Question
what time of year is best to trim Rose a Sharon.  Also, other than trimming horizontal, how do you trim for a tree form.?

Answer
Colleen,
The best time to prune is in the spring when you can either prune back hard to produce many flowers or you can leave 2 or 3 buds per branch to create larger flowers.
The easiest way to train the Rose of Sharon into a tree is to start pruning the lower limbs up the main trunk until you have the size canopy that you want. Then it is just a matter of maintaining the shape through periodic pruning and thinning.