AN ILLUSTRATED HISTORY OF AMERICA’S PLANT HARDINESS ZONES (With Closing Comment on Zone 7(b) and…
By: Tom Packer
Tags: Climate Change, Weather, Plants, Gardening, North Carolina

AN ILLUSTRATED HISTORY OF AMERICA’S PLANT HARDINESS ZONES (With Closing Comment on Zone 7(b) and Wake County, North Carolina)

By Tom Packer, certified advanced trained gardener, citizen scientist, and longtime backyard farmer in North Carolina and Northern California. All this article’s content is that of Mr. Packer alone as an individual and not as a member of or on behalf of any organizations or groups with whom he is affiliated.

While browsing at the nursery, you pluck the pointed plastic label from the plant’s container, wipe off the moist soil with your finger and look at the plant’s name, sun requirements and its “USDA Zones.” Or, while shopping online, a major nursery has “USDA Zone” next to each plant. When you click, the site explains, “There are very few rules in gardening, but perhaps the most important one is to work within your zone.”¹ Experienced gardeners know that by identifying the USDA plant hardiness zone in which they live it can then be determined if a particular plant can survive in the garden because it can withstand the average annual minimum temperatures associated with the zone.

There are of course other factors to be considered when determining the potential viability of a plant in a geographic location, but the use of the average annual minimum temperature provides a universal baseline to be used by all. As one of the early pioneers in the use of plant hardiness maps explained in 1940:

“Hardiness of plants is an indeterminable quantity, based not only on a plant’s resistance to minimum temperatures, but also on its resistance to maximum temperatures, and other factors such as lack of water, exposure, soil conditions, length of growing season, etc. It would be impossible to prepare a map depicting all these factors, though several might be included on a complex one. However, since a map based on the average annual minimum temperatures agrees in many instances with the known limit of hardiness of certain plants, these data were adopted as the basis for hardiness zones.”²

This view has endured and recently was expressed as:

“The extremes of winter cold are a major determinant of natural plant distribution and the successful cultivation and survival of long-lived plants. In the United States, the USDA Plant Hardiness Zone Map is the primary reference for defining and communicating the geospatial patterns of extreme winter cold to the horticulture and nursery industries, agrometeorologists, and plant scientists.”³

The history of how these zones came to be developed involves pioneering horticulturalists and colorful, hand-drawn maps from decades ago. This is the story of how the “USDA Zones __” on your plant tag came to be, what they mean and why they are still in use almost 100 years after the first plant hardiness maps were developed. There do exist what is becoming a myriad of various “plant hardiness” type maps which focus on other weather data, specific regions and different purposes, and which are beyond the scope of this article. Nonetheless, the USDA Plant Hardiness map remains the most commonly used standard by the plant industry and gardeners for assessing plant hardiness in the United States.⁴

This is the first known plant hardiness map, published in the Manual of Cultivated Trees and Shrubs Hardy in North America in 1927.

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It was devised by Alfred Rehder

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who for much of the first half of the 20th century years was an internationally renowned botanical taxonomist and dendrologist at Harvard University’s Arnold Arboretum in Boston.

Rehder divided the United States and southern Canada into eight zones consisting of 5° bands of differences in lowest mean temperatures of the coldest month of the year. As the initial attempt at establishing hardiness zones, the map today looks a bit primitive, as it should given its vintage, using broad approximations and based on relatively limited weather data. It also happens to be missing most of Florida and the lower parts of Louisiana and Texas.

Here is the same map with coloring hand drawn by Rehder himself.

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This shading, perhaps with a crayon or colored pencil, reveals an intimate personal touch. As will be discussed later, this human involvement fortunately still exists even in developing modern day plant hardiness zones maps.

Rehder’s sweeping bands of hardiness had their origin in the work of German geographer Alexander von Humboldt

who is believed to have developed the first map with zones of equal climatic conditions, or isotherms. His first known map was published in an early world-wide atlas in 1845.

