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| https://planthardiness.ars.usda.gov/ |
The United States Department of Agriculture recently revised its Plant Hardiness Zone map. Much of the Denver metro area was changed from a zone 5b to a zone 6a. As a result, some plants that were previously considered unable to survive the winter here (annual plants) are now viewed as able to survive the winter here (perennial plants.)
The Plant Hardiness Zone map works like this: plants that can survive the harshest weather
survivable by a plant are classified as zone 1a plants (this is high altitude
Alaska). The most tender plants that can
only survive the winter where temperatures never fall below freezing are
classified as zone 10b plants (this is south Florida). Each zone on the map covers the average lowest
temperature range that could be expected to occur in that zone during the
winter (not the lowest temperature that has ever occurred or will possibly
occur in the future.) The Denver metro’s
new 6a average low temperature range is minus 10 to minus 5 degrees Fahrenheit.
In its press release announcing the new map, the USDA said
that about half of the US shifted into a warmer half-zone (that includes us,) while the remainder stayed in the same zone.
The USDA says that the shift into a warmer zone does not necessarily
reflect global climate change. The new
map was created based on data from thousands more weather stations and using better
mapping technology than the previous map, which was issued in 2012. That is an explanation for the shift.
So, when plant shopping this spring, should you load up on zone
6a plants and expect them to survive the winter? The answer is, it depends. The following are some thoughts on how to use
the Plant Hardiness Zone map in making your plant purchasing decisions.
First, the Plant Hardiness Zone map doesn’t take into
account extreme weather events. For
example, you who lived around here in 2014 might remember the polar vortex
event we experienced in the fall. The
fall weather had been warm and sunny when a polar vortex roared in, dropping
air temperatures by about 70 degrees in 24 hours. Trees and shrubs that normally would have
survived the winter weren’t prepared for the rapid temperature change and died
as a result.
Second, the Plant Hardiness Zone map doesn’t take into
account microclimates. Microclimates are
everywhere. An easy microclimate to
envision is areas that receive direct sunlight in winter versus areas that
don’t. Snow melts much more quickly in
the areas that receive direct sunlight. In
the spring, plants break dormancy and start growing in direct sunlight areas
before they do in shaded areas. The soil
stays warmer throughout the winter in direct sunlight areas.
All of this is to say that if you buy plants in the more
tender temperature ranges, here zones 5 and 6, they have a better chance of
surviving the winter if they’re planted in south or west facing areas that
receive direct sunlight throughout the winter.
Also, covering these plants helps them to survive an imminent extreme weather
event. I have a front yard that faces
northeast and a backyard that faces southwest.
In theory, because the Colorado front range is a zone 6a, I should be
able to grow perennial plants labeled for that zone in my front yard, but I
can’t. They die because my front yard is
a cold, dark place in the winter. The
same plants come back year after year in my very winter-sunny back yard.
Other considerations about plant shopping using the Plant
Hardiness Zone map are as follows. Most
garden centers and big box stores will lay out their plants for sale in two
areas, annuals and perennials. Perennials
will be labeled with tags saying they’re for zone 6a or below. You buy these plants with the expectation
that you’ll plant them in the ground, they’ll die off in the winter and then
re-grow the following spring. Annuals
are labeled for zones 6b and above.
These plants aren’t expected to survive the winter. They’re often the plants you buy for flower
pots and vegetable gardens.
When shopping for plants in the spring, always read the tag
to see the plant’s Plant Hardiness Zone number.
In the hubbub of spring plant selling, plants can be misplaced in the
store (annuals in the perennial section and vice versa.) Where you find the plant in the store might
not accurately reflect its hardiness zone.
Also, the name of the plant won’t necessarily tell you whether it’s an
annual or perennial. Lavender is a good
example of this phenomenon. Some
varieties of lavender are perennials in Colorado and others are annual. You need to find the hardiness zone number on
the plant’s tag to know what lavender you’re buying (I’ve been burned by this
mistake before – don’t let it happen to you!)
Last but not least, the Plant Hardiness Zone map really only applies to plants planted in the ground, not to plants planted in pots. In winter, the soil in the ground is warmer than the soil in a pot. So, a perennial that would likely survive the winter planted in the ground is less likely to survive planted in a pot. If you’re planting a perennial in a pot and hoping it will come back the next year (a worthwhile effort to make, especially if the pot is a large focal point in your outdoor space,) buy a perennial at least two zones lower and cover the plant during extreme winter cold spells.
Local news about our new Plant Hardiness Zone 6a:
https://www.denverpost.com/2024/02/19/colorado-growing-zone-change-map-temperature/
Plant Hardiness Zone map and how to
use it:
https://planthardiness.ars.usda.gov
More information about plant hardiness, dormancy and
temperature related plant injury:
Hort.extension.wisc.edu/files/2023/12/New-USDA-Cold-Hardiness-Zone-Map-copy100.pdf