
The zones in the U.S. are classified on the basis of their temperatures in winter. The maps of these zones presume that all the other factors that are needed for the growth and development of plants like number of days without frosts, soil quality, water supply, sunlight, etc are perfectly ideal. The coldest of winter temperatures in zone 5 may reach anywhere between -20 to -10 F. There are very few plants that can survive in these temperatures.
