By Becca Pray, B.S. in Geography 2019
If you're not already familiar, the United States Department of Agriculture (USDA) has a map of different growing zones here in the U.S. which is intended to help gardeners and farmers understand what crops will likely overwinter in their outdoor temperatures. For example, I can overwinter a hardier Zone 5 Rosemary in my yard here in Floyd, but that same Rosemary would die in a Zone 3 yard such as one found in Nothern Minnesota. Each plant species -- and even variety -- has a unique hardiness level that must be considered before attempting to overwinter the plant outdoors in the elements.
Last year, the USDA updated this map, and much of Floyd County shifted from Zone 6b to Zone 7a instead.
Why did this happen? And what does it mean for Floydians?
Firstly, let's establish what this map is based on. This map uses one simple piece of information to calculate these zones: the average annual extreme minimum Winter temperature.
Second, the reason an update was made to this map was the addition of recent data in the first place. Earth's Climate has never been 100% static. It is common knowledge that our not-so-distant ancestors endured the recent Ice Age. When new weather data is applied to existing climate models, it changes the averages. It's basic math. The average annual extreme minimum temperature (the coldest average temperature in Winter) is calculated by adding up every low temperature for the 365 day calendar, and then dividing that number by 365.
As you can see in the color key in the above map, if Floyd was originally Zone 6b and is now Zone 7a, it means that means that our average annual extreme minimum temperature decreased from the range of:
-5°F to -0°F
to a range of
0°F to 5°F
Many people are interpreting this as a reason to go ahead and plant Zone 7 plants, trees, and shrubs here in Floyd whereas they weren't doing so before. I would personally caution everyone against this. Let me tell you why.
This change in Zone was determined by an average minimum temperature, NOT the actual lowest temperature.
Here is an example dataset to show how an average is very different from the dataset's lowest number:
Here we can see how Location A and Location B both reached a minimum temperature of 1°F, but the average of Location B is higher because of one higher datapoint (4°F).
This is why I will not personally be planting Zone 7 plants in my garden. Even though our average annual extreme minimum temperature has increased, we will still experience our trusty old -5°F temps with -20°F wind chill in the Wintertime, which will still harm or kill Zone 7 plants.
In addition to the math part of it, the microclimates of Floyd County could not possibly be calculated or mapped. I have clients whose pumpkin vines don't die until weeks after mine do. There are also areas that will be snowy or icy when the rest of the county is sloppy and moist.
In summary, the best people to ask about pushing these Zone limits is your neighbors and nearby farmers. It is best to use your firsthand experience, and listen to your gut instinct about your own microclimate. For more ways to work around your first and last frost dates, check out this Blog post: Gardening is Gambling - How to Mitigate Your Losses.
My final point is that... folks... this is the government. They have done a wonderful job of establishing a foundation model for us to consider, but if you are expecting perfection from the government, we can't be friends. And if you're using the government for your gardening tips, we can be friends but we need a serious coffee date to discuss your trust in authority. Lol! Thanks for reading if you made it this far.
Happy gardening!
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