Unfortunately a cold frame can work against you. It needs to be open to not build heat
during the day, and you absolutely must water the plants inside when the need it.
In zone 7 you do not need a cold frame for juniper. I killed most of my collection of juniper
using a cold frame about 18 yrs ago. It’s trickier using a cold frame than not
I agree with part of this statement and disagree with part of this statement.
I am in the same zone as the OP. I have had J. procumbens nana junipers die when left out without protection all winter.
So I disagree that a cold frame or some sort of protection is not necessary.
With so many things in Bonsai, it depends. Some winters I would have been able to get away without putting them in the cold frame. Other winters, not.
Here we never know if we are going to get adequate snow cover or any snow at all to give insulation against the very frigid periods we sometimes get.
This year we didnt get really any snow at all but we did have a few days where the temperatures dropped to 20 deg F and below. We always have wind that is detrimental.
Every winter is different and you just cant predict how its going to go until it happens.
I will agree that, again like most things in bonsai, you have to be vigilant and think about what you are doing and what is happening, not just put the trees in the cold frame and forget about them until spring.
I monitor the weather constantly and cover or uncover the cold frame as necessary. If the cold frame is covered for too long, I open it and water my trees.
I did that once this winter and its the first time I had to do it in the 5-6 years I have been using the cold frame.
My cold frame is located on the north side of my house and gets very little if any sun at all so it really has no chance at overheating.
None of my trees have ever been lost from being in the cold frame. In fact I have had better winter survival in the cold frame than not.
Again it boils down to monitoring and thinking about what is happening and being proactive.