Last week, the US Department of Agriculture released its updated Plant Hardiness Zone Map (PHZM). This well-known resource for home gardeners and large-scale producers alike has undergone an overhaul from its last version, which was published in 1990.
Click the image above to view a larger version of the map.
The new interactive online PHZM utilizes the PRISM climate data set from Oregon State University. This data is at very high-resolution, with a data point at every 800 m, or approximately half of a mile. This provides for a zip code lookup to find the exact zone in your area, and also allows for a more accurate map that reflects terrain across the state and nation. The underlying data are 30-year averages of the extreme minimum temperatures. The old map included just 12 years of data, 1974-1986.
One clear difference between the old and new map is the larger area now covered by zones 4a and 4b. The newly released map reflects the warmer winter minimum temperatures for the 30 year period of 1976-2005. There is very little area in the 3b zone, primarily located in the higher elevation Black Hills, and not in the northern part of the state as the old map showed. This is consistent with the warming temperature trend that we have seen across much of South Dakota over the last few decades. It is perhaps ironic to note that since 2005 we have had a few years with much colder minimum wintertime temperatures.
Keep in mind that no matter what hardiness zone you live in, that there are microclimates everywhere, at less than half a mile in width. For example, you may find that sunny and shady parts of your yard may allow for different vegetation to grow. You may be able to “push” the hardiness zone for your area by planting a 5a plant in a warm and sunny spot in your yard, even if you are designated in a 4b zone. Also, plant hardiness can differ from year to year, as plants may acclimate better during seasons where cooling occurs more gradually and not in an abrupt fashion.
Click here for more information on the creation of the new PHZM, and to download maps and your zone by zip code.