Is your plant inside or outside? Humidity trays are completely useless, a gimmick sold to those who are less experienced. A small tray of water cannot appreciably increase the humidity in a room, much less the Earth's atmosphere (if outside). I live in the Southeast U.S. and crepe myrtle grow here both as landscape and bonsai, I have them as both. Put your tree outside, make sure it is watered, and it will be OK. Crepes will not survive inside.
John