How Climate Change May Affect the Plants in Your Yard – The New York Times
As temperatures warm across America, growing zones for flowers, shrubs, and trees are shifting northward.
Plant Hardiness Zones
Based on 30-year average ending in:
The maps above 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 — based on the coldest temperature of the year at each location, averaged over a 30-year period — help gardeners and growers determine which plants are likely to thrive, and which are likely to die from winter cold.
Hardiness zones “are creeping north systemically” to higher latitudes and elevations, said Russell Vose, who leads the Analysis and Synthesis Branch in NOAA’s National Centers for Environmental Information. That means “you can probably grow some things farther north than you used to be able to,” he said. (But, he added, you still can’t “plant a banana tree outside in Central Park.”)
How cold it gets in winter is an important factor determining what plants are able to survive 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°F.
Other factors, like light, precipitation and soil type, also affect how well plants can survive in any specific location.
Average Winter Lows in Each Hardiness Zone
When 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,” wrote William Borucki, of Buffalo.
Raynard Vinson, of Hampton, Va., wrote: “I overwinter plants that once had to be dug up and protected.”
In some cases, readers noted changes to their official plant hardiness zones, citing values from another federal agency: the United States Department of Agriculture, which maintains a similar, but more detailed map of hardiness zones.
The Agriculture Department’s map is the official standard for determining what to grow in your garden today, but it won’t tell you much about climate change. The agency made significant changes to its mapmaking process between the latest version, released in 2012, and an earlier map from 1990, making it impossible to tease out the effects of global warming from other methodological differences.
NOAA’s maps were intended to answer the climate question more specifically. Agency scientists applied the same methodology to each thirty-year time period (1971 to 2000 and 1981 to 2010) so they could compare between maps. And they projected the trend into the future to get a better idea of how hardiness zones could continue to shift during the period spanning 2011 to 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 that, while growing zones may be gradually pushing northward, a single cold snap can still wipe out less-hardy plants. NOAA’s hardiness maps capture how winter lows are warming on average, he said, but they don’t tell us about potential changes in the year-to-year volatility of extreme cold.
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 said. “Hopefully they’re not invasive species, like kudzu, but it’s a possibility.”