Are you confused about gardening zones, also known as USDA hardiness zones? What do they mean? Why are they important? Let me explain what it all means and how to find your gardening zone.
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What is a gardening zone?
A gardening zone, also called a hardiness zone, is an area that has similar climate conditions. It is usually used for landscaping and gardening to help determine what plants will grow in each area. There are 13 zones and they are broken into these zones by the annual extreme minimum temperatures. (This just means the lowest temperature that area can expect.) There can be variations within each zone and these variations are referred to as micro-climates.
Occasionally, you will find that a zone is further divided into zones “a” and “b.” This is sometimes done if a particular zone is found to have a large deviation in each range. For instance, I am near Charlotte, North Carolina which is listed as zone 7. However, many times it is divided into 7a and 7b.
Why do you need to know your gardening zone?
First, knowing your gardening zone helps you decide WHAT to plant. Some varieties of crops will do better in some locations than others.
Most plant tags or seed catalogs will tell you what zone the plants will grow in. Local gardening centers usually only carry plants that do well in your area. However, if you are ordering online, that will be a different story. Most companies carry seeds and plants for all locations all over the United States.
Varieties bred for certain areas
There are even plants that are bred specifically for certain areas or varieties that are bred to combat particular growing problems of one area. For instance, in the humid south where I live, powdery mildew can be a problem, especially on cucumbers. But there are certain varieties that have resistance to powdery mildew, so I usually try to grow those types.
There are also plants that really don’t grow successfully in some areas. In the north, it can be difficult to get watermelons to grow because they take too long from seed to harvest. However, breeders are coming up with new varieties each year. Looking through my most recent seed catalogs, I found several varieties of watermelons that have a shorter growing season. So my northern friends may be able to harvest a few watermelons from their own backyard this year.
In the far south, it can be difficult to grow tomatoes during the summer when the rest of the United States is growing them by the bushels. It can be too hot for most varieties. I can remember by Aunt Pauline, who lived in Florida, lamenting the fact that she couldn’t get a decent tomato in the summer. When she would come to visit, my Mom always saved her some. But now, there are quite a few varieties that have been bred to handle the hot, humid summers of the deep South.
So without these zones, you would have to rely on trial and error (or seasoned farmers and gardeners) to help you determine what would and what would not work in your growing area.
A word of warning about these zones
There are several growing conditions these gardening zones don’t address.
What about high temps?
These planting zones say nothing about how long the growing season is each area. It also doesn’t address how high temperatures may get in a particular area. It is based solely on low temperatures. So even though a plant could handle the low temperatures and the cold, it still may not survive the summer heat.
For instance, Seattle, Washington and Charleston, South Carolina are both zone 8, yet their growing conditions are wildly different. Seattle typically has an average high of 76 degrees in July while Charleston’s average July high is 91! That’s a huge difference!
And sometimes rainfall can affect these zones as well. Some vegetables can handle more summer heat if they have adequate water. In our Seattle and Charleston example above, Charleston actually receives more rain than Seattle, but Seattle has more days with precipitation than Charleston. (For more details on these climates you can check out the U.S. Climate Data on Charleston and Seattle.)
Recently, some companies have also started adding the Heat Zone to the plant tags and catalog descriptions. This is a map that is produced by the American Horticultural Society (AHS). Although not as widely used as the USDA Hardiness map, the AHS Heat Zone map can give you an idea of how well a plant can handle hot temperatures.
There are 12 zones in the AHS Heat Zone map and it gives you the number of days a certain location has a temperature above 86 degrees. These days are called “heat days.” For more information and to find your Heat Zone, you can visit the AHS Heat Zone map.
Snow cover can also affect these zones, especially where snow fluctuates from year to year. The snow cover can actually help insulate plants so sometimes, winter crops can be grown in a zone or two colder than what is recommended. Even shrubs not intended for that zone can sometimes survive the cold if there is adequate snow cover. Problem arise however, if you have a really cold snap, but no snow cover. Plants grown out of their zone don’t seem to do as well.
Even in a particular zone a micro climate can exist. Peaks of mountains and the valleys between them can have wildly different climates even though the gardening zone is likely the same. Cold air is heavier than warm air, so it tends to settle in valleys leading to a more likely frost.
Even in my own backyard, there can be slightly different growing conditions. I’ve had things freeze in one area of my garden, yet remain unscathed in another area.
The wind and humidity can also affect your zone and growing conditions. In our example above, valleys tend to be sheltered from the wind, which also means a more likely frost. Think of it this way: Have you ever gone out to your car one cold morning and had to scrape the windows, yet on another cold windy morning, you haven’t had to defrost the windows? When the wind blows, it keeps the atmosphere mixed and the frost can’t settle on the plants (or your car windows.)
We can also have problems with growing cold weather crops in our zone 7 garden. I have been experimenting with growing lettuce all winter. Occasionally, we will have a 70 degree day in December or January and this is really not good for the cold weather crops we have planted. They tend to bolt at the first hint of warm weather.
So while these gardening zones usually tell whether a plant will grow and survive, it doesn’t always mean it will actually flower and produce fruits or vegetables.
Gardening zones don’t really tell you when to plant
Knowing your gardening zone doesn’t really help you know WHEN to plant your vegetables. Lots of vegetables are sensitive to the frost, so they must be planted after your last expected frost date or harvested before your first frost in the fall. But you still need to know these dates (in addition to your gardening zone) to know when to plant. You can find your first and last expected frost dates using this map.
How to find your gardening zone
(This information was taken from the USDA Web site and is considered public domain information. Public domain information may be freely distributed or copied. Attribution: U.S. Department of Agriculture. USDA Plant Hardiness Zone Map, 2012. Agricultural Research Service, U.S. Department of Agriculture. Accessed from https://planthardiness.ars.usda.gov.)
This system is definitely not perfect but it is a guideline to help you determine what you can and can’t grow successfully in your area. So if you are ready to determine your gardening zone, you can check out this handy map from the USDA. This will guide you in knowing what vegetables you can grow in your garden.
And while there is nothing wrong with experimenting with plants outside your hardiness zone, just be prepared in case the vegetables don’t do as expected.
Have you grown anything outside your growing zone? Did you experience success with the vegetable?