Karen, your crepe myrtle is beautiful!! So many blooms! The way we do it here is just to leave it to it's own vices. But dead heading can't hurt it especially since you can reach the flowers. I can't my trees are two stories tall. We trim while the branches are bare in February but if you have snow, I would wait til after the snow.
I planted a small dwarf last year. But it was during the drought and it suffered badly. It is doing better this year.
Our is not in a profuse blooming year but they still give us some color.
