Plant hardiness zones are used to determine if a plant can thrive in a given climate — and your own backyard. The USDA Plant Hardiness Zone Map is a tool used to approximate growing zones and has been updated as our climate shifts, most recently in 2023. The map can be a helpful place to start, but there are many things to think about when determining what can grow in your garden.

1. A plant hardiness zone is a region of the United States that sustains an average extreme minimum cold temperature within a specific range each year. For example, the range for USDA zone 5a, which includes parts of southwestern, central, and northeastern Wisconsin, is −20°F to −15°F. The larger the zone number, the higher the average temperature range. But because the range is an average, it’s important to remember that the zone may experience even colder temperatures.

2. Our climate is getting warmer. If you compare the old map (published in 2012, using data from 1976 to 2005) to the new one (published in 2023, using data from 1991 to 2020), you’ll notice a trend of zones moving farther north. Even in areas that haven’t moved to an entirely new zone, the minimum cold temperature might be a few degrees warmer, and you might
still see a shift in what plants you can grow in your garden.

3. The new map is more accurate. The USDA increased the amount of data incorporated into the map. Weather stations, research greenhouses, university buildings, and scientists in the field all contributed data points that increased the accuracy of zone delineations. Now we have a clearer picture than ever before of the changes we are seeing in our climate.

4. Microclimates are not reflected in the data. Even though it is more accurate, the new map is still only a guideline. If you’re a gardener, you may have noticed that plants do better in certain parts of your garden. Topography has profound effects on the temperatures that plants experience. Certain parts of the landscape may experience more or less cooling due to wind conditions or the locations of nearby buildings, hills, and bodies of water. For example, Allen Centennial Garden is close to Lake Mendota and has a protected perimeter of trees and buildings, so its microclimate allows some zone 6 plants to grow and survive despite being in zone 5.

5. Hardiness doesn’t guarantee survivability. Plants survive cold by building up cold tolerance to their environments. This means they rely on normal patterns of winter, with slowly cooling weather in the fall and slowly warming weather in the spring. So a plant considered to be hardy in your zone might be caught off guard by sudden or unseasonable changes in temperature, causing it to lose cold hardiness and be damaged or die when cold temperatures return, even if the cold temperatures aren’t below its hardiness zone threshold. Species that flower early in the spring, such as forsythias and flowering cherries, are especially prone to these false springs and late frost occurrences. Additional safeguards, such as plastic or cloth covers overnight, may be needed to protect these plants from sudden meteorological shifts.

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Isaac Zaman BS’22 is the horticulturalist for Allen Centennial Garden, and Reba Luiken is the garden’s director. Al Kovaleski is assistant professor in the Department of Plant and Agroecosystem Sciences and runs the Plant Resilience Lab.

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This article was posted in Changing Climate, Food Systems, Front List, Healthy Ecosystems, Summer 2024 and tagged Al Kovaleski, Allen Centennial Garden, climate change, garden, gardening, Isaac Zaman, plant and agroecosystem sciences, plant hardiness zones, Reba Luiken.