For years, Arbor Day Foundation members have told us the climate conditions for tree planting were changing. According to last week’s release of new plant hardiness zones from the U.S. Department of Agriculture, they’re right.
A hardiness zone documents the average lowest winter temperature in a given region. This is key information for growing, as it determines what level of cold-related stress a tree can handle.
The official map was long overdue for an overhaul. Those who had relied on the USDA’s 1990 map found it to be severely out of date, prompting the Arbor Day Foundation to produce its own map in 2006 to more accurately reflect conditions in backyards and neighborhoods across the country.
The shift, for most of the country, is palpable. A comparison between the USDA’s 1990 map and the Arbor Day Foundation’s 2006 map reveals large swaths of the country moving up a zone (see below) The new 2012 USDA map broadly resembles what the Arbor Day Foundation map indicated back in 2006.
Here in Nebraska, most of the state used to be in Zone 4 but is now in Zone 5, signifying a shift in the interval of coldest average temperature from between -30 and -20 to between -20 and -10 degrees Fahrenheit. Zone 11 used to be limited to Hawaii, but now can be found in parts of Southern California and Florida.
While a shift by one zone doesn’t always have a huge impact on what can grow, it definitely matters at the margins. For instance, the Yoshino Flowering Cherry Tree, a favorite in Washington, DC’s Cherry Blossom Festival, is recommended for hardiness zones 5 through 8. Colder areas like New England and the upper Midwest that shifted from Zone 4 to Zone 5 can now conceivably plant Yoshinos in their backyards. (You can also purchase your own cherry tree – and cast your vote for America’s Favorite Cherry Tree).
Another example: The Southern Magnolia thrives in Zones 6 through 10. Twenty years, it could not have grown in most of Ohio, Michigan and Pennsylvania, but now it can be considered by an adventurous tree planter in his or her yard. Or, take the Arizona Cypress, a zone 7 through 9. Now it is now a viable choice in much of New Mexico and Texas.
The warming indicated by the new map has inevitably renewed discussion about climate change. This is a worthy topic, especially as trees are acknowledged as part of the solution.
When we plant trees in our yard, they immediately go to work for us. The shade provided by trees during hot summer days not only makes life more comfortable, it also reduces the energy consumed through air conditioning by as much as 40 percent. In winter time, well-placed trees can block bitter winds, resulting in reduced energy use from heating. And, a parking lot with 50 percent shading can eliminate one ton of emissions per day, while extending the life of the pavement.
When households demand less energy, we save precious resources and reduce the greenhouse gas emissions that contribute to climate change because utility companies save energy during periods of peak demand. When mature trees absorb carbon dioxide in urban areas, harmful pollutants are removed from the atmosphere and are naturally regenerated.
Many Americans are excited to explore new species of tree previously untenable in their region. Purchasing and planting trees is also a low-tech and low-cost way to mitigate an enormous environmental challenge.