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  • The USDA's New Plant Hardiness Zone Map - Explained

    November 27, 2023 6 min read 0 Comments

    Gardener in Greenhouse Harvesting Tomatoes

    What is the New USDA Updated Plant Hardiness Zone Map?

    Plant Hardiness Map Zones Explained

    Imagine the United States as a big garden. But this garden isn't all the same - some parts are scorching, some parts are abnormally fridged, and some areas have weather that changes a lot. Now, different plants like different types of weather. Some plants love the cold, and some plants need lots of sun and warmth.

    The USDA (United States Department of Agriculture) created a map that splits the country into 13 zones. Each zone represents a different kind of climate or weather pattern. The map is based on the average lowest winter temperature over 30 years. The zones are numbered from 1 to 13. Zone 1 is the coldest, where temperatures can go down to minus 50 degrees Fahrenheit (-45.6°C), and Zone 13 is the warmest, where the temperature rarely goes below 60 degrees Fahrenheit (15.6°C).

    In addition to the main zones numbered from 1 to 13, the USDA Hardiness Zone Map includes 'A' and 'B' subdivisions within each zone. These divisions represent a slightly more detailed breakdown of climates.

    Each zone represents a range of average annual minimum winter temperatures of 10 degrees Fahrenheit. For example, Zone 7 ranges between 0 to 10 degrees Fahrenheit. But that's still a pretty big range! So, the USDA decided to split each zone into two parts: 'A' and 'B,' each representing a 5-degree Fahrenheit difference to make it more precise.

    For instance, if you're in Zone 7:

    • Zone 7a would have winter low temperatures between 0 to 5 degrees Fahrenheit.
    • Zone 7b would have winter low temperatures between 5 to 10 degrees Fahrenheit.

    These subzones 'A' and 'B' allow gardeners to choose plants more likely to thrive in their specific climate.

    So, if you live in Zone 3, you'll want to find plants that can handle really cold winters. But if you live in Zone 10, you'll look for plants that can handle hot weather.

    What’s Different in the New Plant Hardiness Map​​? 

    The new 2023 map for plant hardiness zones has some cool new features. It has data from more weather stations than the old map,13,412 weather stations compared to the 7,983 since the last major update in 2012, so it's more accurate. And for Alaska, Hawaii, and Puerto Rico, the map has much more detail now. Instead of showing a big area, it shows smaller sections. 

    Data resolution is enhanced from a 6 ¼ -square-mile area of detail to a ¼ square mile. This higher resolution helps growers dial in their microclimates and geographical oddities to account for different terrains that can change weather patterns from one neighborhood to the next. 

    The average temperatures are pulled from updated weather reports that show weather changes in the extreme bell curve of the zones. This way, with the help of the USDA Hardiness Zone Map, gardeners can make smart choices about what to plant and expect a better chance of their plants surviving and thriving.

    The map should act as a guide, but not an absolute, when making growing decisions. Other factors, such as available light, soil composition, rainfall, temperature allowances & manipulation, duration of exposure to cold and wind (windchill), relative humidity, pest and wildlife pressure, disease management, and crop selection, all play a role in crop success. However, with vigilance and care from the farmer, crops can thrive.

    How to Use the Plant Hardiness Map Zones: 

    To find your Plant Hardiness Zone, type in your ZIP code and search. Then, click on the map where you want to know the zone.

    If you want to see a map of your whole state, go to the "Map Downloads" tab at the top. You can choose different sizes for the map and even print it out. There are different DPI (Dots per Square Inch) options to print from. Choosing between 150 dpi (dots per inch) and 300 dpi for a map depends on the purpose and quality you need.

    If you're downloading a map for casual use or viewing on a digital screen, 150 dpi should be sufficient. It provides decent detail and clarity while keeping the file size smaller, making it easier to store and share.

    However, if you need a higher-quality map for professional printing or detailed analysis, 300 dpi is recommended. This higher resolution will result in a sharper and more precise image when printed, ensuring that all the fine details and labels are clear and readable.

