The USDA has just released their new Plant Hardiness Zone Map. This is a much-needed update using the 30-year period of 1976-2005 versus the old USDA map which used the period 1974-1986.
One of the outstanding features of the new map is the northward shift of the hardiness zones in Illinois. For example, the boundary between zones 5 and 6 have shifted about 60 miles to the north. Zone 4 has left Illinois in the new map. In addition, the new map shows a lot more detail including the warming effect of the Chicago urban area (see map below).
In the second figure, the minimum winter temperature for each year is shown for Champaign-Urbana. The old USDA plant hardiness zone map used an unusually short period during one of the colder periods in the record (1974-1986). The general trend in the data shows an increase in the minimum winter temperature from the late 1800s to about 1950, followed by a cooling trend to the 1980s, and finishing with a warming trend through 2005. Interestingly, the last few years showed a cooling trend – except for this year when our coldest temperature so far is 4°F.
The other important feature in the second figure is that even if an area changes Zones, there is still a lot of year to year variability in the minimum temperature for winter. For example, while Champaign-Urbana is nearly classified as Zone 6 in the new map, Champaign-Urbana dipped into Zone 4 a few times (-20°F or less) in the last 30 years. In fact, as a gardener I would say that the period from the 1930s to the early 1970s was more benign and less challenging than the 1980s and 1990s with respect to winter temperatures. Summer conditions would be another story since the 1930s and 1950s were marred by severe droughts.