Learning about Gardening Zones

What gardening zone do I live in? Learn to read the gardening zone map to find out which are the best plants to grow in your climate, and which should not be grown at all.

Learning about Gardening Zones – Photo by US National Aboretum

Do you know which gardening zone that you live in? What is the difference between each colored region? Are the color-coded areas considered gardening truths, or are there more factors involved?

The gardening zone map was designed by the US Department of Agriculture to illustrate the different regions that plants could potentially grow well in. Each area is called a hardiness zone. The official USDA map is divided into eleven separate hardiness zones, which span the United States, Mexico, and Canada. A zone is simply an area with similar weather conditions — each zone has an average temperature of ten degrees more or less than the adjacent one. Each numbered zone has been further divided into an a and b section.

How does this zoning system help gardeners? It is a tool to predict if a certain perennial plant, shrub, or tree will survive throughout the year — if it can withstand the typical rainfall distribution and temperature shifts of a particular area. For example, Blue Grape Hyacinth bulbs may not survive the cold winters in zones one through three, or the hot summers of zones ten and eleven. They should therefore only be planted in zones four through eight.

The USDA hardiness map is not completely accurate nor comprehensive. It is a useful reference chart that can help farmers and gardeners make productive decisions. It is however only a general set of guidelines. Average temperatures can fluctuate from one year to the next, which is why zoning boundaries have shifted over the years.

Factors, such as the effect of snow cover and the lack of soil drainage in very cold weather, are not considered. In the West in particular the zones are not a perfect representation of climate. When weather moves over the mountain ranges in the western states it becomes increasingly drier. Elevation and precipitation, which have just as much, if not more to do with the growing patterns in the West, are not factored into the equation.

For the gardening zone map to be useful, view it as a well-researched suggestion. Plants should grow in the regions they are designated for, but elements such elevation, wind, and soil drainage may need to be considered as well. If unsure about your regions abnormal weather conditions, contact a local arborist for help.

How to I find out what gardening zone do I live in? To determine your zone, find your position on the plant hardiness map. The US National Arboretum offers a very detailed chart, which even lists example cities in each area. The National Gardening Association is another great resource. They provide specific information on each zone, including climate characteristics, relevant plants, and growing season trends. Search by entering your zip code.

Especially when planting shrubs, trees, and other plants that will have to survive through the winter months, use this official chart as a reference point to help predict which plants can thrive in your area.

• US National Arboretum: http://www.usna.usda.gov/Hardzone/
• Backyard Gardener: http://www.backyardgardener.com/bulb/climatezone.html
• Photo by US National Aboretum: http://www.usna.usda.gov/Hardzone/ushzmap.html