If they are in a new flower bed, you can use a balanced fertilizer for veggies... the easiest is the time-released pellets, which would be a one-time thing.

Most serious growers stop fertilizing in August, as nitrogen can cause the tubers to rot in storage. Most serious growers also do soil tests to see what to apply for the season. If this is your first year, don't sweat it, and just be conservative with the amount you apply. Too much fertilizer is an easy way to fry your dahlias.

Enjoy your dahlias!

Last year when I grew at a farm that tries to stay organic, the beds were ammended with enormous quantities of composted horse manure. If you have a friend with a horse who is very careful about where the hay/feed comes from it can work very well. Friend makes hay from her own fields and doesn't use herbicides. The plants were a little rich on leaves but there was no shortage of blossoms. Biggest problem is that the paths filled with greenery and overwhelmed my support system. This year the field would probably have leached a lot of nutrients and be just right. And in hind-site, I would have limbed up those babies more aggressively. I learned a lot, the conditions were so different! But my age/health won't let me keep up my own garden and an extra one, too.

Thank you garden web friends, i have a flower farmer fined who soaks her organic fertilizer and then waters with it, like a compost tea. perhaps ill give that a try. As far as a friend with a horse, I do have one. I never thought to ask for her manure before (providing she isn't feeding the horse any chemically treated/grown hay).