Only every decade or so does the U.S. Department of Agriculture release the latest edition of the Plant Hardiness Zone Map, a long-term project that details what kinds of plants thrive under different conditions across the country. By using detailed data on the average lowest annual winter temperatures for the last 30 years, the latest map (released in late 2023 and closely analyzed by gardeners ever since) vividly places parts of the country into color-coded zones split up by 10 degrees Fahrenheit and broken up further into 5-degree Fahrenheit half-zones.
Where the 2023 edition shines the most over its 2012 and 1990 predecessors is in the level of granularity and precision within its color gradients. Zone changes from county to county can now be examined thanks to enhanced data collection and improvements in the computational model used in the map’s creation.
“And we also brought in data from upper air data from radiosondes — balloons that go up in the air and sample temperatures at higher elevations — to help us guide our high elevation interpolation. So we’ve tried to improve the detail as to what you can see more effectively this go around,” says map lead author Christopher Daly, a professor at the College of Engineering at Oregon State University.
A deeper glance at the patterns and zone changes over the years reveals some stark changes. The 2023 map as a whole is about one quarter-zone warmer than it was in 2012. Nationwide, it’s about 2.5 degrees warmer than what was displayed a decade prior. Underpinning it all is a familiar, and inconvenient truth: the planet continues to warm as climate change marches on. And for the 80 million gardeners and growers who rely on the map to determine which perennials will thrive in their area, the future remains uncertain.
Knowing the baseline climate and growing condition a native plant thrives in is the first step in any plan, explains Kathleen Strahan, the horticulture community educator at Cornell Cooperative Extension’s (CCE) Westchester County office.
“Once you understand that, you can unlock stuff about any house plant or any garden plant,” Strahan says. “You end up learning a lot about and understanding the environment as a whole, too, because for the most part, a plant needs light, water, and probably fertilizer.” For experts like Strahan and the local gardeners she and her colleagues assist, the issues they’ve seen are usually disruptions to one of those key elements. The composition hasn’t really changed; the milkweeds, trilliums, goldenrods, and other native plants are all still there, but it’s been harder for some growers to adapt to increasingly rougher weather.
Usually a reliable cool point in the season, the first few months of the year have been seeing more than their fair share of warm spells. Even spring doesn’t guarantee stability; this past August, New York state issued a USDA disaster designation for farms across 31 counties after a particularly harsh deep freeze hit in May. Climate change also brings more extreme swings, and that can include unseasonably cold temperatures in the short term, even though the climate is warming overall. That inconsistency can lead some to try changing up their practices.
“It’s the back and forth that makes it a lot harder to be doing things like feeding in the ground,” Strahan says. “Most people will wait later in the season to do that just to make sure that their plants don’t freeze, but if we’re getting kind of unpredictable springs, it’s really hard to do.” In the fall months, without the gradual cold setting in, warm weather can make troublesome pests and fungi like rusts, volutella blight, and bot rot linger, she explains.
The record rainfall, heatwaves, and other weather events across the Hudson Valley have been a struggle for even the most experienced gardeners. “The entire garden ran about a month behind. Even though I have drip irrigation throughout my garden — that’s off of well water that just kind of keeps things alive — stuff didn’t thrive,” says Christopher Harrison, Warwick resident and principal at Harrison Re-Gen. “The lettuce didn’t pop until we got into June, and then all of a sudden we got a deluge and it never stopped raining.”
While some may try to adapt by changing the types of plants they grow, that can backfire since change is so gradual. For instance, Westchester County is currently at around zone 7A. Planting something meant for a zone 8 might sound like a good strategy, but there’s no guarantee said plant would survive the current zone we’re in now. Seeking out resources like CCE branches and local gardening groups can help gardeners and growers adapt as time goes on.
Anecdotally, the changes these last few years have felt pretty rapid, especially when current “climate normals” are based on 30-year averages. That focus on 30-year averages, instead of something like 10 or 15 years, creates normals that account for natural variations in climate.
“You may be on an upswing or a downswing in that last 15 years, but 30 years is more likely to encompass both an upswing and a downswing,” Daly says. “You know, a peak and a trough, that would be more reflective of what the baseline climatology is actually doing.”
