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Know Your Zone!

Why are plant hardiness zones so important for gardening?

Plant hardiness zones are based on lowest annual temperatures for a region, and help gardeners determine which perennial plants are likely to survive the winter. If you’ve noticed your hardiness zone recently changed, that’s because the zones are based on the average lowest temperatures over a 30-year period. In the past thirty years in Chicagoland, we’ve experienced more extreme temperature fluctuations than in years past.

Plant hardiness zones are also tied to first and last frost dates for a given area. Although much of the area surrounding Chicago is zone 5, the city is considered to be zone 6 – likely because of the temperature-stabilizing effect of Lake Michigan. First and last frost dates are especially important for gardeners in determining when to set out or bring in/cut back annual plants. Although we typically think of our last frost-free date in Chicago as around Mother’s Day and the first frost date as around Halloween, this can vary by 2-3 weeks in a given year.
Source: http://planthardiness.ars.usda.gov

Source: http://planthardiness.ars.usda.gov

To determine your plant hardiness zone, check out this web site. Be sure to enter your zip code for the most accurate hardiness zone – don’t go by the visuals on the map alone!


Wondering where to find information on the first and last frost dates? Get comfortable with your favorite website for local weather and check nighttime low temperatures in the spring and fall. To know what you’re looking for, here’s an explanation of frost and freezes:

The first frosts of fall are usually very light – somewhere between 35 and 32 degrees Fahrenheit. Many plants in the fall garden can usually survive a light frost, as they have been acclimating to cooler temperatures throughout the fall. A light freeze is between 31 and 29 degrees Fahrenheit. A moderate freeze is between 28 and 25 degrees, and a hard freeze is below 24 degrees.

While these temperature ranges are pretty specific, you’ll notice that different plants in your garden respond to the slight temperature variations differently. Keep in mind that sometimes nighttime low temperatures are only reached for an hour or so – you’ll notice this by looking at the hourly temperature on your favorite weather website. When the nighttime low temperature is brief – your plants may survive or be unaffected even though a frost or freeze may have technically occurred. Plant survival also depends on the amount and duration of snow cover, as snow provides some insulation and protection for plants.

Frost on thyme. Photo credit @kctomato

Frost on thyme. Photo credit @kctomato