When frost kills the foliage of your bulbs (here, dahlias), it’s time to bring them back indoors for the winter. Source: canoecorner.blogspot.com
Most summer-flowering bulbs other than lilies, and that includes dahlias, gladiolus, cannas, callas, colocasias, acidantheras and tuberous begonias, are considered “tender bulbs,” that is, they are not hardy enough to survive the winter in cold climates. Many of these bulbs may be able to overwinter outdoors in USDA zones 7–8, especially if well mulched (indeed, some gladiolus thrive in zone 6!) and certainly in zones 9 and above, but most gardeners living in colder zones can only keep them alive from year to year by bringing them indoors in the fall.
Usually the signal to bring them indoors is when the first frost damages their foliage. If there is no frost before mid-November, though, bring them in any way: you don’t want them to be still in the ground when it freezes solid!
To do this, dig up the root ball and shake it to knock off excess soil, then let the bulbs dry on newspaper or cloth for a week or so in a frost-free spot (a garage or shed, for example). Now clean them roughly with a brush to remove most of the dirt. Don’t rinse them, though: they must stay dry.
On some bulbs, stems and foliage will fall off all on their own at this stage. If not, cut them off about 2 inches (5 cm) from the bulb. Also cut off any lingering roots and remove any bulbils (baby bulbs). (You can save the bulbils if you want, but be forewarned that they are usually 3 to 5 years from blooming!) You can also sprinkle garden sulfur on the bulbs: this will help prevent rot over the winter.
Make sure you properly identify your bulbs at this point. There is nothing more confusing than looking at a pile of gladiolus corms or dahlia tubers in the spring and trying to remember which were the red ones and which were the yellow ones! Either write the name on their storage container or add a label. Some people write the name on dahlia tubers with a felt pen!
If possible, store the bulbs in a cardboard box or a plastic container (the latter should be have holes so there’ll be a bit of air circulation), covering them with vermiculite, peat moss, wood shavings, or shredded paper. Some gardeners like to store them in a mesh bag.
The ideal location for winter storage is a cool but not cold spot that remains between 40 and 55? F (5–12° C) for much of the winter. That means that a fridge or a cold room may be too chilly for some tender bulbs, but a basement or lightly heated garage should work well.
You have nowhere cool to store them? Don’t worry: they can be kept at room temperature if necessary, but if so, the bulbs will tend to dry out over the winter, so check them monthly, giving them a spritz of water if they start to shrivel.
One final note: if you grow your tender bulbs in containers, overwintering is even simpler. Just bring the pots indoors, cut off the foliage, let the soil dry out, then pile the pots up in cool, dry spot until spring.