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

[sorce]

Negative 28F just on the ground with no fatalities.
3rd floor windowsills, no casualties.

Let the trees be trees.

Just don't repot in spring.

All summer repots. No casualties.

Sorce

 

[sorce]

And don't cut after mid August.

Sorce

 

[WNC Bonsai]

I understand that.

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

Yes, that’s what I meant. I have 2 vents in my greenhouse that use these. You want them to open at a temp low enough to prevent the plants from cooking and 55 should do that. Companies like Gardeners Supply used to sell cold frames with these openers. I have checked a lot of listings on Amazon and they all must use the same wax filler since they all open at about the same temp.

 

[Bonsai Nut]

Ahhh ok. Not an electrical opener. Noted.

Like this. Note that this cold frame is being used to start seeds in February - and is intended to be warm and is using bottom heat beneath the seed trays. It is NOT an over-wintering setup.

 

[WNC Bonsai]

My greenhouse came with the wax filled openers for the ridge vents but since they don’t open until it gets over 70 degrees I go out and prop the vents open on sunny days. However on Dec 18 it was 69 in my back yard and the greenhouse maxed out at 73 even with both vents open and the door cracked. This is the kind of issue you deal with with small greenhouses and cold frames. If my trees break dormancy too early I may look into using temp controlled electrical openers but even they couldn’t do anything when a winter warm spell hits. Hopefully you will see more consistent cold in your area.