Landscape Planting Zones

Range

• The range of the USDA hardiness zone map extends from Mexico through the United States and into Canada. In the warmest zone, zone 11, average annual minimum temperatures exceed 40 degrees F. In the coldest zone, zone 1, they fall to below minus 50 degrees F. Regions with the same hardiness zone number are not necessarily adjacent. For instance, zone 10 includes parts of Florida and areas of California, but not all the intervening states.

Hardy

• References or labels may describe a plant's hardiness by saying that it is "hardy up to zone 7." This means the plant should survive the temperature range in zone 7 as well as the warmer zones on the hardiness scale. If a plant as "hardy in zones 3 through 7," it means the plant is likely to survive only in those zones.

Microclimates

• Gardeners should look at the hardiness zone map as a general guide to their plant selection, not as an absolute authority. Microclimates, areas warmer than their surroundings, exist in every zone and could allow a plant to thrive even when the regional climate is not favorable, states Jerry Goodspeed of the Utah State University Extension in an article on the extension website. For instance, a plant facing a south wall may benefit from temperatures 10 degrees warmer than a plant next to a north wall. This temperature difference could determine the plant's survival.

Other Maps

• The USDA's map is not the only hardiness zone or growing zone map. Other countries and regions have their own maps. Even though southern Canada is included in the USDA map, Canada has its own map that divides the country into nine zones. China, South America and Europe also have their own regional maps.

  Another U.S. planting zone resource is the series of climate guides created by "Sunset" magazine. These maps consider such elements as latitude, elevation and rainfall along with annual minimum temperatures.