top dress with good stuff and then water it in. The nutrients will leach down.
I also like Jobe's tomato food spikes. Dahlias love it. Easy to use, doesn't overfeed as it breaks down slowly.
Other more knowledgable folks than myself will have a better idea as to what to top dress WITH, though finished compost is usually a good place to start.

Yep..what he said about compost..add 3" of compost in a 12" dia. or just fill the entire area. They will love it and grow very strong.

I save my leafs every fall and import and neighbors bags that are easy to move. I had a pile 4ft. high by 6 ft. in dia. It will not be enough as it breaks down and disappears by summers end. But the difference of plants with compost top dressing versus the pkants in bare dirt is amazing.

Maybe a test is in order for you to learn from? Compost some and not the others? The results will be apparent by late July.

I would avoid the cheap compost available at the big hardware stores. Find a local nursery that sells the "black gold". They usually make it themselves.

CrAzY LaRrY

Dig the thing out of the crappy soil, then dig a hole 2' wide and 1' deep where you want to plant them. Put decent soil in the hole, and put the tuber (and sprout) back in.

IMO, nothing can be done in any other way if the plant grows spindly and pathetic. Put as many chemicals into clay as you want and it will still be clay, and won't drain properly, and therefore won't service the Dahlia well. Not saying you have clay, but whatever you do have isn't suitable for Dahlias. So change it.

Cheers,
Russ

Hey Larry, guess what? Triple_b is a SHE!!
How do ya do?