This article first appeared in the St. Louis Beacon, March 7, 2012 - When I dropped my husband off at the airport the other day, the saucer magnolias were already in bloom, nearly a month early. So, we’ve all noticed this winter has been particularly mild. Don’t get spoiled by it. Let’s all just hope that this is one of those occasional, end-of-the-sigmoid-curve extremes and not evidence of catastrophic meteorological change on a grand scale.
Change is surely afoot, as this review of the new USDA 2012 Plant Hardiness Zone Map (PHZMap) will show, but it is a more gradual shift and not one that puts Alabama weather in our region as a regular routine … yet.
What does a Plant Hardiness Zone Map show?
These maps are illustrations of winter minimum temperatures over time. One of the main predictors of plant survival, winter cold can wipe out a plant in one icy slash. Other factors that affect survival, in combination with cold, include summer heat, total degree-days, rainfall, soil type and pH. A standard PHZMap will average low temperatures over a number of years and then depict these numbers through color-coded zones on a base map. Each zone, now standardized at 10°F increments, defines the northern limits for winter plant hardiness.
There have been many map versions through the years, from the USDA, the Arnold Arboretum at Harvard and The Arbor Day Foundation. The oldest copy I’ve seen (from 1960) was shared with me by Kim Kaplan at the USDA. This generation of map was vague in the zone boundaries, with long, smooth, sweeping sectors. They were all hand drawn and based on unpublished data. They had no common definition of zones as we do today. The main map we gardeners have been using, the one on the back of seed packets and in reference books, is also from the USDA and was released in 1990.
In 1999, the first heat-zone map was developed and featured in the book "Heat Zone Gardening" by H. Marc Cathey, former director of the National Arboretum in Washington, D.C. In it, he presented a new mapping concept based on annual high temperatures and the ramifications of them on defining the southern boundary of plant distributions. This map may be found online at the American Horticulture Society website . (The link goes to a PDF, and the map is on page 5 of that document.)
With the measureable increases in average annual temperatures, we have seen many indictors that our growing conditions have been changing. An incremental USDA map update, the first designed for use on the web and with county lines and zip code indexing, was published in 2003. But for the most part, this change has come quietly, as a subtle shifting baseline, and most folks would overlook it.
As a seasoned mariner might gaze at the night sky and be able to circumnavigate the globe, a well-traveled gardener can look at the landscape and tell by the plants growing there which zone one is in. Indicator species, groups of plants that grow well together at particular temperatures, help us identify these zones. My husband and I often laugh at movies that have been set, for filming convenience, in places that the vegetation doesn’t match the story line. We spot things like the 20th century weed, kudzu, in a film supposedly set in 1895.
Peter has even been used as an expert to identify the locale of classified government photographs by the background plants included in them. This knowledge of plants and their growing habits clued us in years ago that the temperatures here in St. Louis were changing measurably. And the USDA was not able to keep up. It has no designated staff for producing these complicated maps.
As the climate warmed, it was obvious that updated maps were needed to guide us in safe plant selection. The Arbor Day Foundation (ADF) took the public domain federal weather data through 2005 and introduced a revised map on the internet in 2006. It was this PHZMap version that started making people understand how quickly our gardening world was changing. From the ADF press release of the day “The new 2006 arborday.org Hardiness Zone Map (note: The map that is linked is animated to show the change.) is consistent with the consensus of climate scientists that global warming is underway. Tree planting is among the positive actions that people can take to reverse the trend.”
In a major new tree planting initiative by Arbor Day Foundation, with the U.S. Forest Service and funded by a stunning gift from Enterprise Rent-a-Car, plans were unveiled to plant 50 million trees, at a million trees a year, over the next 50 years. Talk about a golden anniversary present!
Now, six years later and based on much the same data (also ending in 2005), the newest 2012 USDA version is out and, with sweeping changes, invites comparison. The three national maps I have used for this review are the 1990 and 2012 USDA Plant Hardiness Zone Maps and the 2006 map from The Arbor Day Foundation (ADF). The 1967 Arnold Arboretum map has not been included because the calibration of zones is not on the same degree register making apples-to-apples comparisons difficult.
The press release announcing the new USDA map said “The new map is generally one 5-degree Fahrenheit half-zone warmer than the previous map throughout much of the United States.” And that, ladies and gentlemen, is a significant and substantial shift.
When the 1990 USDA map was first published, St. Louis was on the edge of Zones 5 and 6, with urban city blocks and their heat-retaining hardscapes creating microclimates that would support some plants in sheltered locations to 6B. Today, much of Missouri is in Zone 6, and we are slipping quickly through 6B on the way to Zone 7 in protected locations. That is … according to the map.
Some Zone 7 plants include aucuba, mahonia, nandina, camellia, crape myrtle, English holly and osmanthus are already being grown here in sheltered spots. All of these are my favorites from childhood in a then-Zone-7 Charlotte, N.C., (which, by the way, is now in Zone 8). We have overwintered here for several years broadleaved aucuba and evergreen nandina. Glossy abelia, another Southern standard, easily makes it through. My yellow-flowered mahonia is in full bud, ready to burst open on the next warm day.
To see how these maps play out over time, follow the edge of Zones 5 and 6. Much of Missouri was Zone 5 in the 1990 map. The southern edge slid all the way down into Douglas County, beyond Fort Leonard Wood. In the 2012 map, that margin is now all the way up to Macon County, a distance of more than 200 miles. Totally shocking is the 2006 ADF map, which pictures a Zone 5 that has retreated all the way to Keokuk, Iowa, a northerly shift of almost 80 additional miles. If you divide the change in distance by the years between data sets, it comes to a scary shift of zone edge migration of more than 10 miles a year moving northward.
