One problem with dahlias that I assume you know because you grew them, is the tubers are subject to rot, and the leaves to fungus. Earwigs and slugs love them. OTOH, we do have great drainage here. I found some dahlia tips relating to growing them in frost free conditions.
http://www.dahliabarn.com/care.phtml
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Digging Dahlias
It's important to wait for a frost in fall before digging your dahlias. Wait at least one week after a frost before digging your dahlias. During this time, the dahlia is 'ripening' and preparing itself for winter. Dahlias dug too early will not store over winter. If you live in an area that does not freeze, withhold water from your dahlias in fall to force them into dormancy in the fall. Cut down the dahlias stems to a height of 6"-8" and use a pitchfork to gently loosen the soil around the dahlia and lift the dahlias out of the soil.
Do I really need to dig up my dahlias?
Digging and dividing will keep your dahlias returning year after year when they are dug and stored properly. Dahlias left in the ground will create a massive tuber clump underground that will send up many weak, unproductive stalks that have small blooms. Eventually, if left undug year after year, they won't come back at all. You can treat your dahlias as annuals and purchase new tuber stock each year.
If you live in an area that does not have harsh, freezing winters, your dahlias may survive winter without digging. In the fall, wait at least 1 week after a freeze and after your dahlia stalks have died back completely. Then cut the stalks down to the ground and place a 10"-12" layer of mulch such as grass clippings, leaves, straw or compost. This will protect your dahlias over winter. In March, remove the layer of mulch so that the soil can begin to warm up. Dig up the tubers, divide, let dry for 3-5 days, and replant them.
Although, we do recommend digging, dividing and storing in the fall, you should have success with this method as well.
