zxcvbob wrote:

Do they need to wait until the soil is warm, or plant as soon as it
thaws and nights are usually above freezing? Or do they need to be
started indoors in pots, then set out like tender annuals?

I almost bought a dozen bedding dahlias at Sam's Club today, but then
thought it might be too early. I know they are day-length sensitive;
that's why I've never tried starting 'em from seeds (days are long
while it's still cold up here)

As with other plants usually dahlias are sold in their correct
planting zone, and almost always directions are on the packaging
material. There's plenty of info on the net, I suggest checking
several sites to glean the facts that best pertain to your parameters:
http://www.almanac.com/plant/dahlias