In this posting, I cover three systems for determining which plants to grow in your area.
In order to help gardeners determine the right plants for their area, a number of organizations have developed systems that specify the types of plants that can be grown in a given geography. Here are the most important three systems used in the United States.
USDA Hardiness Zones
The oldest, and most widely adopted system, was created by the United States Department of Agriculture in conjunction with the U.S. National Arboretum in 1960. The USDA Hardiness zone map divides the U.S. into 11 zones based on the average minimum temperature. This is a reasonable methodology because each plant specie has a temperature threshold below which it can't survive.
However, the USDA system is not without problems. The most notable of these is that it groups geographies together that may have the same average minimal temperature, but whose high temperature in Summer can be dramatically different. For example, Coastal Oregon and South Texas are both zone 9 according to the USDA Zone Hardiness map because the minimum average temperature for both regions is in the range of 20-30 degree Fahrenheit. But these two areas differ dramatically in terms of Summer high temperatures (average high temperatures in Corvallis are around 80 degrees Fahrenheit while they are 95 degrees in San Antonio)
In general, the USDA Map is most accurate for the Eastern half of the US and least accurate for the West. If you want to know your hardiness zone, here's a link to a web site that will tell you the zone based on your postal zip code.
American Horticultural Society (AHS) Heat Zone Map
This system groups like geographies based on the average high temperatures. It is most accurate for Southern states such as Florida and Texas but suffers from the same problems as the USDA but in reverse.
The AHS is trying to establish a system that combines their Heat Map and the USDA methodology to provide a system that specifies which plants will grow in a given geography based on both their cold and heat tolerance. I like this idea, but I have rarely seen anyone using the AHS system so I'm not sure it will ever be adopted. If you would like to determine your AHS zone, check out the organizations zip code AHS zone locator.
Sunset Magazine System
Sunset is a lifestyle magazine dedicated to gardening, travel, food and decorating in the Western United States. Not surprisingly, Sunset and its readership are keenly aware the shortcomings of the USDA Hardiness Zone system for western gardeners. In response, the magazine developed it's own system for dividing up the country....
"A plant's performance is governed by the total climate: length of growing season, timing and amount of rainfall, winter lows, summer highs, wind, and humidity. Sunset's climate zone maps take all these factors into account"
While not as widely used by the plant industry as the USDA system, Sunset provides a very good on-line tool that will help you both determine your zone and find plants that do well in that zone.
The Best System But Still Far From Perfect
In my opinion, Sunset Magazines method and on-line tool make it the strongest of the available zone classification systems. In the future, I will try to list both the USDA zone and Sunset zone when I publish information about a given plant on this web site.
That said, I think I should point out that there's still plenty of room for improvement. Specifically, Sunset's on-line tool provides data on whole species of plants. The systems will tell you if roses or tomatoes will grow in your area but, as any gardener knows, there's a wide range of tolerances within each of these species. For example, certain rose varieties might survive in a moderate climate like Northern California, but only thrive in a places where there are really cold Winters. Temperate climate gardeners would therefore be better off selecting a variety of rose that does well in warmer climates. This level of detail is not really contained in Sunset's on-line tool.
I'm optimistic that future gardeners will have access to very detailed information about plant varieties that grow well in their area. The internet opens the way for gardeners to easily contribute information to that can be broadly shared by everyone. All that's required is an on-line tool to enter this information and a database to store and retrieve it. I'm positive someone will create such a tool; it's just a matter of time.
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