Hi Linda, my understanding is that they are left until the first frost to give the tubers maximum time to mature ready for next season. So as they will no longer be growing, unless it's really mild, I would lift them now to store as the tubers may rot if the pots are not draining. The danger of leaving them in pots outside is that the soil/compost may freeze in the pot and that will kill the tubers.

It is mild but wet here Barbara. I have waited and waited for them to turn black. I have a fruit box and I was going to wrap each one in newspaper and put them in the greenhouse or my outer hall. Do you put them in earth too or let them dry out?

I dug mine out at beginning of November as I wantedthe space for new bulbs. I Cut off the top growth leaving about 6 inches of stem , turn the tuber upside down to drain for a few days and then wrap with newspaper and packed into a box in a shed. You could try just leaving them in the pots with the growth cut back if they are already dry and put the pots in the cool place. Just make sure the compost is just damp not wet or they will rot.

Overwintering Dahlia tubers is more applicable to when they are grown in the ground and where it is likely to freeze over. Many Dahlias survive when left there if they are protected by a thick mulch.

As you grow your Dahlias in pots I would leave them in there over winter Let the soil dry out now and wrap the pots in newspaper or suitable insulating material. Once leaves blacken you can bring the insulated pots indoors leaving the stems on into a cold, dark place like a garage. If you leave then in a greenhouse over winter and there is a mild, sunny spell they might come out of dormancy too early. You really want them to remain dormant until March, then put them in your greenhouse.

No need to wait for a frost. No need for paper or insulating material. It's up to you if you remove them from their pots now or in spring. If space isn't a problem, just cut the stem to about 3" above the crown and leave them in your greenhouse. Otherwise do as here

http://growingforshowing.blogspot.co.uk/

Linda, Scrumpy grows scores of fantastic show dahlias every year.

I took mine out of the pot and left in the shed to dry.

Thank you all for such wonderful advice. I need the space and the soil in the pots is pretty wet. I will cut them back and dry them upside down put them in a box wrapped in newspaper and put them in the shed until March. I will also read growingforshowing just in case.