC is very different than you in the colder ranges. I am fortunate to have lows in the teens but only for a few hours at most during the whole winter (it will be 70 degrees next week).

I'm just curious how much of a temperature differential you can reasonably expect from ground contact and/or mulching.
One winter in zone 6 MA, I placed a temp probe beneath the mulch around my pots in my cement garage floor. Air temps fell into the single digits for several days, and the mulch was frozen for almost 4 months straight. Temps below the mulch stayed at about 32F the whole winter.
 

Lars Grimm

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One winter in zone 6 MA, I placed a temp probe beneath the mulch around my pots in my cement garage floor. Air temps fell into the single digits for several days, and the mulch was frozen for almost 4 months straight. Temps below the mulch stayed at about 32F the whole winter.
Super helpful. I think I am going to run some similar experiments here in NC where the temperatures are not as extreme.
 

coh

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One winter in zone 6 MA, I placed a temp probe beneath the mulch around my pots in my cement garage floor. Air temps fell into the single digits for several days, and the mulch was frozen for almost 4 months straight. Temps below the mulch stayed at about 32F the whole winter.
Was the air temp in the single digits inside the garage? Our garage rarely gets lower than the teens even in the coldest weather.
 

coh

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Agree, NC is very different than you in the colder ranges. I am fortunate to have lows in the teens but only for a few hours at most during the whole winter (it will be 70 degrees next week).

I'm just curious how much of a temperature differential you can reasonably expect from ground contact and/or mulching.
Yeah, I really don't know and it would be interesting to test under various conditions.

I did the mulching thing one winter, I set my trees on the floor of the barn and surrounded the pots with mulch. They survived just fine (I didn't measure pot temperatures, but air temps in the barn get down to near outdoor temps during extended cold spells so near 0 F at times). But...what a pain dealing with the mulch is! Setting it out and getting in settled in around the pots, then digging it out in the spring is a lot of work and my back didn't like it. So I went with other methods for wintering that are less labor intensive.
 

Dav4

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Was the air temp in the single digits inside the garage? Our garage rarely gets lower than the teens even in the coldest weather.
Yes... probably 7-8 F each morning for 2-3 days straight... completely unattached structure. Outdoor temps were -4 to -6 F.
 
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Lars Grimm

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Yeah, I really don't know and it would be interesting to test under various conditions.

I did the mulching thing one winter, I set my trees on the floor of the barn and surrounded the pots with mulch. They survived just fine (I didn't measure pot temperatures, but air temps in the barn get down to near outdoor temps during extended cold spells so near 0 F at times). But...what a pain dealing with the mulch is! Setting it out and getting in settled in around the pots, then digging it out in the spring is a lot of work and my back didn't like it. So I went with other methods for wintering that are less labor intensive.
I think I could take a some unused containers and fill with old soil and sphagnum to simulate roots in soil. Maybe two different sized containers (A and B) on the ground (1) and on the bench (2). Measure the temperature in A1, A2, B1, and B2 as well as the ambient air temperature.
 

sorce

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I am going to continue my "what surface" campaign.

Compost can maintain a temperature hot enough to cook foods inside thru Winter.

I recently mapped the path of the waste sewers in the yard of my old house by noting where the ice was melted.

Odd thing was, due to the rapid cooling new to this year, this wasn't as evident in years past, but that line of ground will keep a pre consistent temperature.

Call Julie, see what's underground, then have a conversation about what's real, and really hidden, than what's obviously different, like temperature.

Talk about the universal differences, don't jump to absolutes!🤫😜

Sorce
 

canoeguide

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It's important to note that placing pots on the ground, mulching, etc. is as much or more about keeping temperatures consistent as it is about keeping temperatures higher.

By putting a pot in/on the ground with mulch, you're using the thermal mass of the ground to keep frozen things frozen and thawed things thawed.
 

Forsoothe!

