Frost Dates for 55115 — Mahtomedi, Minnesota

Half of springs at Stillwater 2SW, MN see their last 32°F night by April 30; only one in ten is past it by April 17, and one in ten still has one after May 13. Planting tender crops after May 5 leaves a 30% risk of a frost on them, after May 8 20%, and after May 13 10%. The 13 days between the even-odds date and the one-in-ten date is narrower than at 61% of the 142 Minnesota stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 11 in half of years, as early as September 28 one year in ten, and holds off past October 23 one year in ten. A hard freeze of 28°F follows 12 days later, around October 23; in spring the one-in-ten date for 28°F is May 2, 11 days before the 32°F one. The freeze-free season lasts 163 days in a typical year, longer than at 84% of the 142 Minnesota stations; it reaches 181 days one year in ten and falls short of 143 one year in ten. On average 159 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 8 days longer, to about May 8, and return around October 2. Severe cold of 16°F or below comes every winter, usually between November 18 and March 27.

July is the warmest month at Stillwater 2SW, MN, averaging 79°F by day and 62°F at night; January is the coldest at 22°F and 5°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 47% of years.

Warmth for growing adds up to 2,473 growing degree days a year (base 50°F), more than at 54% of the 142 Minnesota stations. 33% of them (810) have built up by the end of June, and July adds the most, 644.

The nearest other station, Vadnais Lake, MN, 7 miles west of Mahtomedi, has a last frost 7 days earlier (April 23), a first frost 9 days later (October 20), a season 14 days longer.

The first measurable snow (0.1 inch or more) in Mahtomedi usually arrives around November 27, and the last of the season around March 24. Those are the middle dates across the winters of 1991–2020 at Vadnais Lake, MN, 6.8 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 37 days after the last snow.

ZIP 55115 is 5 miles from Stillwater 2SW, MN, which stands at 898 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 2 other ZIP codes share the station; its figures are presented in full for 55082 (Stillwater). The 2023 USDA Plant Hardiness Zone Map lists ZIP 55115 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.