We have hot summers here too on Long Island, very hot and humid especially the month of August. But by then, the Dahlia has either grown and is flowering well or it just plain does not exhist at all.
My garden is very cottage garden. I have been gardening forever and usually know how to treat plants so outright failures are unusual for me. The soil here is good, nicely but not overly ammended with compost and always mulched.
When I was a kid, my dad used to grow large flowered dahlias. I don't remember him doing anything special at all to the plants and they grew nicely and flowered all the time. He did dig the tubers and lift them in the fall and save for next spring. I have not tried that myself.
Will see what happens when I hit the local nurseries next. If I buy tubers I will pot them like I did with this years cannas and then hope for the best. I have never tried starting them early indoors potted so it is worth a shot.
