If you are new to gardening, you may be a little confused about the diferences between planting zones. The plant hardiness zones, determined officially by the United States Department of Agriculture, are designed to inform consumers which perennial plants will survive the winter in their area. For a better chance at a successful garden, it is important that you know which planting zone you are gardening in and how to purchase the correct varieties of plants.
How the plant hardiness zone charts work
The USDA gathers data from numerous places around the United States to determine an average maximum low temperature for an area. A zone chart divides the country into areas by the coldest winter temperatures they may experience, assigning a numeric value to each one. These zones are your winter hardiness zones for gardening. The colder the winter, the lower the number assigned to the zone. In the US mainland area, we currently have Zones 3 to 11.
Where to find your gardening zone
Plant hardiness zone maps are available from a variety of sources, such as gardening reference books available at your local bookstore or library. A great source for a free, up-to-date map is your favorite gardening catalog. You can also find an online version of the at the US National Arboretum site at www.usna.usda.gov/Hardzone/ushzmap.html
Some places devise their own gardening zone maps, such as the Arbor Day Foundation and Sunset Gardening Publications. Since most plant sellers use the USDA chart to rate the hardiness of their plants, it's probably best to look up your gardening site on the official USDA Zone Map.
How to use the information
The plant hardiness chart is used to inform consumers which perennial plants will survive the winter in their area. Most plant labels and gardening catalogs will put the zone hardiness rating somewhere in the plant description. When you choose plants for your garden, choose plants that will grow in your zone or in a zone with a lower number.
If you are a gardener in Zone Five, you will want to choose plants that are rated hardy to Zone Five, Four or Three. Zone Three is the coldest area of the mainland United States. A plant rated hardy to Zone Seven would probably not survive the winter in your garden.
While most plants that have been in the market for a while will be assigned fairly accurately to their gardening zone, new plants may be less accurately placed. When new plants are put on the market, they will probably have been tried in a number of places, but until thousands of gardeners are growing the plant, the information about their zone hardiness may be only a guess. When purchasing new plants, even new varieties of familiar garden plants, be aware that the zone hardiness indicated may not be accurate.
Be sure you are using the most current version
Because the climate is changing, the USDA will reissue its gardening zone chart from time to time. If the USDA chart follows the lead set by some large gardening firms, many gardeners will find that their gardening zone will change to a higher-numbered zone. In other words, winter has gotten warmer.
The new chart that the USDA will be putting out in 2008 has taken the data from a much greater number of places than previous zone maps into consideration. Even without climate change, the increased range of data may have changed the zone map.
How to cheat in your zone
What if you live in gardening Zone Five and you really, really, want a plant that is listed hardy to Zone Six? If you are determined and don't mind the possibility of losing the plant, you may be able to cheat nature. In every yard there are areas called microzones where the climate is just a little different from the surrounding area. It may be up against the south side of the house, in a courtyard or in a sheltered pocket surrounded by heat-holding rocks or cement. Added warmth and shelter would be the safest bet for a plant that may have a difficult time in your general climate. Be sure to deep mulch it or use a protective cover as an additional safeguard. Sometimes plants that manage to survive a few milder winters in your zone will acclimate enough to survive a harsher one.
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