I would move the pot into a sheltered area if possible, and wrap bubble wrap round it - not over the top, of course. (You could mulch over the top, though). Several layers round the pot might protect the root ball. Good luck.

If they are the type that loose their leaves in autumn just tidy up the plants and move them nearer the house. I have found mine very hardy....even down to -13c so no need to bubble wrap them. My agapanthus is Northern Star, a lovely deep blue, and it is in a 15 inch pot and produced 30 flower heads last year. I have grown these lovely plants for 25 years and never had one lost due to cold and frost.

Bubble wrap's a good plan, and then pop the pot in the greenhouse if you are worried..

I lay my pots on their sides in a sheltered place so the leaves can die back naturally, and the pots don't become waterlogged in the winter. In the spring, bring them out, trim off any remaining dead leaves and let the new shoots romp away. It always works for me.