If you’ve spent any time researching plants or garden ideas, you’ve probably heard the term ‘hardiness zones.’ The United States Department of Agriculture Hardiness zones are simply a guide to help you choose which plants will grow in your area. For example, some plants are not cold tolerant and will not grow in areas with severe winters; other plants will not tolerate the heat commonly found in some areas.
Horticulturist and botanists got together and divided the United States in to zones ranging from 1 to 11, with zone one being extremely cold and zone 11 tropical. This system has saved gardeners from much trial and error but it’s not fool proof. Hardiness zones have more to do with freezing temperatures than heat and do not address all the other factors involved in growing a plant such as humidity, rainfall and day length.
Cities in the same growing zone can have vastly different climates. For example, Seattle, Washington and Tucson, Arizona are both USDA zone 8. Within the same city you will also find micro-climates. An area surround by concrete will be warmer than a garden bordering a lake or river.
In general, if a plant says it’s hardy to zone 6, it can be grown in the warmer zones as zone 6 indicates its cold tolerance. The breakdown is as follows:
Zone 1: below -46 C (below -50 F)
Zone 2: -46 to -40 C (-50 to -40 F)
Zone 3: -40 to -34 C (-40 to -30 F)
Zone 4: -34 to -29 C (-30 to -20 F)
Zone 5: -29 to -23 C (-20 to -10 F)
Zone 6: -23 to -18 C (-10 to 0 F)
Zone 7: -18 to -12 C (0 to 10 F)
Zone 8: -12 to -7 C (10 to 20 F)
Zone 9: -7 to -1 C (20 to 30 F)
Zone 10: -1 to 4 C (30 to 40 F)
Zone 11: above 4 C (above 40 F)
Another problem with hardiness zones are the disparities between the maps. The National Gardening Association lists Fresno as being zone 8 but the Arbor Day foundation lists Fresno as zone 9.
The key to using the USDA hardiness zone map is to remember that while it’s a valuable tool, it’s not the only one in the gardener’s arsenal; experience and education play key roles.