USDA Hardiness Zones - Knowledgebase Question

Palm Springs, CA
Question by teetimes
July 12, 1999
1a.) I need a better understanding of the USDA Hardizone Zone rating which its focus is on low's, while a consideration as large if not larger are highs! I want to know if a plant will survive our 100's+ summer temps.
1b.). I have your 1999 Seeds &Plants plus "Heirloom's" catalogs and w/ rare exception I don't find a hardiness zone rating w/ each description, where should I look?

Answer from NGA
July 12, 1999


Zones are a useful thing to know about. 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.

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!

Because of your hot summer climate you may want to consult Sunset Western Garden Book (ISBN# 0-376-03851-9) and check their listing of 'plants for arid gardens'. This will give you an idea of the kinds of plants you can grow that will thrive in your locale.

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