This method works great in zone 9a. I have made a couple of improvements. I use Promix BX for the top layer of seed starting mix. Johnny's Germination mix is second choice. For the bottom layer, I usually 80 percent pine fines, the other 20 percent is peat, compost and bagged garden or potting soil. Add lime and some all purpose fertilizer like 8-8-8. I cover the pots with a sheet of poly instead of individual bags. The insecticide is key here due to fungus gnats. Water after applying the insecticide on the top and cover.