If that trend is steady, we could have the climate of New Orleans here in our own gardens before the end of this century. Doesn’t that begin to show the extent of the problem? Not only will our existing plants have a hard time adjusting, many native plants will be wiped out. And the influx of invasive species … well, it won’t be pretty.
When I started comparing the maps from 2006 ADF and 2012 USDA, it seemed there was little difference between them. Both show the clear and drastic northward zone boundary migration. Searching for an explanation, I discovered that both maps were based on data sets ending in 2005. The primary difference between the two is that the 2012 map uses 30 years of data and the 2006 map uses 15 years. According to Sean Barry, at the Arbor Day Foundation, the choice of data range was pegged to begin with the 1990 map and run through the most recent data available at that time to illustrate the changes that are occurring. “If you use too large an interval, the informative power is diluted” and the nature of change becomes obscured.
To compare some specifics, I charted a list of cities following a route up the Mississippi River Valley. Comparing places along the way from all three maps, most locations have seen a shift of about half of one zone warmer (or about +5°F) on both the 2006 and 2012 maps over the 1990 version. The Arbor Day Foundation map, however, shows a greater degree of change for New Orleans, (was Zone 9, now 10), Keokuk, Iowa, (was Zone 5, now 6), and La Crosse, Wisc. ( was Zone 4, now 5). The most dramatic zone shift along the river was with Minneapolis, previously placed at the boundary of Zones 3 and 4 in 1990, the 2006 map shows it at Zone 5, the same as northern Missouri from the 1990 map. In all the cases of comparison, the 2006 ADF map showed warmer zone shifts than the 2012 USDA map. This is likely due to the different spans of time in the data sets used to generate the maps.
Why Do These Maps Matter?
If I plant a few vegetable seeds early, it is only a few dollars and hours of time. If you are a large-scale farmer with acres and acres of land, seed is measured in tons and time for planting hundreds of labor hours. To help us figure out the normal planting times and which plants to grow, most modern farmers and gardeners turn to the US Department of Agriculture (USDA) for data-based information about degree-days, sophisticated satellite mapping, precipitation records and plant hardiness zones.
Home gardeners always love to “push the zone” by siting plants in very protected locations or wrapping, mulching and insulating plants to carry them through the cold. Tony Avent, owner of Plant Delights Nursery in NC (and classmate of mine), says that he never believes a plant won’t grow in his garden until he has “kilt it myself three times.” But most commercial growers do not have that luxury. They need to get it right the first time or face financial ruin. We gardeners and farmers all depend on predictable weather to plan, plant and profit from our home gardens and commercial crops.
Most folks like to chat up the benefits of the changing climate in Missouri -- the resort town weather we have been enjoying with longer spring and fall seasons added to the taming of winter from vicious bite to gentle nip. Hey, how about golfing here in mid-winter? And some of the benefits are really nice.
Three years ago, plantings of camellias in the English Woodland Garden, at the Missouri Botanical Garden, overwintered and bloomed. This year, they are stunning! And formerly deciduous “evergreen” magnolias are now consistently evergreen. Twelve years ago, when I lived in Houston, I had to drag my potted Meyer lemon into the kitchen for the six weeks of winter. Now, Houston is near the middle of Zone 9 and citrus trees will grow there in the ground.
Some gardeners will look with delight on the expanding palette of plants we may now grow in our home landscapes. We must look beyond that.
The northward zone front migration is significant. It means that the climate is measurably changing around us while we watch and whistle in the dark. We need to wake up and smell the roses, the roses that will be blooming in St. Louis in December by the time my baby grandson graduates from college.
The seasonal timing of events in the garden and the greater wild world is an elegant, intricate and complicated web with living elements that are highly interdependent. This kind of weather change stresses, then breaks, all of those links and begins to degrade the integrity of the fabric of life.
How do you know what to believe about the whole climate change controversy? I would rather get my information from a century of weather records than from a radio talk show host. This new USDA map, long in coming, takes us one step closer to visualizing how the upward trend in our winter weather will affect our growing conditions.
We need to thank our national legislators and USDA staff in Washington for finally getting this 2012 version out, even if it is based on data more than seven years old. In its new internet presentation and spiffy ½ mile grid of details, it is the best scientific weather picture we have for growing plants, even though it is already seven years out of date. The USDA website is running at a million hits a day, which proves that there is need and interest in the map. But with the rate of change coming so quickly, this map needs to be updated annually.
Eighty million home gardeners and business consumers support a huge landscape and gardening industry. In recent revisions of the U.S. “green industry” revenues by Hodges, Hall and Palma (2011), the economic impact of home gardeners, greenhouse growers, landscape architects, lawn services and other green industry businesses is more than $175 billion in annual output and provides nearly 2 million jobs. If you add commercial forestry and agriculture, the numbers of people who depend on this information are staggering.
Is it too much to ask for an updated map based on contemporary data? The heavy lifting of creating the algorithms and ground-proofing the results with real scientists and growers across the country has been done. Plugging in new weather numbers as they accrue should be the easy part.
There are many, many models predicting the future of temperature change. They use complex algorithms, multiple variables and very sophisticated computer software. No matter which data sets you use or what method of smoothing curves is applied, what all these reports indicate is that the world around us is change 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 should be jumping out of the pot.
Full size 1990 map: planthardiness.ars.usda.gov/PHZMWeb/Images/USZoneMap.jpg
Full size 2012 map: www.planthardiness.ars.usda.gov
Full size Arbor Day foundation map: www.arborday.org/media/zones.cfm
How the Maps Were Made
The 1990 USDA map used data from 1974 to 1986, a 12 year average, with 4 years to publication.
The 2006 Arbor Day map used data from 1990 to 2005, a 15 year average, with 1 year to publication.
The 2012 USDA map used data from 1976 to 2005, a 30 year average, with 7 years to publication.