It is not your imagination – in many areas, spring perennials emerge earlier and plants that once didn’t make it through the winter now flourish.
At least 18 of 34 major U.S. cities landed in new, warmer zones on the updated USDA plant hardiness map including Kansas City, St. Louis, Chicago, Tampa, Des Moines and even Fairbanks, Alaska. The January 2012 update, the first since 1990, affected entire sections of some states, including Texas, Nebraska and Ohio.
Part of Minnesota eked into Zone 5 (winter lows of -20 degrees F and above). Japanese maples now survive in swaths of both Minnesota and Iowa. Folks are growing figs in Boston and Pennsylvania is more hospitable to Southern magnolias.
Here in Nashville, we’re now in Zone 7A, up from a 6B. The difference? An average annual extreme minimum temperature of 0 to 5 degrees (F), from -5 to 0. Those 5 degrees matter if you consider paging through plant catalogues in flannel pajamas window shopping at its finest.
Zone 7 is the outside limit for tropical-tending ornamentals. Cannas, for example, should fare well near the south-facing back wall of our home here in Music City – without the hassle of hauling planters inside for the winter.
The 1990 map was based on temperature data from 1974 to 1986. The 2012 update uses temperature data from 1976 to 2005 and adjusted for prevailing wind, slope and proximity to bodies of water and heat islands.
The new map allows users to input a ZIP code to get more precise averages of the coldest annual temperature. But before you start planting dahlias and banana trees, keep in mind:
- Some regions shifted cooler, especially those in the mountains.
- Slope matters. Cold air settles at the base of an incline or hill.
- Natural and man-made microclimates create pockets of warmth or cold.
The USDA map is a guide – not the last word. It makes official trends that gardeners already observed. Even then, some researchers believe the 2012 map was outdated when released and much of the U.S. belongs to even warmer hardiness zones.
Nir Krakauer, an assistant professor of civil engineering in New York, created an online calculator that provides adjusted temperature changes, based on latitude and longitude. Krakauer, who teaches at the Grove School of Engineering at City College of New York, factored in long-term temperature trends using newer data that shows winter temperatures are increasing more rapidly than summer temperatures. He published his research in September 2012 in Advances in Meteorology.
In New York City, his numbers show the winter low is 2.7 degrees warmer than the USDA map reflects. Of course if more plants survive the winter, one thing is certain – more bugs will survive with them.
What have you noticed grows well now in your area that did not years ago? We’d love to hear about your experiences.