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Another, even earlier, global climatic map was discovered, but it is not known if Rehder was aware of it. This map portrays isothermal bands circling around the globe and was created by

William Channing Woodbridge of Connecticut.⁹

The map appears to have been drawn around 1823 and published in 1830. Significantly, it foreshadows Rehder’s and other’s hardiness zone maps by describing bands moving through regions on the map described as Hot, Warm, Temperate, Cold, Wintery and Frozen. These early maps also are especially significant as reflecting the nascent thought that isothermal lines slice through latitudes at different angles, contrary to the then prevailing notion that climate was directly related to latitude.

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However, while it may appear from the literature that after Rehder’s 1927 map the horticultural community primarily referenced updates published by the Arnold Arboretum, a significant map was published in 1928, found deep inside a dense United States government publication. The 1928 Atlas of American Agriculture, Climate. Temperature, Sunshine, and Wind published by the USDA contained climatic maps prepared by J.B. Kincer, including this map (on top of page).

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While somewhat lost to obscurity, perhaps because it was just one of many maps in the publication or, unlike Rehder’s work, numbered zones were not assigned, it nonetheless appears to be the first USDA map to depict average annual minimum temperatures in zones across the United States. Even more significantly, Kincer used the measurement of the lowest recorded temperature in a year, then averaged those over a number of years, in order to arrive at the zones in the map. This differs from Rehder’s use of the average low temperatures for the coldest month, which would result in higher temperatures being used since it averaged all the low temperatures for the coldest month, instead of using solely the coldest temperature of the year. Kincer’s use of this particular measurement (referred to as the “plant hardiness (PH) statistic”) would later be adopted by the USDA when it started publishing plant hardiness maps by that name in 1960 and continues to the present.

There is not much, if any, reference in the horticultural literature to Kincer’s map after it was published. The next plant hardiness map of import published was in 1938 when horticulturist

Donald Wyman

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of the Arnold Arboretum enhanced Rehder’s map.

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Wyman used previously unavailable temperature data from the U.S. Weather Bureau from the years 1895–1935. Significantly, he based the new isotherm contours on the average annual minimum temperatures, just as Kincer had done in 1928. Wyman’s temperature bands, however, differed by 5°F, 10°F, and 15° F, depending on the latitude. For example, average winter minimums on the map’s zone 3 range from -35° to -20°, a difference of 15°, but Zone 7 for example has only a 5° spread.

Wyman and the Arnold Arboretum continued to revise and update the plant hardiness map, including these 1948 maps, one with Newfoundland added with a taped page and another attractively colored map drawn by hand.

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The above maps likely were drafts of the more polished, finalized 1948 map by Wyman and the Arnold Arboretum which incorporated new weather data.

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Interestingly, in 1950 the Arnold Arboretum even placed notable arboretums and botanical gardens in the United States onto the existing plant hardiness zone map.

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Wyman in 1967 published a new map, updated to reflect new weather data.

Here is what appears to be a draft or working copy of the 1967 map.

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… and here is the final1967 map:

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However, due to the lack of uniformity in the zone intervals in the Arnold Arboretum maps, the USDA developed and published a Plant Hardiness Zone Map [PHZM] in 1960. In an attempt to reconcile the two maps, Wyman subsequently published a table showing the differences between the two numbering systems used in his map and the new USDA map. However, the horticultural community experienced some confusion by being presented with these two different appearing maps and eventually use of the USDA map increased with the USDA’s newly consistent zone designations becoming the standard for assessing plant hardiness in the United States.¹⁹

The 1960 USDA map:

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The USDA updated its PHZM (Plant Hardiness Zone Map) in 1990.

