Very low temps will freeze even the largest containers .Your problem is keeping any containerised plants that are not hardy in a frost free place .

When we get prolonged, very cold weather (last winter our temperaturtes went down as far as -15C for most of November and then for much of December) the soil in the garden can freeze for a depth of several inches. So, yes, the compost in large containers can also freeze. Does this harm the plants - not necessarily! We also have a large number of plants in containers and, whilst it is disapointing to lose one, this doesn't happen very often. You cannot wrap everything in insulation and we don't bother to. If it is possible to keep the containers on the dry side when cold weather is forcast this will help.

Thank you for your answers.

All the plants are hardy except for a couple which I intend to bring into an unheated conservatory over winter.

It might help if you could bring the containers close together, preferably near the house wall, sheltered from cold winds if possible. They will possibly protect each other to some extent if the frosts aren't too severe.

All mine are in pots Chris, I thought I would lose the lot last year...I lost the Bay and Agapanthus that was it. I think I moved the bay to the coldest spot with hind sight. I pack my plants close together as I have no other option.

I am lucky that my gargen faces south so the house protects it from the winter north winds, also it is surrounded by wooden fencing.

This provides me with some sheltered spots to move things to.

I have a couple of plants a Salvia Hot lips for one which I will try to overwinter in an unheated conservatory.

In the colder parts of the USA, the natural soil often freezes more than 3 feet deep--as the water pipes in Flagstaff, AZ can testify--but many hardy plants and bulbs simply shrug it off. The main difference is that the absolute temperatures in an unprotected pot will be lower and more variable than deep in the ground, encouraging the formation of ice crystals in plant tissues.