We use the Hardiness Zone Map as one of the tools to help us choose the right plant for the right place, especially for cold hardiness. The USDA recently updated it. Hooray! This is big news because the last update was more than 10 years ago and things “they are a'changing’”. Unpredictable, may be a good way to think about the highly variable nature of the extreme minimum temperatures of recent years, plus we’ve got more info plugged into the equation than before. The new map is more precise and detailed, now using data collected from 13,412 weather stations across the country, with more than 5400 additional stations than the old map had access to.
The map divides the continental United States, Alaska, Hawaii and Puerto Rico into 13 zones (and two half-zones within each zone) based on average lowest winter temperatures over 30 years, from 1991 to 2020.
Well, surprise, surprise - the 2023 version of the USDA Hardiness Map reveals that about half the country shifted up to the next warmer zone. This indicates that these areas warmed up somewhere between 0 to 5 degrees Fahrenheit over the past 11 years. Hardiness zones in this map are based on the average annual extreme minimum temperature during a 30-year period in the past, not the lowest temperature that has ever occurred in the past or might occur in the future, so keep the definition of “average annual” in your reckoning for using the map’s information.
The most increase in detail is really visible for Alaska, but what about where you live? The 2023 web version offers a Geographic Information System (GIS)-based interactive format and is designed to be user-friendly. To find out which zone you’re in, type in your zip code in the box on the top of the map. Just be aware that although this edition of the map is drawn in the most detailed scale (down to 1/2 mile square) to date, there could still be microclimates that are too small to show up on the map, like is your foundation dark brick and acting like a solar heat collector during the day, radiating heat at night? Or are you at the bottom of a slope and maybe in a “cold pocket” where cold air settles?
Keep in mind that plant hardiness is determined in the landscape and not in containers above ground, where they don’t have the benefit of the temperature modification of the ground, so if you’re into container gardening more protection will be needed when considering “hardiness”.
Hardiness zone information is a good place to start, however consider that wind, soil type, soil moisture, humidity, pollution, snow, and winter sunshine can greatly affect the survival of plants. The way plants are placed in the landscape, how they are planted, and their size and health might also influence their survival. Knowing the factors of your particular site and what they might mean to the growing environment is very important. Benefit from the logic of “Know Before You Grow”. Your wallet will appreciate it.
