|
 But
the newest update to the plant hardiness map, in which half the
country shifted to warmer zones, is a stark reminder that the
climate is changing and changing fast.
“The magnitude of change in the new maps really surprised me since
it just reflects the last 30 years,” said Illinois State
Climatologist Trent Ford. “One of the most consistent changes in
Illinois’ climate directly linked to human-caused global warming is
warming winters.”
In November, the U.S. Department of Agriculture updated its plant
hardiness zone map, giving growers across the nation a better
understanding of which perennial plants will survive a region’s
winter and thrive long-term. The map, available at
planthardiness.ars.
usda.gov, was previously updated in 2012, and the new, more detailed
version is based on 30-year averages of the lowest annual winter
temperatures from almost twice as many weather station sites.
Illinois now has six zones, with 12 sub-zones,
ranging from zone 5a in northwestern Illinois to the newly added 7b
zone in southern counties.
“In Illinois, the most obvious change between the new hardiness zone
map and the 2012 map is a northward progression of zones 6a, 6b, and
7a,” Ford said. “The boundary between zones 5b and 6a migrated 60
miles north, from around Springfield in the 2012 map to around
Peoria in the 2023 map.”
On the new map, most of central Illinois now falls
into zone 6a, with zone 6b moving north into parts of Coles,
Moultrie, and Shelby counties. Zone 7a also migrated 50 miles north
from Cairo into the St. Louis Metro East area. This has also pulled
a small section of 7b into Illinois, which is the first time this
warmer zone has occurred this far north. [to top of second
column] |
Warming winters can reduce stress
on plant populations, helping some landscape plants thrive, but
also bring negative impacts, such as introducing invasive
species from the south, introducing new pest pressure on crops,
or ecological competition in natural areas.
“Zones are a shortcut we use when thinking of an
area’s general climate and how plants will fare,” said Jamie Viebach,
University of Illinois Extension horticulture educator. “The new
changes in the map may actually explain trends gardeners have
already seen.”
If a gardener’s zone shifted with the new map, Viebach says that’s
not a reason to worry or change existing landscaping, but it could
be an opportunity to try to overwinter new perennial plants.
Most landscape plants are hardy across a range of
zones. A plant known to be hardy from zones 4 to 8 will thrive in
zone 6, whereas a plant that is only hardy in zones 6 to 8 may
struggle if a winter extreme temperature tips below the expected
average as a result of year-to-year variability.
Much of the data that prompted the updated map was collected and
submitted by trained members of the public through the Community
Collaborative Rain, Hail & Snow Network. Volunteers are needed in
Illinois, and more information is available at www.cocorahs.org.
For more information about planting in Illinois, visit
extension.illinois.edu/plants or connect with your local county
Illinois Extension office at https://extension.illinois.edu/lms .
[Sources: Jamie Viebach and Ryan Pankau,
horticulture educators. Trent Ford, Illinois State Climatologist.
Images: USDA, planthardiness.ars.usda.gov]
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