A hardiness zone map (HZM) provides information that gardeners and professional horticulturalists use in determining which herbaceous and woody perennial plants will survive cold temperature in a particular geographic area. Last week the United States Department of Agriculture released a new hardiness zone map to replace the older 1990 version.
- is a visual representation of average annual minimum temperatures across the United States. Data points used to create the map were the lowest daily minimum temperatures recorded at thousands of temperature data stations during each of the years sampled.
- divides the U.S. into multiple hardiness zones with 10o F differences in average annual minimum temperatures. Zone 1 is the coldest zone (-50o F to -60o F).
- divides each hardiness zone into "a" & "b" with "a" being the colder half of any zone and "b" the warmer half.
There are changes in the new map and the process that was used to develop it:
- The 1990 HZM was based on data from a 12-year period (1976-1990) while the new HZM is based on data from a 30-year period (1976-2005).
- The data used to create the new map was more complete, and a complex algorithm was used to interpolate between recording stations. Temperature data from more than 8000 temperature data stations belonging to the National Weather Service, the USDA Natural Resources Conservation Service, the USDA Forest Service, the Bureau of Reclamation, and the Bureau of Land Management was used. Average minimum temperatures were then calculated for ½ mile square computer grids for the entire country to create the new hardiness zone map. This was followed by a review process that included climatologists, agricultural meteorologists, and horticultural experts who checked for errors, looked for the source of errors, and corrected errors.
- The new map is Geographic Information System-based. This means that the map is more accurate, is interactive (by zip code) to improve user experiences, and has higher resolution that can show smaller areas of zone delineations than before. While the 1990 map was a static image and was not designed for web use, the new map allows users to zoom in to a local area to see the higher temperatures of cities that are heat sinks, the lower temperatures on mountain tops, and the buffering effects of large bodies of water on temperature.
What does the new hardiness zone map show?
In general, the new map shows what we have all been experiencing in recent history: warmer low temperatures during winter. A shift of ½ of a zone was common for much of the country. Closer to home, here is what happened to Minnesota's hardiness zone map:
- There was a ½ zone shift for much of the state because Minnesota, like the rest of the U.S., has been experiencing warmer annual minimum temperatures during the time period used to create the new HZM.
- Zone 5a (with average minimum temperatures of -15o to -20o) crept up into the south central portion and the far southeastern corner of Minnesota.
- Much of the southern ½ of Minnesota that was formerly divided into zones 4a & 4b is now zone 4b (with average minimum temperatures of -20o to -25o).
- The four pockets of Zone 2b (with average minimum temperatures of -40oto -45o) disappeared from northern Minnesota.
- The amount of Minnesota that is zone 3a (with average minimum temperatures of -35o to -40o) shrank significantly due to an increasing area of zone 3b (with average minimum temperatures of -30o to -35o).
- Parts of the far northern shore of Lake Superior that were formerly zones 4b and 4a are now designated as 4a and 3b, meaning they are colder.
What kind of impact should the new hardiness map have on Minnesota gardeners?
- We can all rest easy knowing that the warmer minimum annual temperatures we have been enjoying over the past years really did happen!
- Remember that a HZM is created based on average annual minimum temperatures and should only be used as a general guide. By the very definition of average, we know that temperatures lower than the average minimum temperature of the zone you live in will occur. Pick your plants accordingly. Maybe we can broaden the palette of plants we choose to grow in Minnesota a bit, but be cautious and wise in your weighing of risk vs. gain as you trial new plants. Losing an herbaceous perennial or quickly-maturing shrub to winter injury may be of little concern in terms of the time it takes to establish a replacement plant. Losing a slow-growing shrub or a tree that takes decades to grow to mature size creates more pain.
- Hardiness zone maps are of no help in predicting plant damage or mortality during acclimation and deacclimation. Remember that hardiness is not just about the lowest temperature a plant must survive during a winter. Every year, starting in late summer, perennial plants goes through a multi-month process called acclimation that prepares them for winter survival. In spring dormant plants go through a reverse process called deacclimation that restores their ability to actively grow during the growing season. Plants can be winter-injured or killed by abnormally low temperatures during the months of acclimation and deacclimation too. This is especially true of marginally hardy plants from warmer parts of the country or world that we may try to grow in Minnesota.
- Hardiness zone maps provide gardeners with one category of plant performance information: winter survival. Good plant performance is not just about winter survival. If the new HZM persuades you to plant cultivars and species new to you, remember that there are other selection categories to consider as you match a plant to your planting site: soil texture, soil moisture, soil pH, light exposure, precipitation, etc.