There are numerous online resources that feature the Plant Hardiness Zone Map created by the U.S. Department of Agriculture, including the USDA and the National Gardening Association websites. This zone map divides the country into different regions based on the average minimum winter temperature.
The 2012 version of the map features 13 zones, each of which is either 10 degrees warmer or colder than the adjacent zone. To make things more precise, each of these zones is then split into A and B sub-zones. In most cases, if a plant description mentions a zone, the USDA map is the one they are using.
The USDA Plant Hardiness Map is a useful tool for planting flowers or any other plants as it allows gardeners to know what types of plants grow best in their particular zones. For plants to thrive, they need to be able to tolerate the conditions in the area throughout the year. This means that the plant needs to be able to withstand both the highest and lowest temperatures, in addition to being suited to the amount of precipitation the area receives.
Although this zone map works quite well for some parts of the country, there are some criticisms that the map doesn't accurately reflect climate differences in the west.