I think that with any crape myrtle the is the chance of die back in zone 6. Sarah is supposed to be one of the more cold tolerant - to -12f . I have never seen a 25 ft crape myrthe in central pa.

Here is a link that might be useful: My blog

I've always loved Crape Myrtles and this year I got one. Well, I must have overwatered or something because it died. It was supposed to be a bush. I finally got the heart to dig up what was dead but at the bottom I saw some small green plants which looked healthy so I let them grow and when it was time for Crape Myrtles to bloom I got flowers on them. It is now coming into Winter and I need to know should I cover it or just mulch it well. My Winters around here sometimes get in the teens but I'm not sure if it can take these temps. I don't know what kind it is. Any help would be most appreciated. It is still just about 12" high.