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 plants that are likely to grow well where you live. Growing the right plants for your climate zone can save you a lot of frustration in the garden and a lot of money since you won't be wasting your cash on plants that just die. To get the most from the hardiness planting zones map, it's important to understand what it tells you and what it doesn't.
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What the USDA Plant Hardiness Zone Map Says
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 colder than the last by 10 degrees Fahrenheit. Each of these zones is then broken into two subsections to account for an area's more specific microclimates.
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 hardiness zone map was last updated in 2012. To update it, the USDA looked at the lowest winter temperatures in each area over the previous 30 years. Based on this information, it then calculated the annual average extreme minimum temperature in each zone.
How to Use This Information
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.
The plants you buy at the nursery or garden center are rated based on these zones, which essentially tell you whether or not the plant you buy will survive the winter. 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.
Another helpful aspect of plant hardiness ratings is that they can help you understand what a plant expects. Deciduous trees, for example, 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.
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.
You Have Wiggle Room
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.
Some plants are perfectly happy living in a pot and moving inside during the winter. These plants can grow anywhere. (Note too that the growing zones don't apply to annuals like petunias, mums and geraniums.) 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.
What the USDA Map Doesn't Say
The USDA 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 may not show your microclimate. 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.
This matters because air temperature affects 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.
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.
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 still count on Google to help you research 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, especially if you plan to push the envelope and grow a plant in a slightly warmer or colder zone than usual.
Light is one such factor. Different plants need different amounts of it, 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.
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 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.
If you're not sure how all these factors will come together for you, talk to your local garden center or agricultural extension office. These valuable resources live, work and garden where you do and can help you pick plants that will work well in your area.