Indeed, zones are expected to change in the coming decades, as our climate gets warmer. If the USDA map were updated today, we might see that a significant chunk of the Lake Michigan coast would be zone 6, just as the ADF map shows. The Wisconsin DNR usually cites the USDA-derived zones due to the fineness of the data (for example, compare the Driftless Area in both maps), but both maps are good. Other cold hardiness zone maps have come and gone, but these are the two most prominent still standing. This author’s home on Milwaukee’s west side, for example, is 5-6. Some places also have multiple zones listed, in recognition of the microclimates making a one-size-fits-all zone classification impossible. The appropriately named Winter is in 3b with the USDA, but 4 with the ADF. Whereas the USDA marks Racine as 5b, it is a 6 in the ADF map. As such, you’ll see higher numbered hardiness zones with the Arbor Day map. The incorporation of newer data (through 2015) was partially pursued to help capture zone shifts due to climate change. Though not challenging the supremacy of the USDA map, the Arbor Day Foundation has produced its own cold hardiness zone map, most recently in 2015. Wisconsin zones, for example, range from 3b (Hayward) to 6a (a miniscule sliver along the Port of Milwaukee). elevation data), the USDA map also differentiates each zone into two subdivisions. In addition to incorporating local variations into its model (e.g. First produced in 1960, this map has undergone a few irregular iterations over the last sixty years, most recently updated in 2012 with temperature data used through 2005. The most widespread and authoritative cold hardiness zone map was released by the USDA. But did you know that there are multiple hardiness zone maps out there, and that where you stand right now might be in zone 6 on one map, but zone 5 on another? Enter the labyrinth, dear reader. These differences are observed in a location’s cold hardiness zone, which represents the average minimum temperature a location is expected to experience.Ĭold hardiness zones are well-known decision-making factors for anybody with a smidge of green on their thumb. Jack Frost descends upon us all in Wisconsin, but the depths to which he brings the mercury differ depending on your latitude, elevation, and proximity to water or urban areas. By Dan Buckler, DNR urban forest assessment specialist, Madison, 60
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