June 29, 2008
Hardiness Zones & Heat Zones
I was asked about hardiness zones, so here's some info on that topic. The USDA created a hardiness zone map to indicate the usual low temperatures for different areas. This is very helpful in determining whether a plant will survive winter in your area. Most plant tags will indicate the plant's hardiness zone. Lower zone numbers are assigned to colder areas. You can grow plants that are hardy in your zone and all lower (colder) zones. For example, I am in a zone 5, so I can grow anything listed as hardy to a zone 5,4,3,etc. Sometimes you can get away with planting something one zone higher than yours, if you plant it in a protected area (like on the south side of your home). You can find the USDA winter hardiness zone map at this link: http://www.growit.com/zones/.
Sunset magazine has also created a hardiness zone map. It uses different numbers and is much more specific than the USDA map. It considers factors like rainfall, length of growing season, humidity and summer heat, but it isn't as commonly used. You can find this map at:
The USDA has also created a heat zone map. This is important in areas that get quite hot in the summer, as some plants that laugh at severe Montana winters will cringe and die during an Arizona summer. You can find the heat zone map at this link: http://www.ahs.org/publications/heat_zone_map.htm.
These zones are very helpful, and I would strongly recommend that you figure out which zone is yours and always check plant tags before buying. My friends in Des Moines, IA and most of the Wasatch Front in Utah are in USDA zone 5. My lucky friends in Santa Clara, CA are in USDA zone 9. Oh, how I miss California . . .