Frost Dates for 52101 — Decorah, Iowa

Half of springs at Decorah, IA see their last 32°F night by May 3; only one in ten is past it by April 19, and one in ten still has one after May 16. Planting tender crops after May 9 leaves a 30% risk of a frost on them, after May 12 20%, and after May 16 10%. The 13 days between the even-odds date and the one-in-ten date is narrower than at 42% of the 132 Iowa stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 7 in half of years, as early as September 25 one year in ten, and holds off past October 21 one year in ten. A hard freeze of 28°F follows 10 days later, around October 17; in spring the one-in-ten date for 28°F is May 8, 8 days before the 32°F one. The freeze-free season lasts 157 days in a typical year, shorter than at 77% of the 132 Iowa stations; it reaches 177 days one year in ten and falls short of 139 one year in ten. On average 153 nights a year drop to 32°F or below. Nights of 36°F, when frost can still settle on leaves in low, open spots, continue 10 days longer, to about May 13, and return around September 28. Severe cold of 16°F or below comes every winter, usually between November 17 and March 21.

July is the warmest month at Decorah, IA, averaging 84°F by day and 63°F at night; January is the coldest at 28°F and 10°F. A freezing night is all but certain in every month from October to April (a 90% chance or more). From June to August the chance of a 32°F night stays under 10% each month. In spring, May still brings one in 58% of years.

Warmth for growing adds up to 3,070 growing degree days a year (base 50°F), more than at 58% of the 132 Iowa stations. 35% of them (1,063) have built up by the end of June, and July adds the most, 728. Precipitation totals 37.7 inches a year, most in June (6.1") and least in January (1.0"); May to September bring 62% of it.

The nearest other station, Waukon, IA, 16 miles east of Decorah and 415 feet higher, has a last frost 1 day earlier (May 2), a first frost 4 days later (October 11), a season 4 days longer.

The first measurable snow (0.1 inch or more) in Decorah usually arrives around November 19, and the last of the season around March 29. Those are the middle dates across the winters of 1991–2020 at Decorah, IA, 3.1 miles away: half of years saw snow earlier, half later. Every one of those winters saw snow. The last 32°F night of spring typically comes 35 days after the last snow.

ZIP 52101 is 3 miles from Decorah, IA, which stands at 860 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 52101 in zone 5a, the band where the coldest night of the year averaged -20 to -15°F (-28.9 to -26.1°C) over 1991–2020; a zone describes winter lows, not frost dates.