Hmmmm. Very good point. I suppose I may have considered that also. Plus I wasn't all that convinced they would really grow and thrive here.
I filled another bit pot with aforementioned mixture and put some spinach and Red Salad Bowl Lettuce starts in it. Hoping the soil will be warmer up higher from the ground and protected from the slugs. Guess the dahlias will just have to look out for themselves and go into the ground. Vegies come first.