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The 1990 edition of the map consisted of the average annual minimum temperatures based on the lowest temperatures recorded for each of the years 1974 to 1986 in the United States. It showed 10 different zones and introduced a zone 11 to represent areas that were essentially frost free. Almost 8,000 weather stations within the involved latitudes and longitudes for the U.S., Canada and Mexico were identified and those with valid average annual minimum temperatures based on at least 10 years of data were used.²² This also is the last USDA PHZM prepared by hand.²³

The most recent USDA Plant Hardiness Zone Map was published in 2012 and can be found here. It has new features, including being able to access hardiness data by zip code.

The 2012 map differs from the 1990 map in that the for the first time it was in a digital format and would be maintained on the internet. The development of the map is explained in detail in a scientifically thorough 2012 article in the Journal of Applied Meteorology and Climatology.²⁴ Highlights from the article include:

* The map focuses solely on the PH statistic, that is, the lowest temperature of each year recorded at weather stations from 1976–2005 and then averaged.

* This one dataset was used due to its widespread adoption both in the United States and internationally. As the article notes, “Accurate prediction of winter injury is a key component to the successful cultivation and survival of long-lived woody and herbaceous perennial plants in many regions of the world. The frequency and severity of winter injury are also important determinants in the natural distributions of many temperate plants.”²⁵

* Instead of the 13-year data set used for the 1990 map, the 2012 map included 30 years’ worth of data. This longer period of time was chosen because “… it was more stable statistically, samples recent climatological variation more completely, and yields a clearer picture of the role that past winters have played in the survival of long-lived plants.”²⁶

* An interesting glimpse into climate reports reveals that the weather station data used is incomplete, but this evidently is acceptable (with limitations) in the meteorology field. Most of the weather stations in the country are referred to as “COOP” stations, which are stations operated by volunteers, typically at most only record findings once a day and not unusually are missing a significant number of weather observations throughout a year. There exist other weather stations, but far fewer in number, which are operated by professionals, record observations hourly and are not known to have missing information.

* The quality threshold for weather stations to be used in development of the 2012 map was that they had to have at least 24 days of minimum temperature data during each of the months between October and March for at least 10 of the 30 years involved in the study, and they were then put through other quality control procedures. So, otherwise stated, a weather station missing 20 percent of its relevant weather data and for as much as 66 percent of the time-period involved would still be used. This is not necessarily a criticism of this methodology, but more of an insight afforded to gardeners into how climate statistics are derived. Of the 7,983 weather stations used in developing the map, only a small portion are professionally operated with consistent recording of observations. Nonetheless, the 2012 map clearly is significantly more sophisticated than prior maps and the result of use of updated, modern technology.

* Despite the use of advanced technology, the human element remained in the formation of the 2012 map. As explained by the USDA, once an initial draft of the map was produced, people from the agricultural, climate and horticultural communities were able to comment on whether certain parts of the map were too warm or too cold in their opinions, and changes were made based on this human input.²⁷ While not the same as colleagues gathered around the drafting table while the 1927 and 1928 maps were drawn up by Rehder and Kincer, it nonetheless is reassuring to see the human touch involved.

* It is important to keep in mind that the 2012 map reflects the average lowest winter temperature for a given geographic area for this 30-year period but does not necessarily represent the coldest it has ever been. This would be due to data from prior to 1976 not being included and because many of the weather stations used have missing days of data.

* The USDA plant hardiness map is not an indicator of whether global warming/climate change/climate crisis is occurring. The USDA states, “Climate changes are usually based on trends in overall average temperatures recorded over 50–100 years. Because the USDA PHZM represents 30-year averages of what are essentially extreme weather events (the coldest temperature of the year), changes in zones are not reliable evidence of whether there has been global warming.”²⁸ The founder and director of Oregon State University’s PRISM Climate Group, which per contract with the USDA created the 2012 map, also commented, “Plant hardiness is ‘a really volatile statistic in time. It’s only one day per year over 30 years. Even if the average is slowly creeping up on us, as a single, volatile statistic it’s just not a very good metric for looking at climate change.’”²⁹

*Even though the data in the 2012 map is as of 2005, the USDA reports that a check of more recent data did not find that adding the more recent years of data would make a significant difference in the definition of the zones.³⁰

