Winter hardiness zones and the warming of the coldest night of the year in the Netherlands

The USDA plant hardiness zone, printed on virtually every plant label, is defined as the 30-year mean of the lowest temperature recorded each year. Using the homogenized daily minimum temperature series (version 2.0, 2026) of the five principal KNMI stations in the Netherlands (1901/1907-2022/2026), we compute annual extreme minima under two year conventions (calendar year, the USDA convention, and a July-June frost year) and compare three 30-year climate normal periods: 1931-1960, 1961-1990 and 1991-2020. The national mean of the annual extreme minimum was nearly unchanged between the first two periods (-10.73 to -10.67 degrees C) and then rose by 2.0 degrees C in a single generation to -8.66 degrees C (calendar year; +2.5 under the frost-year convention), roughly three times the warming of the autumn seasonal mean over the same step. As a result the Netherlands as a whole shifts from zone 8a to 8b. Three of the five stations shift robustly by half a zone under both year conventions and both homogenization versions (De Kooy 8a to 8b, Eelde 7b to 8a, Vlissingen 8b to 9a); De Bilt and Beek end within 0.5 degrees C of the 8a/8b boundary and are reported as boundary cases, not as shifts. Trends over the full series are 0.26 to 0.34 degrees C per decade (Mann-Kendall, p < 0.05 at all stations on the primary series). All data, methods and code are openly available.