Handy Dahlia tips from TNDC
 

4 Summer tips from The National Dahlia Collection Penzance

http://tinyurl.com/ovtqo74

or

http://www.cmlb.net/emailmarketer/di...87&L=189&N=499

My house and allotment is 100km east of Penzance but weatherwise, it's very
different. I'm 5 miles inland, 500' amsl and east facing.

The Winter's can be ferocious, pretty similar to Dartmoor. I used to lose 2
or 3 plants each Winter (out of a total of 20).

To the above 4 Summer tips, I'll add one Winter tip :- Cover the dahlia
patch with tarp or heavy plastic. Since doing this in early Dec 2012, I
haven't lost one to frost damage. I believe the tarp keeps them reasonably
dry.

If you're in a sheltered area, then you should be ok without covering.