Warming shifts gardeners’ maps
By Elizabeth Weise, USA TODAY
Every gardener is familiar with the multicolor U.S. map of climate zones
on the back of seed packets. It’s the Department of Agriculture’s
indicator of whether a flower, bush or tree will survive the winters in
a given region.
It’s also 18 years old. A growing number of meteorologists and
horticulturists say that because of the warming climate, the 1990 map
doesn’t reflect a trend that home gardeners have noticed for more than a
decade: a gradual shift northward of growing zones for many plants.
The map doesn’t show, for example, that the Southern magnolia, once
limited largely to growing zones ranging from Florida to Virginia, now
can thrive as far north as Pennsylvania. Or that kiwis, long hardy only
as far north as Oklahoma, now might give fruit in St. Louis.
Such shifts have put the USDA’s map at the center of a new chapter in
the debate over how government should respond to climate changes that
were described in a report last year by a United Nations-backed panel of
scientists. The panel said there was “unequivocal” evidence of global
warming fueled by carbon dioxide emissions, which have created an excess
of the greenhouse gases that warm the Earth.
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Climate change is reshaping how people garden. Across the agricultural
industry, the issue is driving a dispute over climate maps that involves
economics, politics and meteorological standards.
At nurseries across the nation, it has become common knowledge that the
government’s climate map is out of date. And yet the nursery industry,
which had $16.9 billion in wholesale sales in 2006, has joined the USDA
in taking a conservative approach to changing the map.
A big reason: money.
Nurseries commonly offer money-back guarantees on plants. Analysts say
many in the industry are worried that adjusting the climate maps would
encourage customers in cooler areas to increasingly buy tender,
warm-weather plants unlikely to survive a cold snap.
And growers are worried that their losses won’t be sufficiently covered
by the Federal Crop Insurance Corp.’s Nursery Crop Insurance Program,
which covers them for losses caused by weather-related events such as
flooding. If growing zones move north because of warming there is still
a possibility of cold snaps, and it’s unclear exactly how insurance
programs would deal with that risk.
The USA’s climate zone map designates 11 major belts for growing plants,
from the relative cold of Zone 1 ? which includes Fairbanks, Alaska ? to
midrange temperatures of Zone 6 (which includes parts of Missouri,
Tennessee and southern Pennsylvania) to the heat of Zones 10 and 11,
which include Hawaii and southern Florida.
Changing zone boundaries to reflect warming could “have a significant
impact on certain growers of certain plant species,” says Dave Hall of
National Crop Insurance Services, which represents insurance companies.
Economic factors shouldn’t be placed above the science of climate
change, says meteorologist Mark Kramer, who worked on the 1990 USDA map
that remains in effect, as well as a proposed update in 2003 that showed
a warming trend. The USDA rejected the 2003 map.
“If nature changes, industry should change with it,” Kramer says. “If
the weather changes, we shouldn’t operate with zones and systems that
USDA officials reject suggestions that the agency’s resistance to
changing the 1990 map reflects a reluctance to acknowledge the potential
impact of climate change. They say the agency wants its next map to
reflect a 30-year period that gives a fuller picture of the world’s
climate than the 16-year examination Kramer conducted for his rejected map.
“The majority of the scientific community thought 30 years of credible
data made the most sense,” says Kim Kaplan of the USDA’s Agricultural
Kramer and other skeptics say the USDA’s tactic will lead to an analysis
that mutes the effect of warming trends during the past decade.
The agency’s delay in releasing an updated map has led another group to
release its own climate map. In 2006, the Arbor Day Foundation put out a
map based on data from 1991 to 2005 that shows a significant northward
movement of warm zones for plants and crops.
“Everyone’s entitled to their opinion,” Arbor Day Foundation’s Woodrow
Nelson says of the USDA map. But he says his group, which provides
low-cost trees, was seeing trends that it wanted reflected in a map for
“With the millions of trees that we’re putting into the hands of people
across the country, the most recent data available is important. Data
from 30, 40 years ago is really kind of irrelevant in the life of a
Avid gardener Toni Riley, who lives on a small farm in Hopkinsville,
Ky., with her family and a cadre of dogs, cats, sheep, goats and a
horse, also values the most up-to-date information. “What I plant
depends on the weather,” she says. “I personally am very concerned about
*The data debate *
There’s no denying the warming trend and its increasing impact on
plants, says David Ellis, editor of /The American Gardener/, published
by the American Horticultural Society. “We don’t really need a dramatic
new map to show us this.”
