Dahlias are normally dug up after the first initial frost in your area and then stored indoors for winter. Only in warm, southern regions can these tubers be left in the ground year round. They can be easily lifted with a garden fork or spade shovel prior to the complete die out of their foliage. Break up the clumps and separate the tubers, allowing them to dry out some before storing, usually about a week or two in a cool, dry area. Then, clip off the foliage, shake off any remaining soil and pack the bulbs in dry peat moss or wood shavings within a brown paper bag or cardboard box. Place them in a dark area with cool room temperatures, like a basement, until spring.
As for the mums, this article should help: https://www.gardeningknowhow.com/ornamental/flowers/chrysanthemum/wintering-mums.htm