Q: We have a Rose of Sharon bush. Both this year and last, there have been a considerable amount of blooms on the plant that never fully open. They turn yellow and drop.
A: In my experience, two things cause Rose of Sharon bud drop: soil too dry or soil too wet. Either condition causes root problems that convince the upper part of the plant to abort its blossoms. On the other hand, some experts say some amount of bud drop is a natural thing, similar to the June fruit drop that occurs on peaches. If water stress isn’t your problem, simply enjoy whatever beauty the plant gives you.
look about halfway down this article