Twin Cities growing season, 1871–2025

Y-axis = calendar day (Apr–Nov). A bar sitting low & tall means an early spring thaw and late fall freeze — a long season.

Teleconnection influence

The frost-free window grouped by each spring’s March–May ENSO phase (1950 onward). This is an exploratory association, not evidence that ENSO controls Minnesota freeze dates; pre-1950 phases are uncertain estimates:

• El Niño 170 days on average across 43 seasons — close to the 166-day average of the whole record.
• La Niña 166 days on average across 36 spring La Niña seasons; comparisons with other phases are descriptive, not forecasts.
• Neutral 165 days on average across 76 spring neutral seasons; this does not imply neutral conditions cause shorter seasons.
• The phase averages sit within about 5 days of each other, while individual seasons vary far more than that (standard deviation ≈ 17–18 days). A difference this small sits inside normal year-to-year variability, so no phase reliably delivers a longer or shorter season in this record.
• The growing season spans spring to fall within one calendar year. From 1950–2025, its color uses that year’s March–May ONI, which may differ from the October–December winter phase on the Patterns page. Pre-1950 phase estimates are uncertain.
• NAO & AO are recorded alongside the ENSO phase and listed in each bar's tooltip, but this page does not test whether they relate to frost timing.