So a friend of mine got 3 bluebells without the completed credo. He was missing the last stage where you get double yield. I got the double yield credo finished and i also get 3 bluebells per harvest. Either the double yield is tied to the planter having the finished credo (which it does not say in the tooltip) or the double yield credo is bugged.
It seems that this also applies to other gardening items lik blueberries blood sterns and strawberries. So actually you need the credo when planting? Would be nice to have the tooltip reflect that.