I really struggle with Dahlias despite the fact that our climate is good for them. Could it be the soil? It is basically red clay and hardens quickly after rain.
I believe if I were you I would dig in lots of compost. Steve in Baltimore County
We too have a problem with clay based soil throughout our entire region. I just dug up ALL the clay from the holes I was going to plant any and all of my plants into, including my vegetables, flowers, etc. I threw almost all of the native clay that I dug from the holes elsewhere. Then I made a mixture of soil using bagged topsoil of good quality, plus some peat moss, some Southeastern Pa. mushroom compost and some tree leaf compost. I ended up with a mixture that was about 80% organic and 20% topsoil. That worked well for me.
I made sure that the soil I made wasn't too fine in consistency, and the bagged soil that I used was somewhat coarse, and so was the mushroom compost. That way it drains pretty well, but with the 30% or so of peat moss, it held some moisture.
Then I had to fertilize too later on.
That sounds like whaat I have to do and it is very hard digging into that soil.