    Using the Features on the New USDA Plant Hardiness Maps

    The Plant Hardiness Zone Map comes with numerous interactive features. Below the Zip Code Search on the map, you'll find various "widgets." Besides the standard map widgets that let you zoom into your current location and reset to the default map view, there's the layers widget (represented by a stack of papers). This widget lets you switch any map layer on or off, including roads, placenames, and the Zones layer. Plus, if you click on the ". . ." next to any layer, you'll get a slider that lets you tweak the transparency level of the chosen layer, which can be handy for better visualization. 

    These features help dial in growing locations that growers may find helpful if you are in a zone transition. For all of you spreadsheet types, check out the PRISM Climate Group website for downloadable CVS files in your area. PRISM Climate Group’s data updated the new USDA zone map.  

    Knowing your first and last average frost date will help determine when you can start the hardening-off process to transplant your indoor seed starts outdoors. You can access this tool at climate.gov

    How to Plan with the New USDA Map Changes

    If the new USDA Plant Hardiness Zone Map says your plant's zone has changed, it's okay! You don't have to eliminate plants or change what you're growing. Your plants will most likely keep doing well.

    The zones on the map show the average coldest temperature each year for the past 30 years. They don't show the coldest it's ever been or could be. So, consider this when choosing new plants, especially if you want to grow plants outside your zone.

    This map is very detailed, but there might still be tiny areas called microclimates that are too small to see. These are spots that might be hotter or cooler than their surroundings. 

    Your garden might have these, too. For example, a part of your garden might be warmer or cooler than the rest because it's sheltered or exposed. Also, some parts of your garden might be warmer or cooler than the rest of your yard, like a spot near a south-facing wall or a low spot where cold air collects. A map can only tell you some things about your garden. You learn the most from spending time in it.

    Microclimates are especially common in locations with extreme elevation changes, thinner air, and increased shaded areas. 

    Related:  How to Create and Work with Microclimates in the Garden

    How do Seasons Affect Plant Hardiness?

    Many plants slowly get used to the cold in the fall as days get shorter and it gets cooler. They lose this cold hardiness gradually in late winter as it gets warmer and days get longer. But, a sudden cold snap early in the fall might hurt plants, even if it doesn't get as cold as the average coldest temperature for your zone. Also, a warm spell in the middle of winter, followed by a quick return to average cold weather, might harm plants. The USDA Plant Hardiness Zone Map doesn't account for these things. This is where using Frost Blankets during frequent temperature changes is especially helpful. 

    The map is a general guideline for the AVERAGE lowest temperature. Growers are responsible for preparing and equipping themselves for colder and hotter spikes in ever-changing weather patterns and conditions. 

    Bootstrap Farmer’s advice: If this is your first year growing, detailed market evaluations should always be your first step in deciding what crops to bring to market. Once your buyers tell you what they want, you can start researching crop varieties and ordering seeds. For home gardeners, this means asking your family what they will and won’t eat before planting a whole row of butternut squash, only to find out the kids won’t touch it!

    Once your crops are decided and planned by season, talk to a trusted seed source and read the seed labels to determine if their recommendations are a good fit. Commercial seed supplier reps often have intimate familiarity with local nuances. 

    Conclusion

    The USDA’s Plant Hardiness Zone Map is a great starting point. When planning crops, though, remember to consider available light, soil composition, rainfall, temperature allowances & manipulation, duration of exposure to cold and wind, relative humidity, pest and wildlife pressure, and disease management. Crop selection should also always consider the actual amount of work and effort needed. 

    NOAA's Interactive map: average date of last spring freeze across the United States used in conjunction with the USDA’s Plant Hardiness Zone Map, and proper crop selections are your three-step tools for deciding what variety to plant and when it is safe to plant those seedlings outside.  

    Finally, consider seasonal extension equipment like an indoor seed starting station, frost blankets, and hoop houses to add up to two growing zones of flexibility to your growth. 

    Check out Bootstrap Farmer’s Growing Guide Resource for seasonal extension and growing tips.

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