Understanding Gardening Zones

The next time you pick up a garden catalog, pay attention to the hardiness zone number assigned to each plant. Those numbers correspond to specific regions in which the plant will thrive. For instance, the tulips I want to plant are recommended for zones 3 through 8. That means that if I live in any zone outside of that range, I'm taking a chance that the tulips will not grow optimally, will sprout but not produce blooms, or will not sprout at all.

First developed and published by the United States Department of Agriculture in 1960, the hardiness zone map indicates by geographical region minimum and maximum temperatures for that zone. The USDA averages the lowest temperature for a zone over several winters, and concludes an average annual minimum temperature. As an example, if five winters in succession in a region reach a minimum temperature of -8, -5, -12, -3, and -7 (all in Celsius), then the mean coldest temperature is 7, placing the region in zone 7b. In the United States, that zone encompasses parts of Arkansas and Georgia.

In 2006, the Arbor Day Foundation revised the USDA hardiness zone map to reflect the changes in warming in North America, based on fifteen years' data provided by the National Oceanic and Atmospheric Administration. While some zones have not changed, others have warmed two full zones. One of the main differences between the two zone maps is that the Arbor Day map does not include any half-zone delineations and, instead, uses ten zones, as compared to the eleven zones of the USDA map.

An organization named Sunset developed climate zone maps of their own that consider total humidity, rainfall, and summer highs and lows—instead of just the winter lows the USDA map uses. Sunset subscribes to the theory that a plant's performance is controlled by many factors, and their climate zones take into account all of those factors. The USDA zone chart basically lets you know where a plant will survive a winter, while the Sunset chart tells you where the plant will grow year 'round.

Finally, the American Horticultural Society published a Heat Zone Map in 1998. Using data from the National Climatic Data Center, the Society created a map divided into twelve zones. Each zone specifies an average annual number of heat days. Heat days are days with an average temperature over 86 degrees Celsius, and that is the temperature at which plants begin to experience damage from heat.

So, what does all of this information mean for the average gardener? Simply put, if you want to grow a wide variety of plants, cross reference all of the information from all zone maps, and then choose plants that will thrive within your environment. Keep in mind that other factors within your own environment will also determine your success as a gardener, especially if you use a greenhouse or enclosed patio to grow plants.