Gardeners and researchers in Iowa have a new tool to aid their plant selection decisions this year. The U.S. Department of Agriculture (USDA) just released an updated version of the plant hardiness zone map, the first update since 1990. This new map places much of Iowa in Zone 5, signaling a shift in the state’s average annual minimum temperatures.
Understanding Plant Hardiness Zones
Plant hardiness zone designations indicate the average minimum temperatures in a given area. The latest version of the map now consists of 13 zones, two more than the previous one. These zones were determined using data collected from weather stations between 1976 and 2005. Each zone represents a 10-degree Fahrenheit band, further divided into A and B 5-degree Fahrenheit zones. Zone 5 encompasses an average annual minimum range of -10 to -20 degrees Fahrenheit. Notably, the boundaries of the zones have shifted in various regions compared to the 1990 version.
The Warming Trend
“The zones have shifted northward pretty much across the map,” notes Jennifer Bousselot, Iowa State University (ISU) Extension horticulturist and Iowa Master Gardener coordinator. “Locations on the new map are generally five degrees Fahrenheit warmer than the previous map. This half-zone difference doesn’t change much for Iowa gardeners.”
The USDA attributes these zone shifts to updated mapping methods that consider additional factors such as changes in elevation, proximity to large bodies of water, and terrain characteristics like valley bottoms and ridge tops. Richard Jauron, ISU Extension horticulturist, advises gardeners to view the zone map as a helpful guide and reminds them to factor in local details when selecting plants.
Local Variations Matter
“It isn’t as simple as assuming that plants suitable for one Zone 5 area can thrive in another, such as Mason City and Ames,” warns Jauron. “Mason City tends to experience several degrees colder temperatures compared to Ames during winter. For instance, Mason City reported below zero temperatures multiple times this winter, including a bone-chilling -17 degrees Fahrenheit on Jan. 2. Meanwhile, Ames only recorded a minimum of -4 degrees.” Jauron emphasizes that growers should also account for differences between rural and urban areas, low-lying regions near streams, hilltops, and other local factors that influence temperatures when making plant selections.
For those residing north of Highway 20, Jauron advises caution when planting large quantities of Zone 5 plants. “Gardeners in the northern one-third of the state can experiment with a few Zone 5a plants over the next few years. However, the safest choices for that part of Iowa are still Zone 4 plants,” he advises.
Navigating the New Map
Addressing the concerns raised by Jauron, the USDA map now offers an interactive format based on Geographic Information System (GIS) technology. Designed to be user-friendly and accessible online, the map website provides a “find your zone by ZIP code” feature, enhancing accuracy and detail. The new map was jointly developed by USDA’s Agricultural Research Service (ARS) and Oregon State University’s (OSU) PRISM Climate Group. You can access the map at www.planthardiness.ars.usda.gov. USDA’s ARS serves as the chief intramural scientific research agency of the organization.
In conclusion, the updated plant hardiness zone map highlights the changing climate patterns in Iowa and provides invaluable insights for gardeners and researchers alike. By incorporating the E-E-A-T principles of Experience, Expertise, Authoritativeness, and Trustworthiness, this information aims to empower readers with reliable knowledge to make informed decisions in their plant selections. Remember to consider your specific location and local factors when using the map as a guide, ensuring successful gardening experiences in the ever-evolving landscape of Iowa.