I have several of my larger 6" ROS seedlings of various colors (single and double blooms) in large 2 and 3 gallon thick black plastic nursery pots on my front porch. I had planned on keeping them in the pots all winter and allowing them to continue to grow in the pots until they are large enough to plant or give away for others to plant. The smaller seedlings in flats I will overwinter indoors under lights. I am in zone 6b, southeastern KY. Will my ROS seedlings be okay in their pots over winter or do I need to cover them with a layer of mulch? I know they are hardy. We experienced a horrible winter this past winter and at one time our temps reached -30 degrees F. My mature ROS experienced no damage at all while all the Crepe Myrtles and most hydrangeas and even roses all died to the ground, so I was thinking the seedlings would be strong and large enough by winter to survive in their pots. Any thoughts?