Frost Dates for 50158 — Marshalltown, Iowa

Half of springs at Marshalltown, IA see their last 32°F night by April 28; only one in ten is past it by April 13, and one in ten still has one after May 11. Planting tender crops after May 3 leaves a 30% risk of a frost on them, after May 6 20%, and after May 11 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 22 one year in ten, and holds off past October 21 one year in ten. A hard freeze of 28°F follows 11 days later, around October 18; in spring the one-in-ten date for 28°F is May 2, 9 days before the 32°F one. The freeze-free season lasts 162 days in a typical year, shorter than at 61% of the 132 Iowa stations; it reaches 180 days one year in ten and falls short of 143 one year in ten. On average 152 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 9 days longer, to about May 7, and return around September 28. Severe cold of 16°F or below comes every winter, usually between November 16 and March 20.

July is the warmest month at Marshalltown, IA, averaging 84°F by day and 63°F at night; January is the coldest at 28°F and 9°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 38% of years.

Warmth for growing adds up to 2,915 growing degree days a year (base 50°F), fewer than at 60% of the 132 Iowa stations. 35% of them (1,016) have built up by the end of June, and July adds the most, 716. Precipitation totals 38.0 inches a year, most in June (5.9") and least in January (1.0"); May to September bring 62% of it.

The nearest other station, Toledo 3N, IA, 17 miles east of Marshalltown, has a last frost 2 days later (April 30), a first frost 2 days later (October 9).

The first measurable snow (0.1 inch or more) in Marshalltown usually arrives around November 27, and the last of the season around March 21. Those are the middle dates across the winters of 1991–2020 at Marshalltown, IA, 1.8 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 38 days after the last snow.

ZIP 50158 is 2 miles from Marshalltown, IA, which stands at 870 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 50158 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.