Answer: Rose of Sharon or Hibiscus syriacus is usually a low maintenance shrub needing only average soil and lots of sun to bloom nicely. I would be surprised if the problem is lack of nutrients, but you could top dress with good quality compost and then use a general purpose granular or slow release granular such as 10-10-10 in the spring. However, if the shrub is planted adjacent to a lawn that is regularly fertilizer with a high nitrogen fertilizer then you would not want to fertilize it in addition to that.
One of the more common reasons for this shrub to fail to bloom is lack of sun. It does best with full direct sun all day long. Another common reason is pruning at the wrong time. It blooms on new wood or new growth of the year, so pruning should be limited to removal of winter damage or shaping as needed in early spring. Pruning during the summer would remove flowering wood or flower buds.
I hope this helps you trouble shoot.
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