How the Hardiness Zone Map Was Started
Every plant can adapt to a range of environments. Gardeners have learned through experience where the great variety of landscape plants can be
grown. Over the years many schemes have been proposed to help gardeners locate those environments when they introduce new species, forms, and
cultivars. The pooling of many of these schemes culminated in the development of the widely used "Plant Hardiness Zone Map",
under the supervision of Henry T. Skinner, the second director of the U.S. National Arboretum.
I wasn't able to use the USDA Plant Hardiness Zone Map. For my state the zones were too finely defined for me to find any
reference in them. They are included here, in case they are useful to you.
Find out by Zip Code what Your Hardiness Zone is:
I get 2 different Hardiness zones when I put my current zip code in AborDay (the iframe to the right = my result is warmer 5 - 6),
then when I check my zip code with The National Garden Association (my result is colder, a 4).
So I suggest trying both Zip Code Finder Tools, to figure out which ones best for you.
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I went digging. There had to be some kind of reasonable explanation for the discrepancy in my Hardiness Zones. There is.
Checking my Zip Code with the US Postal Service it turns up not just the town I live in, but also turns up what it calls an
"Acceptable City name" for a city that is 1,880 feet lower in elevation (5,600), then I am (7,400). Not only is that
"Acceptable City name" warmer, by virtue of elevation, but its climate is impacted by the Rocky Mountains in a whole different way,
then the elevation I live at. A good example is the blizzard, this past December 2006, where they were reporting 22 inches of
snowfall in town (the lower elevation), but we picked up 42 inches of snow.
I tried to get Google Earth to define the boundaries of my Zip Code, and turned up many mixed results. Google Earth correctly found the main post office for my town.
Google Earth was showing that "Acceptable City name" post office, as just outside of Kittredge. A little hard to believe, as Kittredge has its own postal code.
Never-the-less, Kittredge is 600 feet lower then I am in elevation. About this time, I decided my whole county was the issue. My county covers the eastern plain,
right up against the Rocky Mountain Foothills, and into the foothills themselves. I found this web page
Summits of Colorado’s Jefferson County spanning summits as low as 5,772 and as high as 11,589. I have no way of knowing whether these summits are in my Zip Code,
but I could see from Google Earth that my neighboring county "Clear Creek County" is also in my Zip Code. Clear Creak County is most certainly much higher in elevation,
and much more rugged in terrain.
In my case, of this Hardiness Zone discrepancy, I am going to use the colder zone 4, rather then the warmer zone 5-6 I'm also given. I will be gardening
at 7,400 feet in the Rocky Mountain foothills, just west of Denver.
I believe the discrepancy is in how The National Garden Association
describes Zone Map Drawbacks where in the western USA (west of the 100th meridian - mid North Dakota, to mid Texas), the USDA map fails.
You can read more (from Better Homes and Gardens) on the
Benefits and Limitations of USDA Hardiness Zones And to read specifically why I may be having troubles with accurate USDA Hardiness Zone reporting -
read the same Better Homes and Gardens article, but scroll down to the bottom:
Problems in the West.
All that said (in the name of information) there is no weather station in my small mountain town, and the county I live in has a vertical differential between
5,600 feet, all the way to the highest summit at 11,589. I live at 7,400 feet, which must be colder then down below at 5,600 feet. I will garden with a
Zone Hardiness of 4. I don't want to loose tender plants to winter cold. I think I'm saying that I think The National Garden Association
Find Your Hardiness Zone by Zip Code, may be the more accurate of the two (2).
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You need to know your last spring frost date, and your first autumn frost date.
- Call Your Local University Cooperative Extension for last spring frost date, and
your first autumn frost date information.
- I called My Local University Cooperative Extension, and it turned out, they had no
weather data on my location. Exact location in Colorado matters, due to the
influence the mountains exert on both temperature and moisture --- I needed frost dates, minimally, for my elevation. I found the best data I know of that
can be found (outside the Local University Cooperative Extension). I went to Accuweather.com
and entered my Zip Code. From there I pulled down the menu of "Forecast" and selected "Historical Weather" and then selected from a
side menu "Typical Weather". This brought up Typical Weather for Entire Year --- a 4 by 3 grouping of all the months in a year, with a
little summary. I drove down deeper in April, and this brought up Typical Weather for April and a grid of all the days in the month. I can see
that by April 11th, the average nighttime low is 33 degrees Fahrenheit for my Zip Code. April 11th is only the average frost date.