WAKE COUNTY, NORTH CAROLINA

As this piece was written in the state capital of North Carolina, Raleigh (Wake County), a brief note is made about its plant hardiness zone status. Wake County for decades lived in the Zone 8, 10–20° average annual minimum temperature zone, starting with Kincer’s USDA map of 1928 and in the succession of Arnold Arboretum maps. Wake County continued to reside in this Zone 8 through the USDA map of 1960, where it was within Zone 8(a). However, it was placed into Zone 7(b) (5–10°) in the USDA maps of 1990 and 2012, where it remains today.³¹ So, whenever the USDA map is next updated Wake County may find itself in Zone 7(a), still in Zone 7(b), or Zone 8 may come back to Wake County, where it previously was for over 60 years, from 1928 until the 1990 USDA map was published.

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[1] Monrovia.com

[2] Wyman, Donald. “A HARDINESS MAP FOR THE UNITED STATES.” Bulletin of Popular Information (Arnold Arboretum, Harvard University), vol. 8, no. 12, 1940, pp. 61–64. JSTOR, www.jstor.org/stable/42962115. Accessed 4 Oct. 2020

[3] Daly, C., M.P. Widrlechner, M.D. Halbleib, J.I. Smith, and W.P. Gibson. 2012. Development of a new USDA Plant Hardiness Zone Map for the United States. Journal of Applied Meteorology and Climatology, 51: 242–264 at 259 Link to article

[4] https://www.almanac.com/content/plant-hardiness-zones#

[5] https://www.flickr.com/photos/usdagov/6862204426/in/photostream/

[6] Image courtesy of Arnold Arboretum Archives

[7] Image courtesy of Arnold Arboretum Archives

[8] System Der Isotherm-Kurven in Physikalischer Atlas. Heinrich Berghaus, 1845. Image from Geography and Map Division, Library of Congress

[9] https://en.wikipedia.org/wiki/William_Channing_Woodbridge

[10] Isothermal Chart, or View of Climates & Production in Woodbridge’s School Atlas to Accompany Woodbridge’s Rudiments of Geography: Atlas on a New Plan . . . William C. Woodbridge, 1830. Geography and Map Division, Library of Congress. See also, https://blogs.loc.gov/maps/2018/04/the-first-isothermic-world-maps/

[11] https://archive.org/details/CAT31378527007/page/n17/mode/2up See, p. 10.

[12] Image courtesy of Arnold Arboretum Archives

[13] https://www.flickr.com/photos/usdagov/7008319727/in/photostream/

[14] Image courtesy of Arnold Arboretum Archives

[15] https://www.flickr.com/photos/usdagov/7008320087/in/album-72157627857689389/

[16] Image courtesy of Arnold Arboretum Archives

[17] Image courtesy of Arnold Arboretum Archives

[18] https://www.flickr.com/photos/usdagov/7008320087/in/album-72157627857689389/

[19] Daly, C. (n 3) at 259

[20] https://www.flickr.com/photos/usdagov/7008290183/in/photostream/

[21] https://www.flickr.com/photos/usdagov/6862173684

[22] United States Department of Agriculture, Agricultural Research, Miscellaneous Publication №1475

[23] Daly, C. (n 3) at 243

[24] Daly, C. (n 3)

[25] Daly, C. (n 3) at 242

[26] Daly, C. (n 3) at 244

[27] https://planthardiness.ars.usda.gov/PHZMWeb/AboutMapMaking.aspx

[28] https://planthardiness.ars.usda.gov/PHZMWeb/AboutWhatsNew.aspx

[29] https://www.wiscontext.org/where-wisconsin-do-hardiness-zone-shifts-reflect-changing-climate

[30] https://planthardiness.ars.usda.gov/PHZMWeb/AboutWhatsNew.aspx

[31] https://planthardiness.ars.usda.gov/PHZMWeb/Images/300dpi/NC.jpg