What do the February 23, 2006
Question What do the "zone" numbers on a plant tag tell me?
Answer The "zone" concept was designed so people could compare the numbers on a plant tag with the numbers on a map. If the numbers match, then that plant should grow in that area of the map.
At this time, the only zone numbers on a plant tag are the USDA hardiness zones. These numbers represent designated windows of average low temperatures for a particular area. For example, zone 7 has an average low temperature between 0 and 10 ^o F. Sometimes the zones are broken into smaller windows by adding an "a" or "b". For example, zone 7b has an average low temperature of 5 to 10°F. When a hardiness zone number is assigned to a plant, it gives people an idea of the lower temperature range of that particular plant. It is important to know the lower temperature limit of plants because cold temperatures can kill plants fast.
There is another temperature extreme that can kill plants. Most northerners quickly recognize this second extreme when they move to the South. Heat does not usually kill plants as quickly as cold, but they end up just as dead.
In 1997, the American Horticultural Society (AHS) devised a system to identify heat zones within the US. The AHS Heat Zone Map contains 12 different zones in the US. Instead of average temperature, AHS used the average number of days that the temperature is above 86°F. For example, heat zone 8 has an average of 90 to 120 days per year of temperatures above 86°F.
Why 86°F? This is the temperature where cellular proteins in plants start experiencing damage. It is also the temperature where air conditioners run, sweat pours, hairspray fails, and dogs dig holes.
Hardiness zone numbers tell you how far plants can go north and heat zone numbers tell you how far plants can go south. Before long, plants will have both hardiness and heat zone numbers on their tag. The averages that comprise these zones do not show the extreme temperatures, differences between night and day temperatures, or duration of extremes. However, they do give us some frame of reference. These numbers can help us match plants to our weather conditions.
We have other extremes in North Carolina as well. The coast has salt, the mountains have acid rain, cities have smog, rural areas have deer and everybody has good ol' humidity. I can't wait until we get zone numbers for all these problems.
If you want more information about temperatures, then call a meteorologist. If you want more info about heat killing plants, then call me at 910-893-7533, write me at PO Box 1089, Lillington, NC 27546
or email me at gary_pierce@ncsu.edu Hardiness and heat zone maps can be found on the web at http://www.gardeningplaces.com/heatzonemap/ and http://www.usna.usda.gov/Hardzone/ushzmap.html
There are other zones for specific places in the US. For example, the gulf states are in a disaster zone, California is in an earthquake zone and Washington DC is in the Twilight Zone. | 
