I remember visiting my grandfather when I was just out of horticultural school, and sitting with him in his enormous dahlia garden. The garden was a large, fenced off rectangle in his sloping yard, which was for the most part overrun with a teeming pack of golden labs. He lived across the road from tiny Quennell Lake but all of his own water came from a well that he constantly worried would dry up over the summer months. With this worry, most folks would not even consider using the extra water that a garden full of dahlias would require. But not my grandfather--to him, the blooms were worth the trouble.
It seems to me now that there were hundreds of dahlias in that garden, and so many colours! There were dahlias that stood five feet tall with huge, white double blooms, some that were just tiny, single yellow flowers, and still others that stood in the middle, and were reds, pinks, whites, and yellows.
The garden was impressive, to be sure. What impressed me even more though, was that my grandfather lifted and stored each one of these dahlias for the winter, only to plant them again in the spring. Hundreds of them, every year. I vowed then to never bother with dahlias, and i never did, turning my nose up slightly at their garish, bragging double blooms and bright colours.
And then this spring I realized that I had inherited my own little patch of dahlias. One day I looked out and there they were, growing right under the living room window. There were four or five clumps of them and they just kept on growing and growing and growing, until one of them was as tall as me (which to be honest, is not saying so much really) and had great huge white blooms. And I didn't find them garish at all. Throughout the summer we picked big gorgeous bouquets, we deadheaded, we staked up the flopping stems too heavy with blooms to stand on their own, and we made sure our little patch of dahlias got plenty of water. Not a low-maintenance plant--not in the least. But really, these guys give back with their crazy blooms.
And so I knew that in the late fall, and even as late as December here on the west coast, when all of their foliage has turned black, and any new buds still hanging on has turned brown before they can open, it is time to lift the dahlias and put them to bed for the winter.
Here's how.
First, you want to cut back the dahlia stems to about 4-6" above ground, and then compost the stems. Then take a spade and loosen the soil about 10-12" all around the dahlia stem, and carefully lift out the tubers, removing and discarding any that are rotten or split.
Shake off as much of the soil as you can, then wash the tubers with water and let them dry thoroughly. Once they are dry, you'll need to store them. There are a few different ways to do this.
If you have a few dahlia varieties, it is best to keep them labeled separately from each other, so you can place each type in a breathable ziplock storage bag. Add sand, vermiculite, wood chips or peat to the bag to keep the bulbs dry, and then place the bag in a box that can be closed to keep out any light.
In this fashion, your tubers can be kept in a cool, dry place like a garage or shed where the temperature will not dip below about 3 degrees Celsius. It is a good idea to check on them once a month or so to be sure that there are no rotting tubers. If any tubers are rotting, discard them and then let the back dry out for an hour or so before closing it up again.
Now, let it be known that this was not my grandfather's method. His was simply to shake the dirt off the tubers, put them into a covered cardboard box in the garage, and then leave them there till spring. And as I mentioned, his dahlia garden was pretty darn nice.