If planning to use a plunge bed it should be remembered that it’s depth should be dictated by the weather conditions in your area!
Here in Standish UK my bed is in a frost pocket half way down my garden and in "normal years" -5C or -6C is about as bad as it gets! (But did have a -12C about 10 years ago which removed the basal plate from every pot of daffodils I had, about 100. Oooooch)
So I only need to cover my bed with 2" to 3" of peat for the pots to be protected.
I had at this point hoped to attach a picture of my plunge, but am unable to due Outlook Express playing up.
I do not use the plunge to advance my flowers, only to protect them from the frost. I would not concider this as growing pots undercover as the plunge is in the open, but can see why some may disagree.
To help Chriss with an answer to her question, if you fill in between the pots in the plunge with newspaper it save you time and money when topping it up!
Regards to all,