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Anthony is right on the money. The problem is cycles of freeze/thaw/freeze/thaw, you don't want that over and over. Roots that are fiberous like pines are also drier than fleshy roots which don't handle temperatures much below 20°F. Pines can take much lower temps. The volume of water in the roots is probably key to how frozen is frozen. The length of the lowest temps in combination with the depth of the low is important, too. While it gets just as cold in NC upper elevations as in SE Michigan, the deep cold in NC starts a month later and finishes a month sooner. Also, there are good winters and bad winters. There's 10°F, and there's 10°F for 2 weeks and the difference is night and day. The problem with characterizing what you might have based upon what you had in the past recent enough to remember is that weather cycles come in decades, like the high/low water cycles here which are about 34 years long. We had two bad years in a row in 2011 & '12 and I lost several maples in the landscape and a few bonsai. I loose a couple bonsai every year. It's a numbers game with trees that are in a stressful hobby, -if you have enough finished and goona-bes, that 3 or 4% will pass on every year. All this is to say, you never really know, so touching dirt, good dry mulch high as the nebari all winter, some wind protection, and for me in full sun so daytime temps modify the depth of cold and things leaf out with the rest of the local environment. Even Japanese maples can take all the sun I have in SE MI until June 1st, so plenty of time to protect 4 to 6 weeks after benching.
 

Lars Grimm

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Anthony is right on the money. The problem is cycles of freeze/thaw/freeze/thaw, you don't want that over and over. Roots that are fiberous like pines are also drier than fleshy roots which don't handle temperatures much below 20°F. Pines can take much lower temps. The volume of water in the roots is probably key to how frozen is frozen. The length of the lowest temps in combination with the depth of the low is important, too. While it gets just as cold in NC upper elevations as in SE Michigan, the deep cold in NC starts a month later and finishes a month sooner. Also, there are good winters and bad winters. There's 10°F, and there's 10°F for 2 weeks and the difference is night and day. The problem with characterizing what you might have based upon what you had in the past recent enough to remember is that weather cycles come in decades, like the high/low water cycles here which are about 34 years long. We had two bad years in a row in 2011 & '12 and I lost several maples in the landscape and a few bonsai. I loose a couple bonsai every year. It's a numbers game with trees that are in a stressful hobby, -if you have enough finished and goona-bes, that 3 or 4% will pass on every year. All this is to say, you never really know, so touching dirt, good dry mulch high as the nebari all winter, some wind protection, and for me in full sun so daytime temps modify the depth of cold and things leaf out with the rest of the local environment. Even Japanese maples can take all the sun I have in SE MI until June 1st, so plenty of time to protect 4 to 6 weeks after benching.
Ahh, the cycling is a very compelling argument that I hadn't considered thoroughly. Thank you for the explanation.
 

coh

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I'm not sure how important the freeze-thaw cycling is. Even in my shelters (especially the one in the barn), my trees are subjected to numerous freeze-thaw cycles during a typical winter. The main benefit of the shelter is protecting against extremely low temps (for me below about 25). There is some buffering against outdoor temps but when we go into a mild period the temps in the shelter will eventually get above freezing and the soil that was completely frozen will thaw, at least partly. I can't say that I've seen evidence of problems from this. Maybe this increases the risk of damage to pots, but I haven't seen that either.

Most of my trees have already been through at least 2 or 3 cycles this winter - they were briefly frozen while still outside in the late fall, then were frozen solid during the extreme cold we had in late November. Now with this mild spell everything is thawed again.
 

Cable

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I'm not sure how important the freeze-thaw cycling is. Even in my shelters (especially the one in the barn), my trees are subjected to numerous freeze-thaw cycles during a typical winter.
Depends on the extent of the thaw. If temperate trees have met their cooling requirements and the temperatures stay above 42° long enough for the tree to break dormancy and then the temperatures drop then you risk damaging the new growth.
 

coh

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Depends on the extent of the thaw. If temperate trees have met their cooling requirements and the temperatures stay above 42° long enough for the tree to break dormancy and then the temperatures drop then you risk damaging the new growth.
Sure, but I'm talking about impact on the roots. I just have never seen evidence with my own trees that freeze-thaw cycles are a big problem.
 
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