Any gardener or horticulturist with meaningful experience shopping for plants is familiar with the USDA plant hardiness zone map. (If not, check out the online version at https://planthardiness.ars.usda.gov.) Like an imposing arbiter of vegetation dispersal, the color-coded map breaks the United States and Puerto Rico into 13 zones based on the average annual extreme minimum temperature in other words, the lowest temperature each zone experiences in a typical year. Each zone is further divided into two half-zones, labeled "a" (colder) and "b" (warmer). These zones are used to label the hardiness of plants, determining whether they're suited for planting in any given region. A variety of raspberry bush, for example, might be rated for success in zones 5 through 9.
Some zones are too warm for certain plants; apple trees, for example, need a certain number of "chill hours" (below 45-degrees Fahrenheit) during the winter in order to grow properly and produce fruit. But for much of the United States, cold temperatures are arguably the greater limiting factor than heat. Cold temperatures can quickly damage and kill plants that aren't suitably cold-tolerant. But before you assume you can never grow a plant that isn't rated for survival in your hardiness zone, keep in mind the zone delineations aren't perfect. Zone borders can change over time (the last update to the map in 2012 labeled many areas half a zone warmer than the previous map), and no nationwide map can capture every local temperature nuance.
THE MAGIC OF MICROCLIMATES
The last point is worth reiterating, because it's good news for those of you with ambitious green thumbs. Just because a general region fits within one hardiness zone doesn't mean every acre of land within the region occupies that zone. A "microclimate" is exactly what its name suggests: a small area with a different climate than nearby or surrounding areas.
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