What are Hardiness Zones

What are Hardiness Zones? Why are they needed?

Plant hardiness zones are shown as colored sections over a map, each with a specific average minimum temperature. As plants, trees and shrubs thrive in different temperatures, the map will help show you which plants will grow in your planting area. Using the zone system will help avoid planting shrubs that will die, because they can't handle your zones minimum temperatures.

The 1990 USDA hardiness zone map version shows in detail the lowest temperatures that can be expected each year in the United States, Canada, and Mexico. These temperatures are referred to as "average annual minimum temperatures" and are based on the lowest temperatures recorded for each of the years 1974 to 1986 in the United States and Canada and 1971 to 1984 in Mexico.

The map shows 10 different zones, each of which represents an area of winter hardiness for the plants of agriculture and our natural landscape. It also introduces zone 11 to represent areas that have average annual minimum temperatures above 40 F (4.4 C) and that are therefore essentially frost free. (USDA Miscellaneous Publication No. 1475, Issued January 1990)

Zones 2-10 in the map have been subdivided into light and dark colored sections (a and b) that represent 5 F (2.8 C) differences within the 10 F (5.6 C) zone. The light color of each zone represents the colder section, the dark color, the warmer section.

Plants all differ in the temperature extremes they can withstand, some can endure colder winters than others. Some plants have a wide range of zones in which they will grow, say zone 3 to 9, and some will only tolerate a difference across 2 to 3 zones.

USDA Hardiness Zones and Average Annual Minimum Temperature Range

Zone	Fahrenheit	Celsius
1	Below -50 F	Below -45.6 C
2a	-50 to -45 F	-42.8 to -45.5 C
2b	-45 to -40 F	-40.0 to -42.7 C
3a	-40 to -35 F	-37.3 to -39.9 C
3b	-35 to -30 F	-34.5 to -37.2 C
4a	-30 to -25 F	-31.7 to -34.4 C
4b	-25 to -20 F	-28.9 to -31.6 C
5a	-20 to -15 F	-26.2 to -28.8 C
5b	-15 to -10 F	-23.4 to -26.1 C
6a	-10 to -5 F	-20.6 to -23.3 C
6b	-5 to 0 F	-17.8 to -20.5 C
7a	0 to 5 F	-15.0 to -17.7 C
7b	5 to 10 F	-12.3 to -14.9 C
8a	10 to 15 F	-9.5 to -12.2 C
8b	15 to 20 F	-6.7 to -9.4 C
9a	20 to 25 F	-3.9 to -6.6 C
9b	25 to 30 F	-1.2 to -3.8 C
10a	30 to 35 F	1.6 to -1.1 C
10b	35 to 40 F	4.4 to 1.7 C
11	above 40 F	above 4.5 C