It's best if we know where you're located. For example, in warmer climates it's best to prune hydrangea's after flowering in the fall. In colder climates it's best to leave the old flower heads on over the winter and prune in midspring. Also there are different kinds of hydrangea's but I'm assuming you have the more common one, hydrangea Macrophylla. (Big leaf hydrangea)

It's not REALLy necessary to prune the other plants you mention accept to train them. However, the generall rule of thumb is to prune branches that cross or that are touching. Also, branches that grow straight up or down are best pruned off. And suckers (that grow from the base of the tree are best removed.

You can google a search for individual plants by typing "pruning rose of sharon." The link below should help also.

Here is a link that might be useful: Pruning

when we bought our house there were 32 crepe myrtles inthe yard. the previous owner had NEVER trimmed any of them nor any of the other trees other than 1 apple tree. the CMs along the drive are roughly 20 ft apart, yet when we moved inthey were all grown together almost in a hedge! i had to do sever pruning to get them to look like a CM should look, and even now 3 years later some look like crap because they have not filled back in.

mild pruning is best, never take more than about 1/3 the total volume of the tree away at any one cutting. mine suffered because i took about 60% and it took them a couple years to start recovering. you can cut the tops completely out of a CM, leaving just the main trunks, but i prefer to just selectively prune them instead of commiting "Crepe Murder" as the local horts call it.

as far as the compost heap, just keep piling it on, it will start back cooking again.