Perhaps, but there’s been a fair amount of drama as plant, weather and
agriculture specialists have wrangled over the climate map.
The debate is rooted in the type of analytical divide that separates
scientists who disagree over whether enough data are available to show
whether the Earth’s warming trend of the past two decades is a long-term
Weather patterns tend to run in cycles, usually 10 to 15 years. Among
meteorologists, 30 years is widely considered to be a good indicator of
the overall climate.
“It’s been the custom in climatology for a long time to represent
long-term averages or ‘normals’ by a 30-year average,” says George
Taylor, a state climatologist for Oregon. “When you have a 15-year
period, you can get some squirrelly numbers.”
The United Nations World Meteorological Organization standard for
assessing the climate is 30 years, says Kelly Redmond, a climatologist
with the Desert Research Institute in Reno. But “that was before issues
of climate change seriously put themselves on the plate.”
The recent pace of climate change ? the U.N. Intergovernmental Panel on
Climate Change says 11 of the 12 warmest years since 1850 came between
1995 and 2006 ? means gardeners must be more flexible, Redmond says.
“We could be heading into a time where the temperature is always above
‘normal,’ ” he says. “If a plant has a short lifetime, what are the odds
of that plant being killed by a climate event? If it’s a tree or
something that you want to live longer, you’re probably a little more
conservative (in choosing your plants) because even if the (climate)
zones are slowly migrating, that doesn’t mean there won’t be cold spells.”
Crop growers want the safest possible estimate of how cold it might get
because they don’t want to lose plants. Because the USDA’s constituency
is farmers and growers, the agency decided to use a 30-year standard for
data in putting together its new climate map, which could be released as
soon as the fall, according to Kaplan.
“The majority of the scientific community thought 30 years of credible
data made the most sense,” she says. “The conspiracy theorists think the
reason we went to 30 years was that it would dilute the effects of
global warming. That’s flat-out wrong. No one has ever sat on the
plant-hardiness map because they wanted to deny global warming.”
Even so, meteorologists and horticulturists say it is the USDA’s duty to
more accurately show how the climate affects plants and crops. They
include those who devised the 1990 map: Kramer and Marc Cathey,
then-president of the American Horticultural Society.
*A question of accuracy *
The 1990 map was based on just 13 years of weather data, Kramer says. He
and Cathey had hoped to do a new map every 10 years to reflect shifts in
Kramer’s 2003 map rejected by the USDA was based on data from 1986 to
2002 and showed a significant march northward of boundaries for
warm-weather plants. For example, plants that for decades had frozen and
died in Nebraska suddenly were doing just fine.
Kramer isn’t convinced the decades of data the USDA insists on having
provide the most accurate picture of the climate that gardeners face now.
“If I was going to the garden center today, I’d want to have the most
current, updated information. I don’t want to know what happened 50
Some see the changing horticultural landscape as a good thing.
“There are nurserymen who are excited about the new market” for plants
in the northern half of the United States, Ellis says. “There are the
ones who see ? it as a marketing opportunity.”
That helps explain why, without fanfare, the horticultural society
posted on its website the 2003 climate map rejected by USDA and dubbed
it “The American Horticultural Society /draft /USDA plant hardiness zone
The map to be released soon by the USDA is being prepared by the Prism
group at Oregon State University, known for doing sophisticated climate
modeling. The 1990 map designated growing zones as small as counties;
the new one will narrow the focus to square miles.
So what’s a gardener supposed to do in the meantime?
Sometimes, says the National Arboretum’s Scott Aker, the best thing to
do is talk to someone who’s really down in your local dirt. Nurseries
and public gardens are good resources, he says.
Joan Pond Laisney of Carlsbad, Calif., consulted a garden-center expert
before planting her tree-shaded garden. “We researched what grows well
out here and what will live long-term,” she says.
Aker says your neighbors can be a big help, too.
“Nobody is more familiar with soil and weather conditions in your yard
than the person down the street with the beautiful garden,” he says,
“because usually what went into making that garden was a lot of mistakes
and dead plants.”
/Contributing: Anthony DeBarros/
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