There are few people & mainly one person that I am aware of that mainly spells it the way you have spelled it. He is a PhD in Oklahoma who has done much horticultural research and has created several very good hybrids……. beautiful trees.
Yet, The Crape Myrtle Trails of McKinney did extensive research with the U.S. National Arboretum, noted authors with growers & PhD.s from leading horticultural universities and with its literary history. This is why we spell it the way we do.
However, horticulturalist do not spend their time or efforts on common names. Horticulturalist, botanist and landscape architects base their decisions on scientific binomial nomenclature. Everyone agrees that the real name is Lagerstroemia indica. There are approximately 50 species of Lagerstroemia. There is only one scientific name for each plant on earth. For instance in the United States there are eight trees called a tulip tree yet no one argues over which tree is spelled which way or which is truly “The” tulip tree.
What I hope what you will take from our group is the education and the more than 50,000 crape myrtles that have been planted in the McKinney area in the last several years. We have affected a community in a very positive way through beautification, research in insects, and freeze damage control, through tourism, and the more than 10,000 Mother’s Day crape myrtles given to the mothers of 5th graders in our wonderful city.
For another take on the spelling you can visit: http://www.garden.com/garden-articles/is-it-crape-myrtle-or-crepe-myrtle/140/