North America is divided into 10 cold-hardiness zones by the U.S. Department of Agriculture. (It’s also divided into 12 heat zones, but those are not usually a concern in northern climates.) The hardiness zones are based on the lowest low temperature an area is likely to experience. So, if you live in Rochester, Minn., your lowest low, according to the USDA hardiness map, is going to be between -20 and -25 degrees F. If you live in Cook, Minn., the lowest low will likely be between -35 and -40 degrees F.
Knowing your hardiness zone matters because plants are rated for hardiness (whether they will survive winter) based on these zones. The accompanying map shows the north central region of the United States, which ranges from zone 7 in far southern Missouri to zone 3 in northern Minnesota and North Dakota.