Growing Zones Chart - What Does It Mean
This is the standard chart for determining plant hardiness.
Find where you live on the chart, determine the maximum cold temperature in your region to choose plants that will survive at temperatures above that level.
Any plants below your maximum low temperatures may be at risk of dieing if left outside in cold weather.
You can find each plant's tolerable temperatures by looking at the plant tags that are attached to each plant. Sometimes they will tell you at what temperature they will survive. Other times the tag will only mention the hardiness zone appropriate for the plant.
For instance, if a plant will tolerate anything up to zone 6, it means that zones above 6 are OK for the plant. Zones 5 and below are not.