What are "ZONES"? - Knowledgebase Question

Pittsburgh, PA
Question by sgranberg
May 27, 1998
Just to let you know, I'm VERY new at this. I don't even have the house built yet, let alone a garden, but I'm learning. When I read comments in this and other garden sites, the respondants mention "zones". What are zones, and how do they pertain to Armstrong County, PA?


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Answer from NGA
May 27, 1998

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Zones are a useful thing to know about, you're right! They can help you select plants able to withstand the winter temperatures in your area. It is also almost a code between gardeners because it helps other gardeners relate to your local climate conditions.

The United States Department of Agriculture (USDA) developed a map showing average winter low temperatures across the country based on ten degree intervals or Zones. For example, USDA Zone 6 has winter lows between -10 and 0 degrees F, and USDA Zone 5 has winter lows between -20 and -10 degrees F. A conservative gardener in your area would always opt for plants rated hardy to zone 5 or colder.

The map gives you a good general reference guide to go by, but in your own garden, the winter weather and temperatures will also be affected by your local microclimate. This means for example, a sheltered spot in town will be more moderate than a windy, exposed spot in the country even if both are rated the same zone on the map. As a new gardener it wll also help you to ask experienced gardeners right in your neighborhood about the wind and precipitation patterns for your microclimate, too.

Most plants are rated with a minimum zone of cold hardiness. These are the numbers you see in catalogs or books: "hardy to zone 3", or sometimes simply, "zone 3". A plant rated hardy to zone 3 can withstand temperatures as low as 40 below zero!

In contrast, a plant rated to zone 8 will only tolerate lows of ten degrees above zero. Sometimes you will find a listing such as "zones 3 to 8" This means the plant is cold hardy all the way to zone 3 but does not do well in zones warmer than zone 8.

As a beginning gardener, you will find that the plants rated "zones 3 to 8" tend to be very reliable and dependable under a broad range of conditions. This may be a good overall group to start with until you gain a little more experience in your new location. In the meantime, keep reading, visit all the gardens you can, and keep asking questions!

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