Horticulture and Hardiness Zones

In late 2023, the U.S. Department of Agriculture announced that it had retooled its Plant Hardiness Zone Map for the first time since 2012, with help from Oregon State University’s PRISM Climate Group. If you’re not familiar with this map, it’s described by the USDA as “the standard by which gardeners and growers can determine which plants are most likely to thrive at a location.”

The map is separated into 13 10-degree Fahrenheit zones, based on 30-year averages of the lowest annual winter temperatures. Each zone is further broken up into “a” and “b” half zones. If you walk into any flower shop or look at online information on a particular plant species you’d like to put in your yard, you can find seed packs and plant tags that list the appropriate hardiness zone to provide optimal growth for the plant.

This map, the USDA says, incorporates data from 13,412 weather stations, which is quite a bit more than the previous iteration. The USDA made additional changes to ensure that the map is more useful to gardeners around the country. Read on to learn about how these changes affect your home garden as well as the accredited gardens throughout Lincoln Park Zoo.

Is Climate a Factor in These Map Changes?

If you look at the map now, compared to the previous version, you would see that half the country or so has shifted into the next “a” or “b” half zone. In other words, those areas are warmer than they used to be by 0-5 degrees.

Now, this doesn’t necessarily mean climate change is a factor in the way this specific map looks. Extreme temperature variations, along with more sophisticated information-gathering methods and the data from more stations, could simply reflect that the map is becoming more accurate.

But that doesn’t mean the environment is staying the same either. Any gardener can tell you anecdotally they’ve seen some changes in the growth patterns of plants over the past few years. At Lincoln Park Zoo, which is a science-led organization, we’re gathering data to document the changes.

Changes in Chicago

Since 2012, the 60614 zip code in Chicago (where Lincoln Park Zoo is located) has been in Zone 6a, with low annual temperatures between -10 to -5 degrees Fahrenheit. Now, it has moved to zone 6b, with its annual low being -5 to 0 degrees F on average. The map notes this is a +5 degree change. But this isn’t true for all of Chicago. The 60622 area code, which covers the Wicker Park neighborhood, remains in 6a—although it did have a +4 degree change.

USDA Hardiness Zone Map, Chicago zip code 60614 area

“We anticipate that we will start seeing a shift in the plants we can grow here,” says Katrina Quint, the zoo’s director of horticulture. Zone 6 hibiscus, which grow on zoo grounds, continue to thrive, but experts expect to see conditions more conducive to the growth of southern plants appear in the Chicago area. However, without a consistent weather pattern, it’s hard to predict what the actual conditions may be.

In fact, the information gathered has been surprising so far. She points to data from the last four years, tracking all the accessions of hibiscus that have taken place on grounds with the zoo’s nationally accredited collection. Quint says zoo researchers are seeing something experts might not predict in the face of warming weather, which should cause earlier peak bloom times. Instead, she explains, “We are seeing peak bloom time shifting later into the season by a week and a half.”

What This Means for City Gardeners

Local gardeners don’t necessarily have to do anything as a result of the map changes. If there are plants that have thrived in your yard for years, they probably will continue to do so.

One unique aspect of city gardens is that urban areas often experience slightly different conditions based on where they’re located. Differences in temperature, light, wind speed, moisture, and other factors are meaningful in determining how well certain plants do in an area. Because they cause atmospheric conditions to vary in certain small regions, these areas are called microclimates.

So, even though the city of Chicago lies mostly in Zone 6 (with suburbs further out veering into colder Zone 5 territory), small portions may lie in microclimates due to the presence of concrete, tall buildings, fenced-in areas, and other features. In other words — the USDA Hardiness Zone map is a good guide, but your mileage may vary and you’ll have to adjust based on the unique conditions where you live.

Tulips are flowers that thrive in Zone 6 conditions. Image courtesy of Katrina Quint

When you’re choosing new plants, though, you might want to keep the zone classification adjustments in mind. The new information does mean different hybrids or other cultivars that you weren’t used to being able to grow here might flourish. “It opens up the potential gardener’s palette,” Quint says. “Although with a caution: we are still probably going to get low temperatures that are detrimental to many plants.”

Grow With Lincoln Park Zoo

With climate conditions moving so quickly, research needs to be done to understand the alterations that are being reflected in the USDA Plant Hardiness Zone Map. That’s because the more we know, the more we can adjust how we live, work, and play to meet the effects of climate events. Building climate resiliency begins with knowledge.

To that end, the zoo’s Horticulture team is keeping track of local trends, along with phenology data (about the timing and cyclical patterns of natural events). As an accredited arboretum, Lincoln Park Zoo is using its resources and expertise to collect data that can help scientists get a handle on how the planet is changing, and how we can change with it.

In the meantime, there are definitely ways everyone can help and Take Action With Us! Make climate-friendly choices such as eating more plants, reducing food waste, and spreading the word about easy ways to change our food habits.

For more information on the plants that thrive here on grounds in Zone 6b, visit the Plants & Gardens section of the zoo’s website. You can also discover exactly where to find them at zoo’s Garden Explorer.  And as spring draws ever closer, consider taking one of the free Second Saturday tours that run May-September with a member of the Horticulture staff.