Posted: 23/08/2013 at 11:16
Often the dahlia will recover from being flattened as long as the stems have not been snapped or damaged too badly. This will depend on how new the affected growth is of course. With very large specimens it is always best to stake them or offer some kind of support to avoid rain damage.
I was always told that dahlias should be lifted and stored over winter but my attempts were unsuccessful and all but one succumbed to mould. I think I didn't check them enough over the winter so I never tried it again.
These days, I simply leave them in the ground as if they are planted deep enough and mulched they will survive. In the East of England we had temperatures down to -15 last winter but a higher percentage of my dahlias survived than were lost to the cold this year.
I also take seed as a precaution but as Red Dahlia says, seed doesn't grow true to the original plant. This is part of the fun though as you never know what you are going to get. The seed grows very easily, so it is never difficult to fill any spaces if a couple haven't survived the winter.
That said, bringing dahlias indoors in February will give you a head start if you have lifted them.
It will depend on how much time you have as to which route you decide to take.