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.