As a beginning gardener, I made a mistake in my thinking that many gardeners make. I thought my garden zone dictated when I should plant my crops.
In Zone 7, for instance, I assumed all those in zone 7 would plant at the same time. Likewise, people living in zone 8 would plant earlier and those in zone 6 would plant later.
And while this isn’t always inaccurate, I realized as I dove deeper in my garden journey that this isn’t always accurate, either. As I’ll share today, both in the podcast episode and blog post below, garden zones were never created to tell you precisely when to plant. Instead, they serve an entirely different function. While it’s good to know your zone, it’s even more critical you know your growing season — that is, your average last and average first frost dates. Let me explain the difference.
(Click below to listen to the podcast episode, or keep reading.)
What are Garden Zones?
In the United States, garden zones were developed by the United States Department of Agriculture (USDA) to help growers understand, “which plants are most likely to thrive at a location based on the average annual minimum winter temperature” from a recent 30-year period of time. Given these averages, the USDA created a map to help us understand which garden zone we live in:
Updated in 2012, this map takes into account many factors in addition to the average minimum winter temperature, such as:
- the effect cities with lots of concrete and asphalt has on temperature (they are warmer)
- elevation (higher elevations are colder than surrounding areas)
- proximity of large bodies of unfrozen water (producing a milder winter)
Where Garden Zones Fall Short
For the most basic information on your average low temperature for any given year, this map is a great starting point. But it can’t account for certain variables:
- Unseasonable shifts. Plants perform best when gradually acclimated to changing temperatures. This means a sudden cold snap in fall or a spike in temperature in the late winter (followed by seasonable cold) could stress plants that, given normal cool-down and warm-up conditions, would acclimate well.
- Snow cover. In colder zones, snow cover acts as an insulator to plants. But, snow cover can also be unreliable. If an area doesn’t get a normal amount of snow cover, plants that typically overwinter with ease may find themselves at risk due to the lack of insulation.
- Light. Too much sun for a plant that is normally covered by a leafed-out tree might struggle to survive the winter sun.
- Soil moisture. Lack of fall rains can cause stress to plants as they transition into winter dormancy, placing them at risk over the winter.
- Duration of cold exposure. While plants can tolerate cold snaps (even lower than the average), long periods of sustained cold could prove detrimental.
- Humidity. Areas with higher humidity will see a decrease in stress due to the lack of water loss from the leaves.
- Geographical Area. In the eastern US, garden zones prove more reliable than in the western US. In the western US, more climactic conditions come into play due to the weather coming in from the Pacific Ocean.
- Microclimates. While the map does take into account larger swaths of microclimate-induced differences, it can’t account for smaller, localized microclimates. For instance, heat pockets due to asphalt or cool spots caused by small hills and valleys may cause variances in the minimum low temperature.
- The Law of Averages. Remember, the minimum temperatures used to place you in a specific zone is based on an average. You could get an unseasonably cold snap that is lower than your average minimum temperature, especially if you’re at the upper end of your zone. It’s possible you could lose plants you never had to worry about before if this happens.
Purpose of Garden Zones
In this brief explanation of garden zones, you may have noticed a theme: garden zones focus on your minimum low temperature.
This is helpful when you’re thinking about perennial plantings — those plants that grow year after year without replanting. You definitely need to understand your zone when choosing shrubs, trees, general landscape plants, and certain herbs.
For example, by knowing my zone, I understand that if I want to grow a lemon tree, it won’t survive outside, so I’ll need to grow it in a pot and bring it indoors in the winter. Knowing my garden zone also helped me know which blueberry bushes to purchase. I also know to expect certain herbs, like rosemary, oregano, sage, thyme, and chives, to survive over the winter.
Let me dive a little deeper here. Although I have never lost any of those herbs above over the winter, those of you in a colder zone might struggle a bit. In that case, you may choose to plant those herbs in the ground instead of containers (because the ground provides more insulation). Or, you may choose to plant these herbs on a south-facing area of your house.
The Missing Element of Garden Zones
In all this information, you may have noticed a lack of detail in one particular area: when to plant.
That’s where I think gardening “zones” — and all the information online about them — can cause confusion. I’m a big fan of Pinterest (follow me here) and I find some fantastic gardening articles on there. But the ones I usually skip over are the ones that say something like, “What to plant in August in Zone 7.” The problem is, your zone does not tell you WHEN to plant. Its primary purpose is to tell you WHAT will survive the winter.
And while those “when to plant” guidelines can serve a starting point, you need to understand that your questions of “when to plant” are best answered by something other than your gardening zone: your growing season.
The Importance of Knowing Your Growing Season
In general, most backyard gardeners will use their growing season much more in their garden planning than their garden zones. How do you find your growing season?
Your growing season is determined by knowing your last and first average frost dates.
While many plants can be grown outside of these dates, it is these dates that the timing of your plantings revolve around. Some plants you must plant, for example, after your average last frost, and others you can plant before. The same is true for your fall garden and your average first frost.
Let me give you an example of how your growing season and your zone may not always mean the same thing.
You may have heard my friend and fellow gardener Melissa K. Norris here on my podcast. Melissa lives in Zone 7 (like me), but she lives in Washington State (and I’m in Arkansas). Even though our gardening zones are the same, her growing season is much shorter than mine. While I can plant my summer crops in mid-April, she has to wait til late May. And while my harvest of summer crops goes well into October, her first killing frost occurs weeks before mine.
As you can see, looking at a general “What to plant in Zone 7 in August” could potentially fail one of us, especially when it comes to annual crops (those that are planted each season).
So instead of focusing so much on your zone when it comes to your annual crops, you instead need to know your average last frost and your average first frost.
Easy Tools to Know Your Frost Dates
Where can you find your average last and first frost dates? This is a simple site where you can enter your zip code and it will tell you: Farmer’s Almanac First and Last Frost Dates
If you want a little more detail, you can enter your zip code and not only find your frost dates but also see the probability that the temperature will get to a certain point at certain dates around the beginning and end of your growing season: Frost Dates by Garden.org
And, for an even handier tool, the Farmer’s Almanac has customized a planting calendar for your growing season. Enter your zip code here to see when you should plant what all season. Farmer’s Almanac Planting Calendar
I hope this has helped demystify the garden zones for you and has given you the information you need to more accurately plan your garden!
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