Answer: The biggest challenge for potted plants on balconies during the winter months is that the roots are exposed to frigid temperatures and frozen roots can kill perennials. When you plant in the ground or sink the pots into the ground, the soil provides some insulation for the roots. So, if you'd rather not transport your plants to your parent's place, you can try to provide enough insulation to the pots to simulate planting in the ground. I've invested in a few sheets of 2" styrofoam (available at Home Depot near the rolled insulation in the building department). I use a serrated knife to cut the sheets into whatever size I need. I don't actually make boxes for each of my plants but I do lay half a sheet of styrofoam on the floor of my deck and then set the pots on top of it (grouping the pots is perfectly acceptable in the winter months because most of the tops of the plants will die down with the first frost). Then I cut the sheet to fit four sides of my makeshift box. The rest I cut into blocks or strips to set between the pots, with enough room to stuff additional insulating material between the pots and the styrofoam. When I'm finished, the bottom and sides of my group of pots will have a sheet of styrofoam insulation to protect them from the elements and each pot will have 4 sides of insulating styrofoam plus shredded leaves stuffed between the pot and the styrofoam.
Hope this works for you!
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