I believe it may be Caesalpinia ferrea. http://images.google.com/images?hl=...pinia+ferrea+leaf&btnG=Search+images&aq=f&oq=
I may well be wrong, but this site mentions the leaves closing at night. http://www.bonsaihunk.us/info/CaesalpinaFerrea.html
Check again later when it blooms. When botanical names are suggested, copy and past those into a search engine for more information.
I was concerned that Caesalpinia ferrea might be a bit to tropical to turn up in a garden. Saltcedar will be correct with Senna obtusifolia.