Use the Hardiness Map above by locating where you live and matching the color with the chart to the left of the average annual extreme minimum temperature in that area. The color will tell you what Zone you live in. When researching plants to grow in your container garden, use this information to know if the plant will survive throughout the winter outdoors or if you will need to bring the plant indoors while it is still cold outside.
Container plants are less insulated than plants that have their roots in the ground soil, so container plants may freeze quicker and not do as well during the winter as other plants. Container gardeners who live in Zone 6 may want to play it safe and only keep container plants outdoors that are hardy to Zone 5, Zone 4, Zone 3 and so on. Any other plants can be overwintered indoors under grow lights.
Hardiness Zone maps can be a little difficult to read. If you live in a very populous city, read "What's My Hardiness Zone?" to see the Zones of the 20 most populated cities in the United States.