First overwintering - Chicago (zone 6a) - Cold Frame

[ChicagoBonsaiNovice]

I have three potted red oaks (pre-bonsai) that I started from local acorns I gathered last October. I have prepared a cedar cold frame packed with cedar mulch and am waiting for the frost before placing the young trees inside.

Is this an adequate overwintering set-up? I am aware that many people keep them in their garages, but I much prefer this method if it is feasible. I hope that because red oak trees are hardy into zone 3, this will be okay, but I am unsure. Chicago (6a) gets pretty damn cold, often below 0 degrees, and I worry about the cold frame being elevated; air flows freely beneath it. However, there is a quite a bit of much in there, 7 cubic feet, plus the 1 cubic foot of sand/peat moss on the sides.

On a side note, I am also preparing to take dozens of hardwood cuttings of nearby maple, conifer, and other species and root them in the sand/peat moss sections as soon as the trees in my neighborhood go into dormancy. You can see these layers on the sides, which go 6 inches deep above the mulch.

Thank you for your input!

 

[Dav4]

Cold frames are intended to be on or even in the ground, utilizing the massive thermal sink of the earth to prevent massive temperature swings through the winter. An elevated area will be subjected to the ambient temperature of the air and negates most of the benefits of the box you've built. I'd advise finding a spot on the ground to place this, preferably in the shade. Also, I wouldn't put your seedlings in there until the weather has turned decidedly more like winter and the soil is frozen. If mice were to get in there sooner than that, they might decide it would be a nice place to spend the winter.

 

[penumbra]

There is a long learning curve to using coldframes or green houses. I nixed the green house, but I do use low hoop houses and coldframes. They all depend on ground temps. I am also in cone 6 and most of my plants go on the ground in the open and some sit on the bench all winter. My cold frames are set up to avoid most sun and are there to moderate, not raise temps.
It is a personal journey.

 

[Bonsai Nut]

I used to live in Chicago, though I overwintered my trees in an unheated garage.

First, you don't need to do anything (with cold hardy deciduous trees) until your nights are starting to get well below freezing. Wait until your trees are fully dormant and have dropped all their leaves. Also note - once the trees have dropped their leaves they don't need ANY light until they bud out in the spring.

Second, it is important to note that you are trying to protect your tree roots from temps well below freezing - generally in the neighborhood of sustained soil temps of 20F or less. Many tree roots can survive dips as low as 15F for short periods of time, as long as the soil is moist (not dry) and as long as temps rise again during the day. I have left my trees outside here in NC on my benches without protection, and as long as they are watered they have survived temps into the low 20's regularly. I only started to have problems (once in four years) when temps dropped below 20F and I let the soil get drier than it should have - and this was for warmer deciduous trees; I have never lost a conifer due to the cold. If you are getting frosts in the 30F range, while daytime highs are in the 40's, your trees don't need any protection at all.

Third, it is really important to maintain consistent temps as much as possible. Unheated garages work well because they heat up slowly during the day, and cool off slowly at night. If you use a cold frame, place it in direct contact with the ground and make sure it is in full shade. Don't let your cold frame cook your trees in the spring!

Finally - snow makes a great insulator. It never hurts to pile snow around the sides of your cold frame, and even on top of the lid. If your cold frame is completely buried in snow, all the better. I used to place snowballs on the soil surfaces of all of my pots in my garage. Whenever the temps would rise above freezing the snow would start to melt - keeping the soil moist but not saturated.

Cold hardiness varies by tree species - and by age and condition of the tree. Make sure you are aware of the growing zones of your trees in nature, and be more cautious with your trees that naturally live in the warmer ends of your zone.

 

[ChicagoBonsaiNovice]

Cold frames are intended to be on or even in the ground, utilizing the massive thermal sink of the earth to prevent massive temperature swings through the winter. An elevated area will be subjected to the ambient temperature of the air and negates most of the benefits of the box you've built. I'd advise finding a spot on the ground to place this, preferably in the shade. Also, I wouldn't put your seedlings in there until the weather has turned decidedly more like winter and the soil is frozen. If mice were to get in there sooner than that, they might decide it would be a nice place to spend the winter.

Thanks for the advice! You confirmed my worry. I am going to remove the wood planks on the bottom of the frame, dig out some holes next to my house to sink the posts into, and place it on the ground. I’ll post the updated picture later today.

I am actually going to use cuttings from hardwood of this year’s growth, not seedlings. My plan is to root them. But point taken.

 

[ChicagoBonsaiNovice]

There is a long learning curve to using coldframes or green houses. I nixed the green house, but I do use low hoop houses and coldframes. They all depend on ground temps. I am also in cone 6 and most of my plants go on the ground in the open and some sit on the bench all winter. My cold frames are set up to avoid most sun and are there to moderate, not raise temps.
It is a personal journey.

Thanks! See my reply to Dav4; I’m going to remove the wood slats on the bottom, dig holes for the posts, and place it on the ground in a shady area. I will update with a pic by end of the day.

 

[ChicagoBonsaiNovice]

I used to live in Chicago, though I overwintered my trees in an unheated garage.

First, you don't need to do anything (with cold hardy deciduous trees) until your nights are starting to get well below freezing. Wait until your trees are fully dormant and have dropped all their leaves. Also note - once the trees have dropped their leaves they don't need ANY light until they bud out in the spring.

