Autumn warming and the bulb-planting window in the Netherlands

We analyse the homogenised daily temperature series (version 2.0, 2026) of the five principal stations of the Royal Netherlands Meteorological Institute (KNMI) to quantify how the autumn climate, and with it the window for planting spring-flowering bulbs, has shifted over the last century. Comparing three standard 30-year climate normals (1931-1960, 1961-1990 and 1991-2020), the national mean autumn (September-November) temperature stayed almost flat between the first two generations (+0.08 C) and then rose +0.76 C in the most recent generation, +0.84 C in total (station range 0.70 to 0.95 C). The trend over the full series is 0.18 C per decade and is significant at all five stations (Mann-Kendall p < 1e-9). The date of the first autumn frost has moved later at every station and is everywhere significant, by about 8 days nationally over two generations, but the per-station and per-generation picture is uneven, so we treat frost as supporting rather than headline evidence. The warming lengthens the mild, workable tail of the autumn planting window; the horticultural rule that bulbs can be planted until the ground freezes is unchanged. All data and methods are open and reproducible.