Answer: There are several cold hardy crepe myrtles developed by the National Arboreteum in Washington DC that seem to do well in their zone 6. The list includes:
'Acoma', 'Apalachee', 'Caddo','Cherokee', 'Chickasaw', 'Choctaw', 'Comanche', 'Hopi', 'Kiowa', 'Lipan', 'Muskogee', 'Natchez', 'Potomac', 'Powhatan', 'Seminole', 'Sioux', 'Tonto', and Zuni'. One of these might be worth a try in your high altitude garden.
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