Growing zones, or Hardiness Zones, is a term often referred to in the gardening world. But what does this really mean and how do you use your growing zone number in gardening?
What Growing Zones Tell You
A growing zone is a value that has been assigned to a specific location and describes the average coldest temperature that location can expect to see during the winter. My growing zone is 8b; that means the lowest temperature I will see in an average winter will be 15 to 20 F, -6.7 to -9.4 C.
To find your growing zone, visit the USDA Hardiness Zone Map.
How Growing Zones are Categorized
Locations are divided into numbers from 1-13 and categorized based on the average lowest winter temperature in a typical year. The lower the number, the colder the average lowest temperature is that you can expect to see during the winter. Zone 4 has a much lower average lowest temperature in winter than a Zone 8 location can expect to see.
This division is how you see someone like me in Western Washington with the same growing zone number as someone in Texas or Georgia. Even though our climates are very different, our winters can expect to reach about the same lowest temperature.
That doesn’t mean that we will be that temperature all winter, or that our winter climates are even close to being the same. Texas isn’t experiencing the almost constant rain that those of us up here in the Pacific Northwest have all winter.
The number is further divided into a and b to make the classifications just a little more accurate to a location. For example, growing zone 5 has average lowest winter temperatures of -10 to -20 F, -23.3 to -28.9 C. To narrow it down further, it’s split into 5a and 5b with 5a being the lower half of that range and 5b being the warmer half of that range.
Each growing zone is depicted with a color. Colder colors, like pink, purple, and blue, are used for the colder zones of 1-5 while warmer colors of yellow, orange, and brown are used for the hotter zones of 8-13. Different shades of green are used for the moderate zones of 6-7.
What Growing Zones Don’t Tell You
Growing zones don’t tell you a lot. Your growing zone does not tell you what your last frost of the year will be in the spring, or when the first frost will hit in the fall. They don’t tell you how long your growing season is or what your general climate is during that growing season.
Growing zones only focus on the coldest temperatures in the winter. They don’t actually tell you much about what you can expect during the spring, summer, and fall of your areas.
Climate is not You may be able to grow year round in a zone 10. Or maybe the climate in one location in zone 10 has such hot summers that it’s hard to grow any crops during the Summer. Maybe they can grow easily during the late winter/spring season and then again in the fall. But the summer may be too harsh for a lot of plants.
However, it’s possible for someone in that same zone in a different location to grow year round because their summers may stay a bit more mild. It’s all about climate during the growing season, and growing zones don’t give you those details.
First and Last Frost Dates
Your first and last frost dates are dependent on the climate in your location. You can infer climate patterns from a growing zone, but it doesn’t actually tell you the reality of what a specific location’s climate looks like.
You can guess that growing zones with a lower number have shorter growing seasons overall than zones with a higher number. Locations farther from the equator tend to have colder climates than locations closer to it. Their colder winters will give them a lower growing zone number.
To get an accurate picture of your climate and growing season, start with finding your first and last frost dates. Those tell you more about your growing season than your growing zone will.
These dates will tell you when to start seeds, when you can start planting outside, how long you are likely to have to grow before the first fall frost kills off all but the cold hardy plants. This information helps you choose varieties that are likely to do well in your area.
The general climate in a location is the other piece that helps you determine what you might successfully grow there. My summers in zone 8b don’t get hot enough to grow things like okra and sweet potatoes very well. According to my growing zone and first/last frost dates I should be able to grow them. But during the summer my location doesn’t get hot enough for long enough for those crops to do well.
Now those people down in the Southern U.S. in my same growing zone are probably able to do much better with those crops because their summers are hotter for longer.
When Does a Growing Zone Matter The Most
The growing zone of a location matters the most when you are dealing with perennial plants. Perennial plants are plants that come back year after year; they don’t completely die off during the winter months. This is different than a self-seeding annual plant that looks like it comes back year after year. These are different plants that are growing each year from the seeds the previous year’s plants dropped.
There are some plants that can be perennial plants in some locations but only annuals in others. Artichokes, for example, will grow year after year in a warmer climate. But in colder climates, the winters get too cold for the plant to survive. They die off when the temperatures drop, and you will have to replant them every year.
This information will tell you whether a specific plant will do well in your area. This is really helpful when looking at fruit trees or cane berries. Nurseries label trees with the growing zones they will survive in. That information tells you whether or not they’ll survive the winter or if they’re likely to die.
If they’re likely to die off, then that’s not a variety that you’ll want to plant in your location. You’re better off picking varieties of fruit trees that will survive your winters so you will actually be able to get a harvest from them when they mature.
Now go get those hands dirty!