Plant Hardiness and Heat Zones
For those who need to grow plants (particularly perennials) for whatever reason, there are several tools available to help to determine the best plants to grow in your particular location in the USA. The following information is provided to help you to better understand and utilize those tools in selecting the appropriate plants that will withstand and survive your winters as well as your summers.
USDA Plant Hardiness Zone Map
The Plant Hardiness Zone Map tells you how cold your winters are, on average. It divides the US into 11 zones indicated by different colors. Each zone is 10°F warmer (or colder) in an average winter than the adjacent zone. The zone map is used to determine what plants are appropriate for specific locations. Most plant catalogs list the hardiness zone(s) that are best for each plant. You can use that information to help determine the best plants to grow in your particular location that can survive the winters, especially permanent landscape plants. This can also be used in conjunction with the The Heat Zone Map (see section below).
Note that USDA maps now split, and color code, zones 2 through10 (zone 11 represents frost-free areas) into 5°F sub-zones: "a" for the colder section [lighter color], "b" for the warmer section [darker color].
The map shows 20 latitude and longitude lines. Areas above an arbitrary elevation are traditionally considered unsuitable for plant cropping and do not bear appropriate zone designations. There are also island zones that, because of elevation differences, are warmer or cooler than the surrounding areas and are given a different zone designation. Many large urban areas carry a warmer zone designation than the surrounding countryside.
USDA Hardiness Zone Map - National Arboretum
USDA Hardiness Zone Maps (National and by State)
USDA Hardiness Zone Finder - National Gardening Association and find plants by zone.
What Are Hardiness Zones? - National Arbor Day Foundation
USDA Plant Hardiness Zone Map - one page showing 11 zones without sub-divisions (Texas A&M):
USDA Plant Hardiness Zone Map - The Tree Center
Canadian Hardiness Zone Map
Heat Zone Map
The American Horticultural Society's (AHS) Plant Heat Zone Map divides the US into 12 zones. The zones of the map indicate the average number of days each year that a given region experiences "heat days"-temperatures over 86 degrees (30 degrees Celsius). That is the point at which plants begin suffering physiological damage from heat. The zones range from Zone 1 (less than one heat day) to Zone 12 (more than 210 heat days). Many plant catalogs are now including this coding for heat tolerance. This is best used in conjunction with the Hardiness Zone Map (see General section above).
AHS Heat Zone Map - Explanation, How To Use and Heat Zone Finder (map is also downloadable in PDF format)
Click here for: A Gardener's Guide To Zone Maps - this gives a good overview of Zone Maps. (Note: Article was produced by the National Gardening Association (NGA), and talks about their Zone Map, but currently it is using the USDA Hardiness Zone Map.) [Alternate Link]
See also: New Maps Reflect Heat Tolerance, Revised Zone Maps Help You Pick Ideal Plants and Selecting Plants: Will That Plant Grow Here? The Reality Of Zone Maps.
Sunset's Climate Zones -- Find Yours - this zone map takes into consideration: length of growing season, timing and amount of rainfall, winter lows, summer highs, humidity. While national in coverage, its primary users are located West of the Rockies.
Climate Zones - at DoItYourself.com, find information on climate zones along with a variety of most recent maps.
Plant and Encyclopedia - find plant information including search by hardiness zone, provided by LandscapeUSA.com.
Trees, Shrubs and Groundcovers - Publications and Educational Resources - information provided by Virginia Tech.
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