There are a few dwarf varieties that are said to only grow to 2 to 3 feet in height so those would be quite easy to move indoors during extremely cold temperatures if you have a location inside for them during the winter months.
Lagerostromeia indica 'Bayou Marie' and 'Pixie White are said to only attain heights of 2 to 3 feet and 'New Orleans' is a ground cover type only attaining a height of 8 inches or so. To see photos, scroll to the bottom of this page:
http://www.crapemyrtle.com/
Also check out some of the dwarf and semi dwarf varieties listed here:
https://www.clemson.edu/extens...
and here:
http://aggie-horticulture.tamu...
