In November 2023, the United States Department of Agriculture (USDA) released an updated version of its popular Plant Hardiness Zone Map.
The USDA Plant Hardiness Zone Map is a standard guide to a plant’s cold tolerance. It has been an important tool when selecting trees, shrubs, and perennials for the landscape for over 50 years.
The USDA Plant Hardiness Zone Map divides the country into 13 zones based on average annual minimum temperatures.
Zone 1 is the coldest area where minimum temperatures, on average, will go down to minus 60°F (-50°C). At the other extreme, Zone 13 is the warmest zone in tropical locations where average lows don’t drop below 60°F (16°C). The zones are divided by 10-degree increments and further divided into half zones by 5-degree increments.
Most American reference books, nursery catalogs, and gardening magazines describe a plant’s ability to survive cold temperatures using USDA plant hardiness zones.
What Has Changed on This New Map?
Compared to the 2012 and 1990 maps, zone boundaries in this 2023 edition have shifted in many areas. The new Plant Hardiness Zone Map is generally about one quarter-zone warmer throughout much of the United States.
Nearly all of Iowa is now in Zone 5, with the northern half in 5a and the southern half in 5b. Small areas in northeast Iowa remain in Zone 4b, and small areas in southeast and southwest Iowa are now in Zone 6a.
The data on which the 2023 map is based was gathered from 1991 to 2020. The previous edition used an earlier 30-year period (1976-2005).
Utilizing data from the previous 30 years was selected by a group of horticultural, botanical, and climatological experts 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.
Because the latest maps were created digitally with GIS technology, they have a higher level of resolution and can show smaller areas of zone delineations than earlier maps. This means the maps do a good job of showing small differences due to elevation, large bodies of water, or the heat island effect that causes cities to be warmer than the surrounding countryside.
This edition of the Plant Hardiness Zone Map was created using PRISM, a highly sophisticated climate mapping technology developed at Oregon State University. This advanced technology used data from a total of 13,625 stations (a substantial increase compared to earlier versions), allowing for a more detailed and accurate map.
How to Use the Map
The USDA Plant Hardiness Zone Map is a guide to help gardeners select plants suitable for their location. However, the maps are only guides. They are based on the average annual minimum temperature. Longtime Iowa residents realize the weather in our state is rarely normal or average. Periodically, these averages will be exceeded, sometimes by a wide margin. Plants may be damaged or destroyed during extreme weather events.
For example, a tree or shrub hardy in USDA Hardiness Zone 5a (average annual minimum temperature of -15 to -20°F (-26 to -29°C)) may grow well for several years as long as winter temperatures are average or warmer than normal. However, a bitterly cold winter (when temperatures drop to -25 to -30°F (-32 to -35°C)) is still possible and may result in serious plant damage or possibly death.
Despite this limitation, the USDA Plant Hardiness Zone Map is a helpful guide. Gardeners who utilize the maps and select plants based on their cold tolerances should see high plant success rates.
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