Dahlias need four things to thrive.
First, they need rich soil. I enrich mine with homemade compost and mushroom compost.
Second, they need a lot of water to grow. Mine are at the bottom of a hill with a berm.
Third, they must be supported. I have been negligent in this so excuse the messy stake job.
The fourth thing that MUST happen is cool weather. Dahlias struggle until it cools down.
Then they show off with giant, heavy blooms.
These are my three favorites, Thomas A Edison, Snow Country and Firepot.
I love the cool weather, also. I have wilted and withered all summer. Hooray fall!