- The USDA Hardiness Zone Map, which was last revised in 1990, shows the lowest expected temperatures in 10 zones across the US, Canada and Mexico. The 1990 map introduced a new zone, zone 11, which is virtually frost-free with average minimum temperatures above 40 degrees F.
- The U.S. National Arboretum maintains online guidelines on how to use the hardiness zone map. The map is divided into sections representing 5-degree F differences within the zones. The lighter the color within a zone, the colder the climate. Darker colors represent a warmer section of a zone.
- USDA suggests that gardeners keep in mind factors such as pollution stressors and new planting techniques and systems when choosing a place to place a landscape plant. Plants, which should thrive in a certain zone may be less hardy if they are subjected to these stressors and systems, cautions the USDA.