I swear I answered this post, but apparently not.

The link is to a long, all inclusive discussion on digging tubers. Unfortunately it is tough to find specific advice for warm weather climates such as yours. Also unfortunate is that the author doesn't recognize the value of a paragraph for easy reading!

I would do a search for Southern CA dahlia societies and email them directly with your questions on climate-related requirements that would provide additional tips beyond the general practices used by many dahlia growers.

cut and paste this for good info as well:
http://www.dahlias.net/dahwebpg/TuberStor/TuberStor1.htm

Good luck!

Here is a link that might be useful: digging tubers/dividing clumps.

several to choose from in California.

Here is a link that might be useful: Dahlia Associations list