Cut off the foliage when you are ready to get them ready for storage. There is no need to wait until frost kills the foliage. Carefully lift (dig up) the tubers being careful not to slice the tubers with your shovel or fork. Gently hose off any soil off the tubers and let them dry for an hour or so. Dust them with a bulb dust (available at Hewitts) to prevent rotting while in storage. Place them into plastic bag like those in your grocery store’s vegetable section. Fill the bag around the tubers with vermiculite or very dry potting soil. Peat moss and sawdust are also used. Place those bags into light-proof boxes and place those boxes in an area that will be between 35° and 50° over winter. Check them a couple times over the winter to make sure they are OK. Remove any that appear to be getting mushy. If they seem to be getting limp from dryness, it is a good idea to mist them a little before they go back into storage.