barely hardy plants

This may be the dumb-question-of-the-day, but here goes...
I covered my  Verbena "Homestead Purple" with cut xmas tree branches to
help it through the winter.  We just got snow the other day and it makes
me wonder, when do I uncover the plants?  Is there a guideline in terms
of lowest temperatures for tender perennials?
TIA
Barb (Southeastern PA, zone 6/7, where I bumming because I was too busy
to make it to the Phila. Flower Show.  maybe next year...)

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