The USDA Plant Hardiness Zone Map has long been the standard by which gardeners can determine which perennial plants are most likely to thrive at a location. The map is based on the average annual extreme minimum winter temperature, displayed as 10 degree F zones and 5 degree F half-zones. Zones in this new edition, released just this fall, are based on 1991-2020 weather data.
Many gardeners anticipated that the updated zone map would confirm their suspicions that the climate has been getting milder over the past few decades. Perennials and shrubs that would not have survived a typical Central Indiana winter a generation ago are now becoming common in area gardens. Crepe myrtles are a good example. Forty years ago, they never survived a winter. Then, adventurous gardeners found that, although their plants were killed to the ground by winter temperatures, they vigorously grew to large blooming shrubs the following spring. Now, some varieties don’t even die back!
Half-hardy perennials which never used to survive the winter, including some artemisias, salvias, rudbeckias, and snapdragons now sprout from roots every spring. For the past decade or so, it’s been noticed that canna and calla lilies, even gladiolas, survived when planted in the “micro-climate” created close to a building facing south or west. Now they are surviving even planted out in the open. Central Indiana used to straddle the line between Zones 5b and 6a. Now, with a few exceptions, the northern two thirds of the state is solidly in Zone 6a. The southern third of Indiana is even warmer, reaching Zone 7a in the southernmost counties! Note that the “heat island” that is Indianapolis creates its own microclimate of Zone 6b.
The USDA cautions that (quote): “The new map’s temperature zones do not represent the coldest it has ever been or ever will be in an area, but it simply is the average lowest winter temperatures for a given location for this 30-year span. Consequently, growing plants at the extreme range of the coldest zone where they are adapted means that they could experience a year with a rare, extreme cold snap that lasts just a day or two, and plants that have thrived happily for several years could be lost. Gardeners need to keep that in mind and understand that past weather records cannot provide a guaranteed forecast for future variations in weather.”
“Microclimates, which are fine-scale climate variations, can be small heat islands—such as those caused by blacktop and concrete—or cool spots (frost pockets) caused by small hills and valleys. Individual gardens also may have very localized microclimates. Your entire yard could be somewhat warmer or cooler than the surrounding area because it is sheltered or exposed. You also could have pockets within your garden that are warmer or cooler than the general zone for your area or for the rest of your yard, such as a sheltered area in front of a south-facing wall or a low spot where cold air pools first. No hardiness zone map can take the place of the detailed knowledge that gardeners learn about their own gardens through hands-on experience.”