I fertilise after the blooms with half strength seeweed liquid feed, no science to it at all, I get it cheap and have found it to be extremely good. I apply when I remember, at most every two weeks and only until end July so as to firm up any lush growth before winter starts.
Non flowering seedlings can be fertilised from onset of new growth, about now!

Cheers Greenmanplants.

I use the same fertilizer for Helleborus that I use on the Hostas: Osmocote + minors. Since the new growth is starting about the same time on both plants, I do the application at the same time. Here in Zone 7, I use the 8-9 mo slow release, but in your area, you would want the 3 or 6 mo release type. I believe the 6 mo type is only available w/minors. I've found that annual application of lime is not really necessary for Helleborus x hybridus (also incorrectly referred to as orientalis in the trade),and I have ammended clay soil that is naturally acidic.
I do add a handful of dolomitic lime to the soil when planting the Helleborus.(If I remember to do so!).
Rb

Note to Greenmanplants----
You must know that on this side of the pond, our ever increasing horde of health nuts consume all of the harvested seaweed, leaving none available for fertilizer!!