Answer: Crepe myrtle should not be cut back in the fall. The branches will help somewhat to protect the lower part of the plant from winter cold damage. Your zip code places you in zone 6A or the coldest part of zone 6. Depending on your microclimate, it may actually be as cold as zone 5. This is really the edge of hardiness for crepe myrtles even if you are growing one of the most cold tolerant cultivars. Be sure to mulch it heavily in late fall. In the spring, be very patient waiting for it to come into active growth again. It is one of the last plants to resume growth. Then cut it back as needed to remove winter killed branches and to restore a somewhat symmetrical look. (You may even have to cut it to the ground after a severe winter, but it should still bloom nonetheless.) It blooms on new growth of the season, so do not prune it during the summer.
Do not prune your clematis in the fall either. Depending on the specific cultivar you are growing, you may need to cut it back quite hard in the spring or wait until after it blooms to prune it back. Here are some tips on clematis you may find helpful. You may need to cut and paste the complete url into your browser to make it work correctly.
I hope this helps!
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