Hello,
I top dress my hardy hibs with a good layer (2-3 inch) of compost every spring and fall as they are heavy feeders. During the summer months they are watered with the rest of the plants via soaker hose and if I am fertilizing the other plants with Miracle grow I will do the hibs too. I have found that just the spray on fertilizer alone does not do anything in particular for the hibs, I find the compost really really helps, as does a layer of mulch on top. If you plan to split or move your hibs at some point, planting them with compost gives them a very good start.