Garden Planning: What is Your New Zone?
As we approach winter, many of us shift to more indoor gardening activities, especially planning for next year’s gardening projects! The U.S. Department of Agriculture (USDA) just released a new version of one of our most useful planning tools, an updated Plant Hardiness Zone Map.
What is a Plant Hardiness Zone?
If you’re unfamiliar with the concept, the USDA began providing plant hardiness zone information in 1960 to help assess what perennial plants would survive winters in 10 areas of the United States. Each zone represented a 10-degree difference in the average annual minimum temperature, the lower the zone, the lower the temperature.
A 1990 update divided each zone into 5-degree increments labelled “a” and “b” for more precise information. The 2012 version, based on 30 years of temperature records, increased the number of zones to 13, each divided into “a” and “b” 5-degree differences.
The latest map includes the same 13 pairs of 5-degree zones and 30 years of temperature data. The update shows that while Buncombe County zones ranged from 5b to 7a in 2012, zones now range from 6a to 7b! If your garden has shifted to a new zone, it may now have an average low temperature of 5 or even 10 °F warmer than it did in 2012.
How to use the Plant Hardiness Zone Map
To get your zone:
- Click on the map link: https://planthardiness.ars.usda.gov
Enter your 5-digit zip code; this will give you the general zone for your area and the key will indicate the average low temperature.
— For example, for the Buncombe County Extension Office Learning Garden, the zip code is 28806.
— The general zone for this area is 7b and the average low temperature is between 5 and 10 °F.
— Zoom in to locate your garden’s specific location.
—Match the color(s) of your location on the map to the color on the key to indicate your zone. In the case of the Learning Garden, the zone is also 7b, with the average low temperature between 5 and 10 °F.
How to use your Plant Hardiness Zone
Your zone helps you know which perennial plants will survive our winters. Most plants and seeds available for sale will come with information about their hardiness zone. The North Carolina Extension Gardener Plant Toolbox entries also provide the range of zones for plant survival: https://plants.ces.ncsu.edu
Other factors affecting plant survival:
The USDA notes factors other than the average low temperature that determine if plants survive:
- Light: Plants need different amounts of light to thrive. Sudden changes in plant temperature from too much sun in winter might injure marginally hardy plants.
- Soil moisture: Plants’ moisture needs might also vary seasonally. If soil moisture is too low in late fall, they might suffer during their dormancy.
- Temperature: Plants have wider or narrow ranges of ideal temperatures—hot as well as cold.
- Duration of exposure to cold: Plants that can tolerate brief cold exposure may not survive longer cold periods.
- Humidity: Higher humidity reduces plants’ moisture loss, limiting cold damage. Low humidity can cause more severe cold injury, especially for evergreens
Adapted from: https://planthardiness.ars.usda.gov/pages/how-to-use-the-maps
Remember that Plant Hardiness Zones are based on average low temperature, not the lowest temperature that occurs in any given year. “Microclimates” are also important in siting plants. You may have protected areas in your garden where your plants may stay a few degrees warmer, or frost pockets where they may experience longer periods of cold. Knowing your Plant Hardiness Zone is a start in creating the right place for your chosen plants.
Article written by Debbie Green, Extension Master GardenerSM Volunteer.
For more information about the recently updated Plant Hardiness Zones:
USDA Announcement: https://www.ars.usda.gov/news-events/news/research-news/2023/usda-unveils-updated-plant-hardiness-zone-map/
How to use the maps: https://planthardiness.ars.usda.gov/pages/how-to-use-the-maps
NC Cooperative Extension, Buncombe County announcement: https://buncombe.ces.ncsu.edu/2023/11/new-hardiness-zone-map-released/
Why Plant Hardiness Zones are Important: https://www.buncombemastergardener.org/plant-hardiness-zones-important/
Some history of the USDA Plant Hardiness Zone Maps: https://ag.umass.edu/sites/ag.umass.edu/files/fact-sheets/pdf/The%20USDA%20Plant%20Hardiness%20Zone%20fact%20sheet.pdf