Second, it is important to note that you are trying to protect your tree roots from temps well below freezing - generally in the neighborhood of sustained soil temps of 20F or less. Many tree roots can survive dips as low as 15F for short periods of time, as long as the soil is moist (not dry) and as long as temps rise again during the day. I have left my trees outside here in NC on my benches without protection, and as long as they are watered they have survived temps into the low 20's regularly. I only started to have problems (once in four years) when temps dropped below 20F and I let the soil get drier than it should have. If you are getting frosts in the 30F range, while daytime highs are in the 40's, your trees don't need any protection at all.

Third, it is really important to maintain consistent temps as much as possible. Unheated garages work well because they heat up slowly during the day, and cool off slowly at night. If you use a cold frame, place it in direct contact with the ground and make sure it is in full shade. Don't let your cold frame cook your trees in the spring!

Finally - snow makes a great insulator. It never hurts to pile snow around the sides of your cold frame, and even on top of the lid. If your cold frame is completely buried in snow, all the better. I used to place snowballs on the soil surfaces of all of my pots in my garage. Whenever the temps would rise above freezing the snow would start to melt - keeping the soil moist but not saturated.

Cold hardiness varies by tree species - and by age and condition of the tree. Make sure you are aware of the growing zones of your trees in nature, and be more cautious with your trees that naturally live in the warmer ends of your zone.

Thanks for the detailed reply! As I wrote elsewhere, I am going to place it directly on the ground in a shady area. I strongly prefer learning the cold frame method instead of using the garage, but I understand your points; hopefully this works. I see that people do it both ways.

Regarding the threat of overheating, this cold frame actually has a temperature-activated lever which vents the frame when it hits a certain temp; I will install it in the spring.

I’ll keep note of your other suggestions. Thanks again.

 

[ChicagoBonsaiNovice]

Cold frames are intended to be on or even in the ground, utilizing the massive thermal sink of the earth to prevent massive temperature swings through the winter. An elevated area will be subjected to the ambient temperature of the air and negates most of the benefits of the box you've built. I'd advise finding a spot on the ground to place this, preferably in the shade. Also, I wouldn't put your seedlings in there until the weather has turned decidedly more like winter and the soil is frozen. If mice were to get in there sooner than that, they might decide it would be a nice place to spend the winter.

Okay, here’s the new set-up. I feel much more confident in this. Thanks for the advice.

 

[WNC Bonsai]

Now consider adding of the auto-openers for the top so your trees don’t cook when the sun hits it.

 

[Orion_metalhead]

Looks good. cover the top with something so it doesnt act as a greenhouse.

 

[Bonsai Nut]

Consider digging it 6" into the ground, or conversely, mound your mulch 6" up the sides.

And as others have said, clear tops are an issue if the sun can reach them. You'll get a greenhouse effect where the ambient temps are 15F but your cold frame warms above freezing due to the sun. You want to avoid this.

 

[ChicagoBonsaiNovice]

Consider digging it 6" into the ground, or conversely, mound your mulch 6" up the sides.

And as others have said, clear tops are an issue if the sun can reach them. You'll get a greenhouse effect where the ambient temps are 15F but your cold frame warms above freezing due to the sun. You want to avoid this.

Okay, I'll do a little bit of both; dig, and I have several more cubic feet of cedar mulch. I was also planning to dig a hole in the ground to place the pots into. I'll obstruct the clear top with the wood planks. Thank you!

 

[ChicagoBonsaiNovice]

I've put my trees in the cold frame, but I'm afraid that the soil in the pots are wicking up moisture from the earth; they're really wet in there. Any advice?

 

[Orion_metalhead]

Raise the pots above the soil so they dont contact. I layer a few inches of mulch below my pots as a buffer.

 

[RJG2]

Now consider adding of the auto-openers for the top so your trees don’t cook when the sun hits it.

I've seen this mentioned before, but every time I look, the only ones I can find didn't open until at least 50° f (most are 60°+).

That seems far too warm. Has anyone found any that open at say, 40° f?

 

[Orion_metalhead]

I've seen this mentioned before, but every time I look, the only ones I can find didn't open until at least 50° f (most are 60°+).

That seems far too warm. Has anyone found any that open at say, 40° f?

Should be able to get a stand-alone stat that can trigger the mechanism...

 

[RJG2]

Should be able to get a stand-alone stat that can trigger the mechanism...

I understand that.

I assumed he was talking about the ones that open automatically based on temperature.

 

[Orion_metalhead]

I understand that.

I assumed he was talking about the ones that open automatically based on temperature.

Right. Modifications necessary.

 

[RJG2]

Right. Modifications necessary.

I don't think we're on the same page.

This is what I'm talking about:

Bayliss Autovent Opener

This non-electric roof vent opener is simple to install and is completely automated. A threaded adjuster bolt can set the Bayliss to start opening the vent at approximately 55 degrees Fahrenheit. This vent opener uses a very simple principal - Special waxes have an expansion rate of about 18%...

www.greenhousemegastore.com

"This vent opener uses a very simple principal - Special waxes have an expansion rate of about 18% when heated by the sun."

It would be cool if they were available with another substance that could do it under a lower temperate, say 40°.

 

[Orion_metalhead]

I don't think we're on the same page.

This is what I'm talking about:

Bayliss Autovent Opener

This non-electric roof vent opener is simple to install and is completely automated. A threaded adjuster bolt can set the Bayliss to start opening the vent at approximately 55 degrees Fahrenheit. This vent opener uses a very simple principal - Special waxes have an expansion rate of about 18%...

www.greenhousemegastore.com

"This vent opener uses a very simple principal - Special waxes have an expansion rate of about 18% when heated by the sun."

It would be cool if they were available with another substance that could do it under a lower temperate, say 40°.

Ahhh ok. Not an electrical opener. Noted.