Our neighbor has this plant and think it may be a crepe myrtle but arent sure.
I cannot be sure on this either, but it may not be crepe myrtle. Crepe myrtle has prominent side veins on the underside of the leaves and they tend to be more blunt at the tips as well. See this site: http://iransafebox.net/crepe-myrtle-leaf-identification/
Your sample could easily be something else. I’d just plant it in the ground and let it develop, then post more pictures later. Sorry I cannot be of more help. I generally deal with our native wildflowers on this site.
Steve Hill, Botanist, SCNPS