Can mold grow in a Pennsylvania winter?

Yes. Cold weather does not sterilize a closed house. Winter growth here is usually condensation: humid indoor air on cold plaster, windows, and attic decks. Closets on north walls and wet window stools are the giveaway.

Lower the indoor humidity into a range an old house can stand, often the 30s in January, run bath fans outdoors, and do not park furniture tight against exterior plaster. Summer humidity is the other season. Both grow mold for opposite reasons.