Our climate’s changing, and so are hardiness maps

“I soon found myself observing when plants first blossomed and leafed, and I followed it up early and late, far and near, several years in succession, running to different sides of the town and into the neighboring towns, often between twenty and thirty miles in a day.”

– Henry David Thoreau (1856)

For eight years during the 1850s, Thoreau observed the phenology of flowering plants growing in the fields and woods around Concord, Mass. He entered detailed observations into monthly charts, recording the first flowering dates of several hundred species. He was fanatic about this project, a trait that prompted his friend Ralph Waldo Emerson to suggest that he had wasted his life away in the woods. “I cannot help counting it a fault in him that he had no ambition,” Emerson said. “Instead of engineering for all America, he was the captain of a huckleberry party.”

Sixteen years after Thoreau’s death, a Concord shopkeeper, Alfred Hosmer, continued Thoreau’s work. From 1888 to 1902, Hosmer recorded the first flowering dates of more than 700 species. Like his predecessor, he created handwritten tables from his field notes. Hosmer died in 1903.

A century later, Boston University scientists Richard Primack and Abe Miller-Rushing initiated their own study of flowering dates, copies of both Thoreau’s and Hosmer’s records in hand. According to an article in the October 2007 issue of Smithsonian magazine, “Primack and Miller-Rushing compared three years of their results with those of Thoreau and Hosmer, focusing on the 43 plant species with the most complete records. They learned that some common plants, such as the highbush blueberry and a species of sorrel, were flowering at least three weeks earlier than in Thoreau’s time. On average, they found, spring flowers in Concord were blooming a full seven days earlier than in the 1850s – and their statistics clearly showed a close relationship between flowering times and rising winter and spring temperatures.”

Like Thoreau and Hosmer, who observed plants in the wild, veteran gardeners have noticed that garden plants are blooming earlier. So have the creators of the National Arbor Day new 2006 arborday.org Hardiness Zone Map. Based on the most recent 15 years’ data available from the National Oceanic and Atmospheric Administration, this new map shows that many areas of the U.S. have become warmer since the 1990 USDA Hardiness Zone Map was published.

Much of northern Maine, listed by the 1990 USDA map as Zone 3 (average annual low temperature of minus 30 F through minus 40 F), is considered a full hardiness zone warmer, Zone 4 (minus 20 F through minus 30 F), on the new map. While other sections of Maine retain their 1990 hardiness zone rankings, this does not mean these areas have not experienced warming similar to northern areas, just that the amount of warming does not yet warrant new ranking. Indeed, Zone 5 (minus 10 through minus 30) has expanded northward from the Bangor area since 1990.

I believe that global warming is real and that it will significantly affect what can be grown in Maine gardens. I remember when I started working at the Orono campus in 1998, the old-timers in horticulture talked about Zone 4 winters in Littlefield Garden. I never saw one, and now Orono is solidly Zone 5 according to the new map.

You can download the new Hardiness Zone Map at arborday.org. There is also a map that shows which regions have warmed and an animated graphic that shows hardiness zone changes since 1990.

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to rmanley@ptc-me.net. Include name, address and telephone number.