I started the wave petunias on the left from seed in early March, and used store plants for the one on the right. Both bags were started the first of April, and were left flat for 4 weeks before hanging.
I also have a pouch of impatiens that is really full that was started in late April.
I mainly used the Miracle grow brand, but added lots of the Terra sorb water absorbing crystals. But I find that I am having to water the bags practically everyday, even with the cooler temps. And I have read several places that it is very important to let the bags lay flat for a minimum of 4 weeks.
I planted my impatiens in a black tie blooming bag which has two separate chambers, and it seems to stay moister. I don't know if it's the bag or that the impatiens are in the shade.