Rose of Sharon
Rose of Sharon blooms on current season’s growth. The ideal time to prune is late winter or early spring before it blooms. Begin by removing older and inner branches, cutting back 2/3rds or so to the ground. Follow that by removing dead, damaged or weak branches. Then you can prune out branches going in the wrong direction, or growing out, to restore the appearance of the Rose of Sharon to a balanced, upright look.
Rose of Sharon is prone to splitting at the trunk when it becomes more woody and thicker at the base. Keeping the bush trimmed each year to keep it appearing upright as described above may help to avoid splitting of the trunks.
Here is a link to a Michigan State University article on pruning.