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.
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