Answer: It won't hurt your purple leaf plum tree if you thin out the branches. Just remember to take less than one-third of the live growth during any one pruning session.
Brown leaf margins on your hydrangea can indicate over-fertilization or lack of water. If you haven't fertilized, then the problem is most likely not enough water. When you water (once or twice a week, depending upon weather), remember to water deeply, applying water slowly so it has a chance to trickle down and saturate the root mass. Your hydrangea should show improvement in its new leaves.
Best wishes with your landscape!
Q&A Library Searching Tips