As most in the horticulture industry have heard, the USDA released an updated plant hardiness zone map just over a year ago. This 2023 version replaces that of 2012 and illustrates that vast areas of the U.S. have shifted half a zone warmer over the past decade or so.
In many cases, this half zone shift brings regions to an entirely new hardiness zone, as is the case at the Chicago Botanic Garden, where we’ve moved from zone 5b to zone 6a. This same change from zone 5 to zone 6 can be seen in a band spanning hundreds of square miles from coastal Maine down through central Pennsylvania and west through northern Kansas, before the change is less prominent further west in and beyond the Rocky Mountains. This same transition can be seen in largely lateral geographic bands for different zones throughout the eastern two-thirds of the country.
It is important to note that. For example, edaphic conditions play a significant role in overwintering. In the plant hardiness zones are only a measure of the average extreme minimum temperature and that several other variables are at play when determining whether a plant will be hardy Chicagoland area, we have heavy clay soils that retain water more than other soil types and give plants what we colloquially refer to as “wet feet”. This, combined with repeated freeze/thaw cycles, makes it difficult for some genera to overwinter. For example, genera like Coreopsis, Delphinium, and Lupinus that struggle to overwinter here tend to be more successful in colder climates than ours if grown in well-drained soil and under consistent, insulating snow cover.
Luckily, it isn’t too difficult to implement some cultural practices to assist marginally hardy plants. Siting is important, as plants situated just a few yards apart may experience significantly different microclimates. Planting something near a building may provide some residual heat during the frigid winter months. Buildings and other structures, including other nearby plants, can also provide protection from desiccating winds.
In the example pictured in the photo gallery on this page, the redosier dogwood (Cornus sericea) is sited up against a house, enjoying a southern exposure. Roughly 10 yards away, the river birch (Betula nigra) has a more western exposure and nothing around it to buffer the wind. Though obviously in the same hardiness zone, these two plants are likely experiencing two different winters.
Once an appropriate site is chosen, other methods can be used to help with soil structure and water management. In our area, I typically mix some sand or gravel and plenty of organic material (often as topsoil) in with our clay soil to improve drainage. I then mulch with shredded leaves or wood chips. While most home gardeners think of mulch for its water retention and weed suppression benefits, it also helps moderate the temperatures that plant roots experience underground, especially in the absence of snow cover. And of course, it doesn’t hurt to plant in spring, as this gives roots a full growing season to establish before winter arrives. In short, proper siting and preparation may allow new taxa to flourish in places they wouldn’t before.
Considering our milder winters and these cultural practices, I think it’s a good time for home gardeners to broaden what they’re willing to trial and nurseries to reevaluate what they decide to offer. Folks in or near areas that have changed hardiness zones should revisit their catalogs and try out that one perennial or shrub they’ve always dreamed of having but always thought was just a bit too temperamental. This is, of course, only possible if nurseries are willing to take the gamble of providing such plants, though the gamble also lies with the gardener once the plant is sold. Even if a given plant remains too tender to survive every winter, it could still expand a garden’s palette by functioning like an annual as long as it grows quickly enough for nurseries and isn’t too expensive for gardeners to buy more than once. But when nurseries decide to carry these marginally hardy plants, it is vital to be transparent about the nature of these new “perennials” so as not to lose the trust of gardeners. Who can forget Coreopsis ‘Limerock Ruby’? I’m no savvy businessman, but I don’t think deceiving consumers is ever the right strategy.
In summary, we may not know what is and isn’t hardy as well as we think we do, but we’ll never learn what is now hardy without growing it. The novelty of a new plant could attract gardeners like bees to sunflowers, though the less-educated gardeners who don’t know as much about hardiness may be the first ones drawn in. Perhaps it would be worth including a “Moving North” section in local garden centers to compile marginally hardy plants that traditionally only survived a bit further south. I would not be able to resist browsing such a section.
Lastly, it’s worth noting that this shift in climate has been gradual and didn’t happen from 2022 to 2023. As such, even if something remains tender in 2025, it may be worth trying again in another few years. I’ve always gardened with the clichéd mindset of prepare for the worst and hope for the best, and this is a simple extension of the same skeptical optimism. To put my money where my mouth is, I’ll be ordering some red spider lilies (Lycoris radiata) this year to see if we’ve truly entered zone 6. We’ll see what happens.
