Some people leave their dahlias in the ground. If your area is particularly cold, and you want to store them, then dig them up when the first frost has blackened the top growth, dry them off in a frost-free place and then store them in dry compost or sawdust in frost-free conditions. Much the same for begonias - but you can take them in as soon as the stems and flowers drop off. Yes, I've still got flowers on mine!