top of page

The USDA Plant Hardiness Zone Map of 2023

The recently updated plant hardiness zone map (PHZM) can give growers a more accurate understanding of what is suitable for growing in their location.

What is the Plant Hardiness Zone Map?


The United States Department of Agriculture (USDA) is well known for its Plant Hardiness Zone Map. This map summarizes the average annual extreme minimum temperatures experienced during wintertime for different regions. Plants hardy enough to survive those winter conditions are assigned a hardiness zone. This essentially identifies the growing zone for a specific plant. Each hardiness zone exists within a range of ten degrees (F and C). Zones are further split into half zones denoted by a or b for more accuracy over a smaller scale. These zones allow gardeners, growers, and farmers to determine if a plant will be suited to a desired location.


The map uses an interactive geographic information system to determine the hardiness zone of any location within the domain of the United States. Hardiness zones are represented by different colors and range from 1 (the coolest) to 13 (the warmest). These zones span over large geographic regions (USDA Plant Hardiness Zone Map, 2023).


USDA Plant Hardiness Zone Map, 2023. Agricultural Research Service, U.S. Department of Agriculture. Accessed from https://planthardiness.ars.usda.gov/

 

So, What's New?


Two previous versions of this map have existed. The first was created in 1990 and was updated again in 2012. The most recent update was this November of 2023. Since the 1990 version, two more zones, 12 and 13, have been added. These zones now describe regions with minimum extreme temperatures above 50 and 60 degrees Fahrenheit respectively. These zones, currently found only in Puerto Rico and Hawaii, are completely frost-free. The 1990 version used data from the span of only one decade in the mid to late 20th century.


The updated map draws new data spanning over thirty years (1991-2020). This time period is used by climatologists to describe current climate conditions/normals, so these maps reflect this baseline more accurately.


Using GIS technology, hardiness zones can be differentiated at a finer scale on the map. Better technology allows a better delineation of how areas relative to one another, like a body of water versus a city environment, may retain heat and influence annual temperature extremes.


With the most recent update, many plant hardiness zones have shifted northward. Many regions are now experiencing warmer hardiness zones in comparison to the past. While this may seem like evidence of climate change, it would be inaccurate to make this conclusion. Climate data is based on trends observed on a 50 to 100-year basis. Currently, the map reflects trends over the past 30 years.

 

What Has Changed In Pennsylvania?

The USDA Plant Hardiness Zone Map from 1976 - 2005. https://planthardiness.ars.usda.gov/system/files/US_Map_2012.jpg

Pennsylvania's hardiness zones, provided by https://planthardiness.ars.usda.gov/.


In Pennsylvania’s past, hardiness zones 6 and 7 described much of the growing condition. A large portion of the state occupied zone 5 and a smaller portion occupied zone 4. Currently, PA’s hardiness zones range as cool as 5a and as warm as 8a. The previous zones are now shifted slightly more north on the new map. While this may expand what growers can add to their gardens or fields, warmer annual temperatures may have negative consequences. Some native species may not be adapted to handle warmer winters and may suffer as a result.

 

Potential Implications


Winter, despite the challenges it poses to trees, has its benefits. In the spring, snowmelt is a significant water source that helps trees transition from dormancy to a season of activity. The large flux of water bolsters tree growth in the spring. Cold winters are also important in reducing invasive pest populations. For example, the hemlock wooly adelgid (Adelges tsugae) has been detrimental to native eastern hemlock (Tsuga canadensis) populations, but cold winters can lower the survival rate of the insect (Jackson, 2021).


Winter is an important cue for a tree's phenology. Phenology is the study of when certain natural phenomena occur, especially as it relates to seasonal change. Cold temperatures are needed for temperate trees to resume growing in the spring. Different species have different chilling requirements that are needed to signal the end of dormancy. Many fruit trees need multiple hours of cold exposure. Some vary in the length of time they need, but ideally, temperatures should be below 47 degrees Fahrenheit fairly consistently (Schupp and Marini, 2023). Trees have mechanisms to tolerate winter, but they need to acclimate to the cold in phases. When warmer temperatures rise during winter, trees can start to de-acclimate their cold tolerance. This can damage unacclimated trees if a cold snap follows a warm period (Salma et. al., 2021).


Warmer winters may lead to early or delayed phenological processes like leaf senescence and flowering (Fadon et. al., 2020). This is problematic for animals that depend on regularly timed cycles. If a tree fruits too early due to a mild winter, migrating birds could miss out on an important food source needed to power the rest of the migration.



The timing of migration for different species is one aspect of phenology.


While the hardiness zone map does not reliably confirm climate change trends, there is evidence that higher latitudes are indeed warming. As a result, scientists are trying to predict the migration of lower-latitude tree species to warmer northern climates. Studies of paleorecords suggest this migration into northern latitudes has resulted in the altered composition of forest communities in the past. Based on these studies, there is concern regarding boreal and tundra forests which are expected to retreat further northward in need of a colder climate. A lack of preferential habitat and space, as well as physical barriers, may limit or prevent the dispersal of some species into new areas (Boisvert-Marsh et. al., 2014). The shifting distribution of forest communities and tree species will change current regional biodiversity, community composition, and ecosystem processes. This will certainly affect all species dependent on our native forests.


 

References


Boisvert-Marsh, L., C. Périé, and S. de Blois. 2014. Shifting with climate? Evidence for recent changes in tree species distribution at high latitudes. Ecosphere 5(7):83. http://dx.doi.org/10.1890/ES14-00111.1


Fadón, E., Fernandez, E., Behn, H. and Luedeling, E. 2020. A Conceptual Framework for Winter Dormancy in Deciduous Trees. Agronomy.10(2):241. https://doi.org/10.3390/agronomy10020241


Jackson, D. 2021. Integrated Approach to Hemlock Wolly Adelgid Mitigation. Penn State Extension.

https://extension.psu.edu/integrated-approach-to-hemlock-woolly-adelgid-mitigation


Salama, A. M., Ezzat, A., El-Ramady, H., Alam-Eldein, S. M., Okba, S. K., Elmenofy, H. M., Hassan, I.F., Illés, A., and Holb, I. J. 2021. Temperate Fruit Trees under Climate Change: Challenges for Dormancy and Chilling Requirements in Warm Winter Regions. Horticulturae. 2021; 7(4):86. https://doi.org/10.3390/horticulturae7040086


Schupp, J. and Marini, R. 2023. Tree fruit cold hardiness – pruning effects. Penn State Extension. https://extension.psu.edu/tree-fruit-cold-hardiness-pruning-effects


USDA Plant Hardiness Zone Map. 2023. United States Department of Agriculture.https://planthardiness.ars.usda.gov/

13 views0 comments

Recent Posts

See All

Comments


bottom of page