In any given year, it may be sooner, or later - in order to make up the average.
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Basic Plant Requirements
All agriculturists, environmentalists, horticulturists, and home gardeners
have one all-abiding question about any plant they wish to introduce into
their growing spaces. Will it flourish?
The ability to predict whether a newly collected species or cultivar can be
successfully grown in a location is fundamental to the continued productivity
of America's agriculture and to the survival of our landscapes. The
plants in our urban spaces, farms, fields, and forests consist of native
vegetation and exotic plants introduced into our land from all over the
world.
All plants must be placed in an environment that meets their
basic requirements:
Day length.
Day length is usually the most critical factor in
regulating vegetative growth, flower initiation and development, and the
induction of dormancy. Plants survive only when the day length promotes
their growth and prepares them for the seasonal changes.
Radiation.
Most plants respond to radiation in the 270- to 3000-nm
region. Cloudy, rainy days coupled with the shade provided by nearby
plants and structures can significantly reduce the amount of radiation
available. Plants survive only where the amount is within a specified
range.
Temperature.
Plants grow best within an optimum range of temperatures;
and the range may be wide for some species, narrow for others. Plants
survive only where temperatures allow them to metabolize.
Frost.
Plants differ in ability to survive frost, their responses
varying from immediate death to sustained performance. The previous
environmental and cultural conditions of plants can often shift, but not
permanently alter, their tolerance to freezing. Plants survive only
when they are adapted to subfreezing weather.
Heat.
The thermal cutoff temperature varies widely from species
to species. By tradition we group plants into sun, partial sun, and
shade types and plant them according to their light and heat tolerances.
Rainfall.
Gardeners need to know how much water a landscape plant
requires in determining its usability in low maintenance landscapes.
Rainfall gardening often greatly limits which species can be used successfully.
Gardeners also need to know how much and how often to water plants in high
maintenance landscapes.
The ability of plant roots to take up water and nutrients depends
on the pH (measure of acidity or alkalinity), presence of soluble and insoluble
salts, and aeration of the growing medium. The successful culture
of all plant species requires that they be grown in a medium within a definite
pH range and with from 10 to 14 essential nutrients in appropriate balance.
Although plants may tolerate some extraneous elements and compounds, every
plant species and cultivar has well prescribed limits.
Note: This publication is not copyrighted, and permission to reproduce all or any part of it is not required.
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The 2003 US National Arboretum "Web Version" of the USDA Plant Hardiness Zone Map
USDA Miscellaneous Publication No. 1475, Issued January 1990
How to use this Hyperlinked Map
** Click on any area within the United States to go to a close-up of that region **
Click your browser's Back button to return here
** Or -- Click on your State abbreviation below to go to that region **
AK
AL AR AZ
CA CO CT
DC DE FL
GA HI IA
ID IL IN
KS KY
LA
MA MD ME
MI MN MO
MS MT NC
ND NE NH
NJ NM NV
NY
OH
OK
OR PA RI
SC SD TN
TX UT VA
VT WA WI
WV WY
USDA Hardiness Zone Map Introduction Page
Listed by zone. -- Names of representative plants
listed under the coldest zones in which they normally succeed.
Listed alphabetically. -- Cold hardiness ratings
(zones) for selected woody plants.
USDA Miscellaneous Publication No. 1475. Issued January 1990.
Authored by Henry M. Cathey while Director, U.S. National Arboretum. Edited, formatted and prepared for the US National Arboretum web site by Ramon
Jordan, March 1998 & Revised March 2001
U.S. National Arboretum, Agricultural Research Service, U.S. Department of Agriculture,
Washington, DC 20002
** Special thanks to Jody Shuart and Scott Bauer, ARS Information Staff
Note: This publication is not copyrighted, and permission to reproduce all or any part of it is not required.
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