I really do think that the black pots cook the dahlia roots. They like sun but I think like their roots cool like clematis. That would explain the ones you have in shade doing okay. Cool roots.
I use the fabric bags for potatoes. I have 8 now, three are 20 gallon bags. Need to hill them up very soon. I just unroll the bag as they grow and add straw and soil mix to cut down on the cost of soil alone. Never thought to try to grow flowers in them.
