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Have you ever wondered about gardening zones? Where did it come from and why? The USDA Hardiness Zone Map of North America is divided into 11 zones and that allows us to know what will survive should we plant it in the area we live in. Each zone is 10 degrees different from the zone next to it.
The zone that starts at 1 is the coldest and as you go down to 11 each zone will increase in temperature by 10 degrees until you come to the hottest area which is the most southern part of the US and that would be zone 11.
A plant that is supposed to thrive in zone 11 will not in any zone above that as a plant for zone 1 would not survive in zone 11. Basically these zones will give you the average dates for the last spring frost for each zone.
This helps us to understand what to buy and how to create our gardens. A gardener will find that most planting directions are based on the average last frost date and the last frost date for an area is the last day in the spring that you might have a killing frost.
I keep a color coded zone map handy so I know exactly what plant selections to make for my zone which is zone 6. Most gardening magazines will have one. Most seed packages will have at least the zone number for that specific seed and often times you will find a color coded zone map on the package also.
Zone 1 would be below -50 F or below -45.6 C and that might be Fairbanks, Alaska or the Northwest territories of Canada.
Zones 2 a & 2 b to 3 a would be the areas between those mentioned in the previous sentence to Pinecreek, Minnesota and they go down the numbers with each number having an a and a b progressively getting 10 degrees warmer in each zone until they get to number 11 which will be Hawaii and Mazatlan, Mexico.
The hardiness map figures in the average winter temperature for each zone. As the weather has changed over my 73 years I have seen the maps change with global warming especially over the past 15 years.
The newest USDA Hardiness map information is based on the weather for the past 60 years.
As I have said these gardening zone maps have great value and they will tell you whether certain plants are suitable for your zone and when to plant them.
If you live outside North America then you can roughly translate the USDA hardiness zones by finding out how low your area's temperatures can reach, and then use the following chart to find your corresponding zone.
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)
Forty years ago the Sunset magazine created it’s own Sunset Hardiness zone map for 13 Western states which many gardeners consider more superior than the USDA zone maps because they not only factor in the winter temperatures, but the summer high temperatures, the rain patterns, humidity and the length of growing seasons all of which is great information for the gardeners in those states.
The American Horticultural Society has created a Plant Heat-Zone Map that consists of 12 zones that allows the gardener to know even more information such as the average number of days each year that a given region experiences "heat days"-temperatures over 86 degrees (30 degrees Celsius). This is the point at which plants begin suffering physiological damage from heat. The zones range from Zone 1 (less than one heat day) to Zone 12 (more than 210 heat days).
It never hurts to have copies of all these zone maps and it was many years as a gardener before I knew that more than one zone map existed.
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Author Bio Box: Arlene Wright Correll
 For more gardening or cooking information click http://www.learn-america.com/ and click on Arlene’s Books you can download or buy my gardening & cook books. All my royalties from the sale of my books go to the St. Jude Children’s Research Hospital and I thank you for your attention to this site.
To see Arlene's Gardening Diaries click here http://www.learn-america.com/stories/storyReader$559
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