June 25 2019 12:00 AM

Climate zones are shifting upward, and global warming is the probable cause.

You may be wondering why some plants in your clients’ yards are blooming earlier than usual, or why plants that don’t normally survive winter are sticking around for spring. According to a recent New York Times story, the reason may be global warming.

As temperatures warm across the globe, growing zones for flowers, shrubs, and trees are shifting northward. That means the climate zone chart you’re using may no longer be accurate.

Maps show how so-called plant hardiness zones have moved over the past four decades and how they could change in the future, according to an analysis by the National Oceanic and Atmospheric Administration.

These zones were established based on the coldest temperature of the year at each location, averaged over a 30-year period to help gardeners and growers determine which plants are likely to thrive, and which are likely to die from winter cold.

Russell Vose leads the Analysis and Synthesis Branch in NOAA’s National Centers for Environmental Information. In the story, he’s quoted as saying “hardiness zones are creeping north systemically” to higher latitudes and elevations. That means “you can probably grow some things farther north than you used to be able to.” But, he added, you still can’t “plant a banana tree outside in Central Park.”

As any landscape professional knows, how cold your winter gets determines what plants survive from year to year. Lemon trees, for example, are very sensitive to frost and best suited for hardiness zones 9 to 11, which tend not to dip below 20 degrees Fahrenheit. Sweet cherry trees, by contrast, can withstand colder winters, thriving even in zone 5, where temperatures can reach -20 degrees Fahrenheit.

Other factors, such as light, precipitation and soil type also affect how well plants survive in any specific location.

Average Winter Lows in Each Hardiness Zone:

Zone 3: -40°F to -30°F

Zone 4: -30 to -20

Zone 5: -20 to -10

Zone 6: -10 to 0

Zone 7: 0 to 10

Zone 8: 10 to 20

Zone 9: 20 to 30

Zone 10: 30 to 40

When New York Times readers were asked to describe how they saw climate change affecting their area, several people reported that they were already changing their planting habits due to balmier winter conditions.

“I am now able to grow perennials that were once two temperate zones south of me,” says a Buffalo, New York resident in the article. And a man who lives in Hampton, Virginia, reportedly says “I overwinter plants that once had to be dug up and protected.”

Some readers noted changes to their official plant hardiness zones. They cited data from the United States Department of Agriculture, which maintains a similar, but more detailed map of hardiness zones.

Although the Agriculture Department’s map is the official standard for determining what to grow in one’s garden today, it doesn’t have much to say about climate change, the story’s author says. The significant changes the agency made to its map-making process between the latest version, released in 2012, and an earlier map from 1990, make it impossible to separate the effects of global warming from other differences.

NOAA’s maps were intended to answer the climate question more specifically. Scientists at the agency applied the same methodology to each thirty-year time period (1971 to 2000 and 1981 to 2010) so they could do comparisons between maps. They projected the trend into the future to get a better idea of how hardiness zones could continue to shift between 2011 and 2040. Data from the past decade was not included in the analysis, but this period has been the warmest on record worldwide.

Recent warming can be largely attributed to human emissions from fossil fuels, according to the National Climate Assessment.

Christopher Daly, a senior research professor at Oregon State University who helped develop the Agriculture Department’s 2012 map, noted in the story that while growing zones may be gradually pushing northward, a single cold snap can still wipe out less-hardy plants. NOAA’s hardiness maps show how winter lows are warming on average, he says, but don’t tell us about potential changes in the year-to-year volatility of extreme cold.

However, in the story, Dr. Vose called the maps “a good place to start” when considering the effects of climate change on local vegetation.

Warming minimum temperatures “might mean I can safely grow things now that I didn’t grow before, but by extension there may be some species that start to naturally grow where I live that didn’t used to grow there,” he’s quoted as saying. “Hopefully they’re not invasive species, like kudzu, but it’s a possibility.”