Garden Guy column: Plant Hardiness Zone change
A new USDA (U.S. Department of Agriculture) Plant Hardiness Zone Map has been released. Last released in 2012, the newly released map shows some stark changes in the last 10 years.
Prior to the 2012 version, Amarillo was in zone 6b. Changing to 7a with the 2012 version it remains 7a in this updated map. However, when I wrote about the 2012 change, I stated that although we were moved to a warmer zone our old “…Zone 6b appears to be in the vicinity of Dowell Road just west of Amarillo”. Now, west of Amarillo, zone 6b begins in western Quay County NM and appears to be about 115 miles west as the crow flies. Closer to home, zone 6b now occurs at about the middle of Hartley County on a line running roughly diagonally from SW to the NE which has also moved northwest since the 2012 map albeit not nearly as far.
The zones are defined by an average temperature range of 5 degrees. 7a is shorthand in the horticultural world for an average low temperature of between 0 and 5°. These recent averages are computed using data from the 30-year period of 1991 – 2020.
The map, which can be seen at https://planthardiness.ars.usda.gov/, is interactive. You can see the hardiness zones to the level of zip code. I like this because you can see how close the next zone is to where you garden. Another benefit is that it enables more easily and accurately looking at “sister areas” in other places that are in the same zone that you are in.
The new map is more accurate not only because updated data has been used but because the data used to identify each zone is more extensive than in the past. “A very sophisticated algorithm was used to interpolate low temperature values between actual weather reporting stations.”
The data “was selected by the group of horticultural, botanical, and climatological experts who led the review of the latest revision as the best balance between smoothing out the fluctuations of year-to-year weather variation and the concept that, during their lifetimes, perennial plants mostly experience what is termed "weather" rather than "climate." The 1991-2020 period also aligns with the period currently in use by climatologists to describe baseline climate “normals” in the U.S.
Climate changes are usually based on trends in overall annual 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.” (See website for citations.)
Regardless of zone designation, do not forget that there are many different factors that affect plant hardiness. Duration of cold temperatures, overall climate conditions, plant condition, plant origin, microclimates, and other things affect a plant’s hardiness. Each plant’s hardiness rating is also simply an estimate based upon the combined experience of many people from different places who have grown the plant and recorded the results.
This article originally appeared on Amarillo Globe-News: Garden Guy column: Plant Hardiness Zone change