I dug a shovel deep rectangle for our two mums. In the space where the shoveled out soil was I added fortified soil and a sand mixture in the hole. I cut the bottoms out of the potted mums pots and set the pots without bottoms on the mixture of fortified soil and a sand mixture in the hole. I added the soil I shoveled out around the pots to winterize the pots with the mums. Over that I added the black mulch.
In other words the mums are just sitting on the ground for the winter.