Generally speaking, potted plants that are hardy to two zones colder than the zone of your climate should be able to withstand an average winter. For instance, someone who lives in a Chicago zone 5 should ideally plant zone 7 or higher if they wish them to withstand a normal winter.
Many of the same things you would do to protect your in ground plants you can also do for your container plants. Including watering, mulching and in some instances covering with a frost cloth or burlap. However, a little extra effort may be in order:
1. Temperature fluctuations in the soil vary greatly due to the relatively small area contained in your pot. If space allows, move your pots off of hard surfaces such as concrete and stone. If possible, place your containers on the ground where the temperature will be more stable.
2. Watering cannot be overemphasized. Making sure the plant has plenty of water before the first several hard frosts will protect the roots from cold and drying out. When winter rain fills your pots, make sure drainage is sufficient. If a frozen ground doesn’t allow for this, tilt your containers so drainage can persist and root rot doesn’t set in.
3. Double up! If you are expecting a particularly cold winter, you may consider placing your plants container and all, into another larger pot. Use the space between the pots to fill with insulation. Nothing fancy is needed, using old blankets, bubble wrap, or whatever you have laying around the garage will do. If you don’t have a larger container – wrap the outside of your pots with blankets and secure with duct tape!
Of course, there are never any guarantees, but these are some easy practices to hedge your bets for success!