Last year I let the frost get the flowers in my flower boxes. It was messy and not very pleasant cleaning them out. This year I took my cuttings and put the rest of the angel wing begonias in the compost bin. It's nicer working with the flowers before the frost hits them.
I put the soil with the roots in around in places where I will like having it. They make rectangles that remain in shape. I'm waiting for a good rain to soften them, then I'll use a shovel to break them apart.
When I took the soil from the flower boxes, the bottom was neatly lined with the disposable diapers I'd put there in the spring. As I peeled the diapers off the soil, I was amazed at the 1/2 inch thickness of gooey moisture stuff. It really worked amazingly well to help retain moisture in the flower boxes and hanging baskets.