Some plants can get yellow leaves with too little water, I don't know whether that is typical of Rose of Sharon. Wilting of leaves and dropping of flower buds are other drought symptoms. Initially you were asking if you should dig around it to dry it out so it sounded like you'd had a lot of rain. We have the same drought problem here too
There are a number of things that can cause yellowing. Part of diagnosing the problem is determining whether the whole leaf is yellow or whether the veins are still green, and whether it is the youngest leaves that are affected or the oldest. If you can answer those questions it would help in the absence of a photo. Mulch can sometimes stop water getting to the roots. It doesn't usually cause nitrogen deficiency when just applied to the surface.
