Climate change
and our gardens
As passionate gardeners, we keep wondering how the changing
climate is affecting our gardens. (We last wrote about climate
change in this space during the heat of summer in 2010.) We
therefore noticed when the U.S. Department of Agriculture (USDA)
updated its plant hardiness zone map on January 25, 2012.
Throughout much of the
U.S., the map is a half-zone (5 degrees Fahrenheit) warmer than
its 1990 predecessor.
Gardeners rely on plant hardiness zone maps ("PHM"), which set
out the different climate zones where specific trees, shrubs and
flowers are likely to survive. The new U.S. map includes 13
zones (up from 11). At
least in part, this change results from data being collected at
many more weather stations than the predecessor map (1990), and
over a longer, more current time period (i.e., 30 years, from
1976 to 2005 as opposed to the original 13 years).
As in the past, the map
was developed using the coldest annual temperature at various
locations. As well, more
sophisticated methods were used, including calculations that
considered factors like elevation changes, proximity to large
bodies of water and terrain position. Map users can locate more relevant data for their zones
by entering their postal codes. The 1990 map included Canada and
Mexico; the 2012 version, alas, does not.
The USDA notes that changes in climate are generally determined
based on average temperatures over a 50 to 100 year period (not
just the coldest days of the year).
They caution that changes
in zones do not accurately reflect whether global warming has
occurred. Others
disagree, arguing that the map reflects warming trend and the
"new normal".
The U.S. Environmental Protection Agency (EPA) compared
hardiness zone maps from 1990 and 2006 (the latter prepared by
the Arbor Day Foundation). Over that 16 year period, the
hardiness zones in the continental U.S. (excluding Alaska)
shifted northward, because of warmer winter temperatures.
Large areas of some
states warmed by at least one hardiness zone, reflecting a
significant increase in average low temperatures.
A few small areas, mainly
in the western U.S., have become cooler by one or two hardiness
zones. As well, the last
spring frost occurs earlier, and the first autumn frosts are
later. Thus, the
average growing season has increased by approximately 2 weeks
since 1900; this increase has been particularly significant
since 1980.
The U.S. EPA estimates that climate change will bring higher
temperatures and a longer growing season in cool regions, which
could permit farmers to diversify their crops and perhaps see
multiple harvests each season. Some
areas may become too hot for traditional crops to grow.
Climate change will alter
regional biodiversity. For
example, invasive plants will move into new areas, harming
native plants, and animals will move from their current
locations to seek their preferred sources of food.
Already, some bird
species have shifted their wintering grounds up to 400 miles
northward. As well,
allergy seasons may arrive earlier and last longer.
(In contrast to the hardiness zone maps, which are based on
coldest temperatures, the American Horticultural Society
recognizes that plants are now often coded according to heat
tolerance. The Society
publishes a plant heat zone map that divides the U.S. into 12
zones and provides the average number of days every year that a
region has temperatures over 86 degrees Fahrenheit (30 degrees
Celsius). The map was
published in 1997 and appears not to have been updated.)
In Canada, plant hardiness maps are based on the average climate
conditions in each region, as well as several on other variables
like minimum and maximum temperatures, the duration of the
frost-free period, amount of rain in the summer, snow cover, and
wind speed. The original hardiness zone maps were developed in
the 1960's. In 2000,
Canada introduced its new plant hardiness zone map, which is
based on "more recent" data (from 1961 to 1990), and on improved
climate prediction models.
Natural Resources Canada wants the public to participate in
making hardiness zone mapping more accurate, by submitting data
about which plants survive at their locations.
This includes identifying
plants, their precise geographical location, and, where known,
details such as how long the species has been at the location,
sun/shade exposure, winter protection, soil type and how the
plant performs at the location. They want to develop and map a climate profile for every
plant species; ultimately, these profiles should indicate the
range where each species will grow.
It is frustrating to try and interpret broad climate trends in
the context of our own piece of turf. It's harder to know when
to plant what. Warmer hardiness zones and longer growing seasons
can be good news for some plants, but the uncertain winters and
lack of snow cover can devastate others. I guess we'll have to
keep experimenting, and remember to be flexible!
Dianne Saxe
Jackie Campbell
Environmental Law Specialist Dr. Dianne Saxe, one of the top 25
environmental lawyers in the world, brings
environmental, global warming, water pollution issues, daily
environmental issues affecting our lives.
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