I follow Roger Zelinski's method and use the mix he recommended. It is unfortunate he closed his business, Raising Rarities; he had excellent plants. Since our 'soil' here is just rock, maple roots, and compressed clay, putting them in the ground was never an option. So they are grown in pots in a raised bed beside the greenhouse where they get a bit of direct sun midday and filtered the rest. I do throw just a bit of mulch over them in the winter.
What is 'GPH'? As for the folks who got Roger's stock, all I can say is I checked them out last year and they had only a few kinds, and had doubled and tripled the pricing. Did not buy. I suspect they're just selling what was left of his stock and not replenishing.