What Is a USDA Zone?

A Beginner's Guide to the USDA Plant Hardiness Zone Map

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Before choosing the plants for your garden, it's a good idea to consult the United States Department of Agriculture (USDA) plant hardiness zone map. This map is a useful tool for helping you choose perennial plants that are likely to grow well where you live (aka your USDA zone).

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Growing the right plants for your climate zone can not only save you a lot of frustration in the garden, but it can also save you a lot of money since you won't be wasting your cash on plants that just die. Although you likely know you can't overwinter the same plants in Minnesota as you can in Florida, it's still important to understand the subtle differences in each zone so that you can grow a successful garden.

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To get the most from the hardiness planting zones map, it's important to understand what it tells you and what it doesn't.

What Is the USDA Plant Hardiness Zone Map?

The USDA plant hardiness zone map is as much a thermometer as it is a map. To make it, the USDA split the nation into 13 primary zones, each one warmer than the previous one by 10 degrees Fahrenheit. Each of these zones is then broken into two subsections to account for an area's more specific microclimates.

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Zone 1 is the coldest zone and covers portions of Alaska where only the toughest plants can survive. Found in Puerto Rico, zone 13 is the furthest south and is the warmest zone. Tropical plants can thrive there.

The U.S. Department of Agriculture created a new map in 2012 due to warming temperatures. To update it, the USDA looked at the lowest winter temperatures in each area over the previous 30 years. Based on this new temperature data, it then calculated the annual average extreme minimum temperature in each growing zone (this moved a lot of areas about a half-zone).

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How to Use Hardiness Zones

Now you know what the USDA plant hardiness zone map is telling you — but what do you do with that information? You take it with you when you go shopping for plants for your landscaping.

The plants you buy at the nursery or garden center are rated based on these zones, which essentially tell you whether the plant you buy as a perennial will have enough cold hardiness to survive where you live. In zone 3, for example, winter temperatures may dip down to -40 degrees Fahrenheit. A plant rated for zone 4, where the winter temperature usually stays above -30 degrees, simply may not survive the winter if it is planted in zone 3.

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Another helpful aspect of plant hardiness ratings is that they can help you understand what a plant expects. Deciduous trees, for example, are adapted to deal with winter weather. One rated for zones 5 through 9 likely expects some cold weather and may even need it to thrive. It likely won't do well in a tropical climate or warmer zone.

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One perfect example of this is chill hours. Many fruit trees require a specified number of hours in cold temperatures. If they don't get them, they won't set fruit in the spring. The USDA plant hardiness zones tell you where these plants will get the cold weather they need and where they won't.

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Planting Outside Your Zone

Although you can't necessarily make a big jump, like planting a zone 9 plant in hardiness zone 3, you do have some wiggle room. Many gardeners succeed at pushing the limits, growing a plant rated for zone 5 in zone 4. Trying this does put you at risk of losing a plant you really like, but you may just get away with enjoying a species you ordinarily wouldn't.

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Some plants are perfectly happy living in a pot and moving inside during the winter. These plants can grow anywhere. Growers assign perennials and evergreens plant hardiness zone numbers to ensure planting where they can survive the winter. Annual plants are enjoyed during the summer and then die in the winter. They aren't expected to survive cold weather, and as such, they don't need a hardiness rating.

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What the USDA Map Doesn't Say

The U.S. Department of Agriculture plant hardiness zone map is quite helpful, but it doesn't tell the whole story. As detailed as it strives to be, the USDA plant hardiness zone map won't reveal your microclimate. For example, if your garden is nestled in a valley, it could be a few degrees warmer than the surrounding area. A large, exposed expanse of land, however, may run a few degrees colder than other areas. A perennial that is marginally hardy in your zone may have increased winter survivability if it's planted near a brick wall because the wall will trap and release heat.

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Air temperature also influences soil temperature. This means that even in the winter when a perennial is nothing but roots, the local microclimate and air temperatures can impact the plant's survival.

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The hardiness map also won't tell you how hot an area can get. Some plants do just as poorly in hot weather as they do in cold. For this, the American Horticultural Society created the plant heat zone map. This map depicts the average number of days every year that the temperature in a certain area rises above 86 degrees.

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Like the plant hardiness map, the heat zone assigns a zone number to each area of the United States. Heat zones, however, are not as widely used as hardiness zones. Some of the plants you see in the nursery may have a label indicating their heat zone, but many will not. If you're concerned that an unlabeled plant may not work well in your heat zone, you can contact your local cooperative extension service to find out the specific needs of the plant you're considering.

Other Factors to Consider

When choosing new plants for your garden, their hardiness zone ratings should absolutely be one of the first things you check. However, following the advice of the USDA plant hardiness zone map alone doesn't guarantee thriving plants. You'll still need to do a bit more research on your particular plant, especially if you plan to push the envelope and grow a plant in a slightly warmer or colder zone than usual.

Here are a few other smart tips when planting your garden:

Light

Different plants need different amounts of light, and your USDA hardiness zone can affect light exposure. If, for example, you live in zone 5 but fall in love with a plant that prefers part shade in zone 6, you'll need to be especially careful of the light your plant will get in the winter. Living on the edge of its comfort zone can stress the plant, and getting too much direct winter sun when other plants have lost their shading foliage could prove too much.

In this example, a zone 6 plant growing in the Midwest wouldn't get much sun in the winter. In Denver, however, which is in zone 5, the winter sun is often more intense. This means that in Denver, your plant will have to deal with a zone that is a bit colder and brighter than it prefers. Even if the cold isn't an issue, the light intensity could be.

Temperature

The temperature can also affect soil conditions. In some areas, the soil dries significantly at the end of the summer heat. Some plants that would ordinarily thrive in your hardiness zone may struggle if you live in a microclimate where the soil gets too dry. A dry fall can stress these plants just as they prepare for winter.

Humidity

Humidity is also a factor for some plants, as is the duration of any cold weather. Sometimes, a plant can tolerate a few hours of freezing but won't survive a few days of it.

Tip

If you're not sure how all these factors will come together for you, talk to your local garden center or cooperative extension service office. These valuable resources live, work, and garden where you do and can help you pick plants that will work well in your area.

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