Are you sure lack of nutrients is the problem?
Roses are heavy feeders. Black-eyed susans generally don't need or want fertilization. Rhodies (by me) want something like Hollytone, NOT 20-20-20. Maybe they do by you, but I never would.
As a general rule of thumb, shrubs\perennials do not respond well to water soluble, fast release ferts. They are not turf grass. There are some exceptions such as roses.
"Science is the belief in the ignorance of the experts" – Richard Feynman