Answer: Both the broom and the Cotoneaster are winter hardy in your growing region, but planting them in containers will expose the roots to excessive cold and they will need to be insulated. You can simply sink the containers into the ground or you can wrap insulation around the pots (don't forget to set them on a sheet of styrofoam or other insulation to keep the bottoms from freezing). I sometimes use the half-inch thick styrofoam sheets (usually found in home improvement stores near the water heaters) to set my containers on. I use the pink spun house insulation for the sides of the pots, paper side out, and then wrap the insulation with plastic. Don't wrap the bottom of the pot or you'll plug up the drainage holes; just set the pots on the styrofoam sheets. Don't cover the plants at all. They will survive and leaf out in the spring.
Best wishes with your garden!
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