I believe growers push there plants with fertilizer because big plants sell but this is not what there size would be in a regular garden . This could also be part of the problem with survival rates especially if they were planted later in the season. No time to harden off for winter. Another interesting read are some of the heuchera books. I don't have the titles but google search should pull them up. One goes through the different species and where they grow so that would give you an understanding of the mixes we have today.
