Plant Hardiness Zones Explained
(by arborday.org)
The Plant Hardiness Zones
divide the United States and
Canada
into 11 areas based on a 10 degree Fahrenheit difference in the average annual
minimum temperature. (The United
States falls within Zones 2 through 10). For
example, the lowest average temperature in Zone 2 is -50 to -40 degrees
Fahrenheit, while the minimum average temperature in zone 10 is +30 to +40
degrees Fahrenheit.
Suggested hardiness zones
have been indicated for all trees and perennials available online from the
Foundation. If a range of zones, for example, zones 4-9, is indicated, the tree
or perennial is known to be hardy in zones 4, 5, 6, 7, 8, and 9. Suitable
hardiness means a plant can be expected to grow in the zone's temperature extremes,
as determined by the lowest average annual temperature.
Keep in mind that local
variations such as moisture, soil, winds, and other conditions might affect the
viability of individual plants.