Pop quiz: What makes Earth different from all the other planets that have been discovered so far?

Location. Location. Location. We happen to be in this magical Goldilocks-zone of being close enough to the sun to have warm(-ish) temperatures and liquid water—at least at some times of the year across most of the globe. And we’re also not so close to the sun that the planet is so scorching hot that water evaporates immediately.

This “just right” spot in the Milky Way is what allows us to have life on Earth—including and especially plant life. We have plants and that they grow well—at least when the weather is warm enough.

The challenge of living in a cold climate is that the temperatures are too cold to sustain plant growth year-round. Air and water temperatures need to be above freezing for photosynthesis to occur and for plants to grow—not to mention that many plants lose their leaves for the winter. The colder the climate, the more plant growth will be challenged.

How cold is your climate?

There are a few methods to help describe how cold—or warm—a climate is. Understanding these different ways of describing your climate will help improve your gardening game.

  • You’ll understand different aspects of climate that limit plant growth for different types of plants—like minimum winter temperatures and the freeze-free season.
  • You’ll be able to efficiently find plants that match your local climate, helping you sift through catalogs more quickly and giving you the best chance of gardening success.
  • You’ll have a common language to share with other gardeners when discussion your gardens or potential plants. Someone can tell you that they are gardening in zone 4b  (like I am!), and you’ll have sense of whether their gardening climate is similar to yours.

Plant Hardiness Zone (Minimum Winter Temperature)

The USDA Plant Hardiness Zone Map provides one of the most common ways of defining growing regions in the U.S. Hardiness Zones are based on the average coldest (minimum) winter temperatures that were observed during the period 1976 to 2005. These average winter minimums are grouped into zones in increments of 10°F, which are represented by numbers. The lower the number, the colder your zone. The zones are further split into two sub-zones, represented by letters. (Follow these links to find information on hardiness zones in Canada and other countries.)

For example, the extreme minimum temperature in Zone 5 in the winter is between -10 and -20°F. Zone 5a is colder, with average minimums dropping down to between -15 and -20°F, and Zone 5b is a bit warmer at -10 and -15°F. It’s important to note that these maps just show averages, and so winter temperatures can be lower than what is mapped, and that the maps can’t capture local climate conditions (i.e., microclimate).

The USDA Plant Hardiness Zone Maps are based on the coldest (minimum) winter temperature for a particular location, and can be searched interactively and by zip code.

Hardiness zones are particularly useful for perennial plants, because many of these plants are damaged or killed by extreme winter temperatures. Once you know your hardiness zone, you can look for plants that will tolerate the cold temperatures where you live. For example, if you live in Minneapolis and want to plant a cherry tree or an azalea, you’ll want to look for a variety that is hardy to zone 4 since Minneapolis is in zone 4b.

In general, hardiness zones can provide a quick way to describe your general climate. Many garden catalogs that sell plants will include a map of the hardiness zones or a similar maps that show “growing zones” or “planting zones” to help with plant selection. And, if you frequent gardening sites, the hardiness zones can be used to quickly get a sense of an area’s climate.

Growing Season (Frost-Free Season)

As useful as it is, plant hardiness zones only describe one aspect of your climate. Another important thing to know your growing season. The growing season, also called the frost-free or freeze-free season, is the time of year between the last frost in the spring and the first frost in the fall. A frost is generally defined as low temperatures reaching freezing (32°F). A freeze can be used to indicate an even lower temperature, such as when temperatures drop to 28°or 30°F; this is what you might call a “hard frost”. Knowing the average number of days in the growing season can be helpful for purchasing seeds or planning your annual planting plan.

It can be a bit harder to locate this data for your location, but Dave’s Garden has a simple zip code search that will find the information for many locations in the US and save you from digging through oodles of climate data. The site provides tables that show the probabilities for the the dates of the last spring frost and the first fall frost at three different temperatures (24°, 28°, and 32°). For example, the spring 32° date with a 10% probability for my local zip code is May 24; this means that there’s less than a 10% chance that a spring freeze of 32°F would occur after that date. It works the same way in the fall—my 32°F fall date for 10% is September 19 and so there’s only a slight chance that a frost would occur before them. The 10% dates are the most conservative. The 50% dates represent the average frost dates—so half of the time frost comes before that date, and half the time it comes after.

You can find a summary of growing season dates for locations in the U.S. from Dave’s Garden. Tables show the risk of frost for different dates.

Feeling overwhelmed with all these probabilities? If so, you might prefer to look up your average frost date with the Farmer’s Almanac. It provides less information on the risk of frost in spring and fall, and will just give you the average dates and the total number of days for your (average) growing season.

Knowing your hardiness zone and growing season are important for understanding the general climate where you live, but each little location has it’s own climate—a microclimate—which we’ll explore in the next post.

One thought on “How cold is your climate? (Part 1) How to Find Your Hardiness Zone and Growing Season

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