I posted back in January about my excitement about the update of the USDA Hardiness zone map. While I acknowledged some of the shortcomings of the new map, I was excited because it was a big improvement over the old map, both in terms of content (more recent temperature data included) and presentation (interactive search features, better graphics).
Dr. Nir Krakauer, assistant professor of civil engineering in The City College of New York’s Grove School of Engineering, was less impressed, however, and has essentially declared the new hardiness map dead on arrival. Dr. Krakauer conducted his own analysis of climatic data and determined that much of the country is already a half zone to one full zone warmer than the new USDA map. Why the difference? The USDA zone estimates were based on a simple average of annual minimum temperatures from the past thirty years. Dr. Krakuar’s applied a regression analysis to account for more recent warming. One of the key observations of the new analysis is that winter minimum temperatures are warming much faster than average temperatures, leading to the new hardiness zone projections.
So, what do we do with this information? All of the limitations that we associate with hardiness ratings still apply. Most landscapes experience micro-climates that may be slightly colder or warmer than the surrounding area. Plus, winter hardiness is just one piece of information in plant selection (we also need to also consider sun exposure, pest pressure, soil factors – drainage, soil pH, and so on). Most importantly, average temperatures don’t kill plants, extreme temperatures do. So whether we look at long-term averages or try to account for the most recent trends; the extreme 1 year in 5 or 1 year in 10 is what will cause issues. For example, if Krakuar’s projection is right and we’re a zone warmer on average, then the greater Lansing area is now zone 6b (0 to -5 deg. F). We’ve been colder than that in 2 out of the last 4 years.
The press release that accompanies Dr. Krakuar’s study starts with an attention-getting line: “Gardeners and landscapers may want to rethink their fall tree plantings.” But really things haven’t changed that much from the past. How homeowners and landscapers use zone information is still a matter of risk tolerance. If I chose a zone 5 plant and we’ve really warmed to zone 6, I’ve lost nothing. If I gamble and plant zone 6 plants and we drop back down to -10 or -15 deg. I may be looking for replacements. And, of course, there’s the tendency of many gardeners to ‘push their zone’, so we may have folks trying to grow zone 7 plants in Lansing. Which is fine – as long as people understand the risk. Actually I think a better approach to hardiness zones may be a probability rating: Are you willing to take a 1 in 10 chance your plant won’t make it? 1 in 4? 50-50? As new climate data are added to the model, garden centers could post and update the odds, sort of like the latest line at a sports book in Vegas. Those that want the ‘tried and true’ can stick with the old zone 5’s; those that want to live on the edge can pay their money and take their chances.