During changing and trying times, it only seems relevant to quote Bob Dylan, as the times are indeed a-changin’. Perhaps we could also have exclaimed “ch-ch-ch-changes” as an ode to David Bowie, if that's more your flavor. In any case, due to a changing climate and another passing of a 10-year period, the USDA has released its newest ‘USDA Cold Hardiness’ map for the contiguous United States. Here, we will review what it is, how to use it, and what it means moving forward for your garden, home landscape, and our changing climate.
On November 15th, 2023, The USDA (United States Department of Agriculture) along with the help of Oregon State University's PRISM Climate Group, released the latest “Cold Hardiness” map for the United States. This map takes the climate data from the past 30 years (split into 10-year ‘normals’) and sets forth a map indicating the average lowest temperatures for every region/zone. Each of the 13 “zones” on the map accounts for a 10°F range, which is further subdivided into 2 half-zones (a and b) accounting for 5°F degrees of range each. The new map uses even more data and newer technology to give consumers, gardeners, and farmers more helpful tools to plan for our outdoor spaces.
Cold temperatures are one of the most helpful indicators to determine if a plant can survive in a given area, as it sets the low end of what it can endure. I’m sure we have all seen a plant turn to mush and die over the winter, never to return. Or perhaps a species just not being able to make it through the entire year’s growth cycle. Both of these are likely because the plant was not cold-tolerant enough. For example, you wouldn't plant a tropical banana tree and expect it to overwinter in Ohio or plant a succulent like Aloe outdoors in Aspen, Colorado. Other factors also have an influence, such as microclimates, soil type, rainfall, Growing Degree Days, and more. However, cold tolerance is one of the biggest factors in determining plant survivability in the most general sense.
It is important to note that the USDA zones map uses 30 years of data to solidify the average coldest temperatures for each zone. It’s important because the map does not determine the extreme “low” end of possibility (or extreme high end, for that matter - hello West Coast, Southwest, and Southeast!). As some of us may know, although climate change is generally warming our planet, it can also create more extreme events in winter due to the disruption of the polar vortex, among a few other factors. That’s another blog post, but check out this piece for more on that: Great Texas Freeze of 2021. The takeaway should be, depending on where in the US you reside, there is always the possibility for the low end to be even lower (or the high end to be higher and longer), so playing it safe when it comes to your plant selection, and going with the plant that could potentially handle even outside those averages of the temperature spectrum is wise.
Using the map is quite simple. You hop onto the USDA plant hardiness website, punch in your zip code into the search bar, and you can see exactly what zone you are in. It should tell you the zone with the average low temps and give you the option to zoom in on the color-coded map for the general region related to the zip code. If you are located right on the threshold of multiple zones, my advice would be to err on the side of caution and go with the lower range of the two zones. If you need further info on how to use the maps, they can be found here.
Once you have determined what USDA zone you are in, you should know the average cold temperatures to expect for your garden and the associated zone. For example, zone 6a or zone 10b. Whether you are planting an ornamental garden, native plant haven, or your mini food forest, you’ll be able to see what to expect.
What makes this even easier is that with this zone/temp information, when you are planning or shopping for plants, it's always something to reference. Almost all plants have an associated cold tolerance zone (sometimes called plant hardiness), and it's usually the USDA zone. However, please note, when researching online, make sure that it is referencing the USDA, as there are also other metric systems, like The Sunset Western Garden Zones, which are even more in-depth than just cold tolerance and used by more avid gardeners.
So now, when you are planning your new garden or looking into options to spruce up an existing garden, you can use this USDA zone number to make sure the plants will survive the winter and thrive through an entire year’s growth cycle. Additionally, although certain garden favorites may not survive the cold winter temperatures, come spring through fall, they may very well still be suitable for the garden. One option is to keep them in pots so they can be moved indoors during the colder months.
Lastly, as noted previously and in other blog posts, make sure you are also taking other things into account when planning your garden, such as microclimates, water availability, and soil.
Beyond all the data and the fact that we have a new map to reference, why are we making a fuss about it? Well, in our minds, this is pretty concrete evidence that the climate is very much being affected by our actions, which in turn is having a negative impact on the planet. As our climate warms due to heat-trapping carbon pollution, planting zones have shifted north toward higher latitudes and elevations. Overall, the map shows data that it is, on average, 2.5 degrees warmer than previously.
This means the days of saying, “Well, it's too hard to tell,” when it comes to climate change, are now far behind us, and we are having to come to terms with the fact that the world is changing quite rapidly due to our environmental footprint. Beyond colder temperatures trending further and further north, we are also facing more intense, frequent, and catastrophic weather events.
Although some gardeners who have shifted from zone 7b to 8a might rejoice in that they can now try their gardening game at mandarins, kumquats, or maybe even a cold-hardy banana, the intense reality is that these shifts are also creating more space for invasive species, insects and pests to take hold and thrive in areas they previously couldn't. For example, gardeners in South Florida are struggling to grow tomato plants outdoors due to the extreme and extended heat scorching their plants and providing the perfect host for fungi to have negative effects on their plants.
Our aim here isn't simply to be alarmist. However, we do think it is critical for us to evaluate once again our carbon footprints, and the environmental impacts we have on our host planet. Making educated decisions about our home’s landscape is one small yet significant step in doing so.