Trees are moving north from global warming. Look up how your city could change.
By the end of the century, Alabama cherry trees might find themselves unwelcome in Montgomery, replaced by blue jacarandas, now native to Latin America. In Washington, D.C., cabbage palmettos — the state tree of Florida and South Carolina — could thrive, while Fraser firs — popular as Christmas trees — could die out.
As greenhouse gas emissions nudge temperatures higher, trees’ growing ranges are shifting northward, projections from the U.S. Forest Service show. Trees near the southern edge of their geographic ranges — what scientists refer to as “plant hardiness zones” — will be left behind, while northern latitudes will welcome new species from the south.
Click to select time period
Plant hardiness zones represent the 30-year average of the coldest temperature at each location across the country. Zones increase with each 10-degree Fahrenheit (5.6-degree Celsius) interval. Factors besides extreme cold — such as soil, rainfall and humidity — influence which trees grow.
“The hardiness zones have always been a useful tool — not necessarily a Bible — but a tool to help people think about what they’re planting and why and where,” said Dan Lambe, the Arbor Day Foundation’s chief executive.
Lambe began contemplating hardiness zones’ slow but inexorable northward migration in 2019 while attending an arboriculture conference in Knoxville, Tenn., where the hardiness zone is projected to change from Zone 7 to Zone 8 by mid-century.
“All these arborists were onstage talking about climate change with zero mention of politics whatsoever,” Lambe recalled. “They were talking about the changing palette of trees that they’re picking because of excessive temperatures.”
By the end of the century, Washington, D.C., could become too hot for trees that live in Zone 7 but not in Zone 8 — including Christmas tree species like Fraser firs and blue spruces. Meanwhile, new arrivals, like the aforementioned cabbage palmettos, also called sabal palmettos, and desert willows, which flower across the southwest and northern Mexico, could survive the D.C. winter.
in Washington, D.C.
Unlike the government’s official plant hardiness zones, which were released in 2012 and are based on temperature observations from 1976 to 2005, the projections shown here include a time range closer to the present day and allow for comparisons over time. Unfortunately, the projections are only for the contiguous United States.
These projections assume temperatures will increase by about 4.5 degrees Fahrenheit (2.5 degrees Celsius) by the end of the century, which is slightly lower than the world’s current warming trajectory, so hardiness zones may actually shift further north. Places where hardiness zones will remain unchanged could still undergo subtle shifts in vegetation as temperatures rise.
Lacking legs, trees migrate one generation at a time. Seeds hitch rides in river currents and gusts of wind or in birds’ beaks and rodents’ mouths. Then, natural selection takes over. If a seed lands in a suitable environment, it spreads roots and grows. Otherwise, it dies.
Increasingly, humans control trees’ migration patterns. Homeowners like to experiment with new species — maples for fall colors, willows for shade, stands of evergreens as windbreaks and holly hedges to block the noise from the neighbors’ garage band.
As the climate warms, horticulturalists are trying out species adapted to more southern climates. Michael Hagen, curator of the native plant and rock gardens at the New York Botanical Garden, told me recently that his colleagues are planting southern live oaks, known for the Spanish moss that drapes, ghostlike, from their limbs.
Live oaks can grow as far north as Zone 7, according to data provided by the Davey Tree Expert Company. By century’s end, they could grow in Chicago and up into Michigan, while south Florida could become too hot for them.
Just because a tree can survive somewhere new doesn’t mean you should grow it there, said Doug Tallamy, professor of agriculture and natural resources at the University of Delaware and author of popular books on ecology.
Trees “are decorations, but they have ecological functions, too,” Tallamy told me. “There is more to consider than will it live here: Will it perform here? Will it function here? Will it support the insects and then the birds that depend on that tree here?”
Some introduced tree species flourish. Common lindens, also called European lindens because of their provenance, are grown as street trees in the United States, where bees adore them. Monterey pines grow in a twisted knot on their native California coast, but in warmer places they have thrived as timber trees, tall and straight as an arrow.
Sometimes, however, species introduced to new areas become invasive. In Ohio, it is illegal to sell or plant the odorous and invasive Callery pear, an Asian native, and the Chinese pistache has become a nuisance in Texas. So before experimenting with northward creeping trees, consult a licensed arborist or your state forestry service.
Unlike most plants, trees outlive us. In this way, planting a tree is less like seeding a rose garden and more like having a child.
“If there’s one thing that inspires us as humans to think about deep time, it is planting a tree,” said Pete Smith, manager of the urban forestry program at the Arbor Day Foundation. “It’s one really tangible thing that we know can outlive us with a little bit of care and protection.”
The code I wrote to produce the plant hardiness zone map is in this computational notebook. The code for the tree species maps is in this notebook. The hardiness zone projections were produced by Stephen N. Matthews, Louis R. Iverson, Matthew P. Peters and Anantha M. Prasad. More information on those projections can be found with the U.S. Department of Agriculture.
I got the list of U.S. city coordinates from this GitHub repository and matched them to coordinates in the hardiness zone data. I filtered out cities that were further than 10 miles from the nearest datum. My code for that operation is in this notebook.
The list of tree species and their hardiness zones was provided by the Davey Tree Expert Company. Out of curiosity, I added entries for European linden, giant sequoia, Monterey pine, sweetbay magnolia and southern magnolia. I found each species’ typical growing range in the U.S. Department of Agriculture’s PLANTs database, which is good enough for government work but does not align perfectly with every species.
You can use the code and data to produce your own analyses and charts — and to make sure mine are accurate. If you do, email me at harry.stevens@washpost.com, and I might share your work in a future column.
Thanks to the talented Simon Ducroquet for the tree illustrations at the top of the story.