The frost will have hit the top portion of most dahlias growing in Canada by now and that is not a bad thing.  The trick to rescuing them from the demise of frost is to get them out of the ground before the frost enters it.  In other words, before the roots freeze.  

I wait for the tops of my dahlias to die down with the early fall frost first, as this knocks the plants out and reduces their size in my composting units [where they belong after you dig them up].  Also, I enjoy the flowers until mother Natures spoils them for me with a killing frost - why not? I have waited this long for them.  

After you dig the frost damaged plants out of the ground with a garden fork or [carefully] with a spade, shake of the loose dirt, hose them down with a course spray of water and let them dry in the warm sun for a day or so.  Do not let the tubers freeze.

Store them in dry peat moss, shredded  newspaper or vermiculite, after you have dusted them with Green Earth garden sulphur.  

Use a paper bag to store them: never plastic as it does not 'breathe.'

And place your tubers in a dark, cool place for the winter.  Your basement likely works well, providing that it is not too damp.

Come March, remove them from their storage place, plant them up in 2 gallon containers and place them in a sunny window to grow on of another season.

Easy.

For more details go to www.markcullen.com.