The climate of my Southern hometown, Charlotte, N.C., has followed me to Missouri. I’m not talking about this winter, since it is an extreme year and not ‘normal’ under any stretch of thinking. What I’m referring to is the shifting of winter plant hardiness zones northward as shown on the 2012 USDA Plant Hardiness Zone Map (PHZM). Recently released, it reveals a dramatic shift in our winter low temperatures that will have a huge impact on our landscaping and local food production.
PHZMs are the gardener’s Holy Grail for figuring out what to grow based on the geographic trends of temperature and a specific plant’s tolerances. While winter cold is not the only factor, it is a defining limit. When the 1990 USDA map was published, St. Louis was on the boundary of Zones 5 and 6, with urban city areas and their heat-retaining hardscapes creating some microclimates that would support plants in protected locations to Zone 6B. Parts of St. Louis that are less heavily developed and at a slightly higher elevation were still in the grips of the retreating colder Zone 5.
Through the years, we have noted the winter survival of plants that have not been hardy for us historically, such as camellias, aucubas, nandinas and tree-form crape myrtles. Some of this new success is the result of breeding for winter hardiness, but much of it has been due to the rising bottom of our winter cold minimums. Anecdotal evidence of temperature change has been accruing for quite a while. Many plants could survive but with extreme winter damage, like evergreen magnolias and chaste trees. Just drive by Tower Grove Park to see glorious southern magnolias now tipping up toward 30 feet. In the past, these trees would lose their leaves every winter and sometimes be killed back to a stump.
The Arbor Day Foundation, alert to this shifting climate, was agile in utilizing the open data of the National Climate Data Center to produce a new map in 2006. It was the first to illustrate, from hard data, the changing scene in our growing conditions. Now, in 2012, the USDA has released its version, in an interactive web format, that uses much of the same data but with finer detail. And the conclusions are clear: Most of the U.S. is considerably warmer now in the winter than it was in the last century. Most of Missouri falls in Zone 6 now, and we are sliding quickly through Zone 6B on the way to Zone 7 in sheltered locations in St. Louis City.
ZONE FRONT MIGRATION – IT IS GETTING WARMER HERE
Looking for the historic interface, the common edge of Zones 5 and 6, I found that most of Missouri was in Zone 5 in 1990. The southern edge of this zone dipped all the way down to Douglas County, south of Ft. Leonard Wood. With the 2012 map, that margin is now all the way up to Macon County, a distance of more than 200 miles. If you divide the distance by the number of years between data sets, it comes to a shocking shift of zone edge of more than 10 miles a year northward. If that trend continues, in 70 years, we could have the climate of New Orleans here in our own gardens!
WHAT DOES THIS ZONE SHIFT MEAN?
Some gardeners will look with delight on the expanding palette of plants we may now grow in our home landscapes. More okra, anyone? Our range of hollies and boxwoods also will expand. More broadleaf evergreens will be available.
The dark side of this scenario is that with the warming winters come the northward migrations of bad actors, as well. We know that armadillos have made it to the Shaw Nature Reserve already. Fire ants arrived in the Bootheel in 2009 and are marching in our direction. Julie advises me that the disgusting little daylily leaf miner is coming to town and that daylily rust has already been here for a decade. Azalea lace bug, from the milder climates of Japan, spent last year making all of the leaves silver with their tunnels and feeding holes. And the most dreaded King Kong of the botanist’s nightmare world, Kudzu, has already crossed the line into Missouri. Lay in a good supply of remedies for mosquitoes and fleas. Lack of winter cold to kill off resident populations of these nasty critters is a particular insult from this super-mild winter.
The worst scene in this temperature-shifting is for native plant species. Plants don’t migrate quickly like many insects and other animals do. Some predictions estimate as many as half our native species may be gone by the end of this century, the changing climate a major cause.
There are many, many models predicting the future of temperature change. These reports, and our own gardening experience, indicate is that the world around us is changing rapidly. We all know that the old folk tale about a frog in hot water, while not literally true, is used as a metaphor for slow change being overlooked. What these new maps show with empirical evidence is that the growing conditions in our St. Louis gardens have changed a great deal over a relatively short period of time. We, as gardeners, are among the first to take real notice.
Patricia Raven, Ph.D., has a doctorate in ornamental horticulture and Julie Hess is senior horticulturist at the Missouri Botanical Garden.