U.S. Frost Dates by Weather Station

NOAA station USC00392302 · South Dakota

Frost Dates for De Smet, SD

NOAA weather station De Smet (USC00392302), South Dakota · elevation 1705 ft (520 m) · hardiness zone 4b*

  • Lower-risk planting dateafter May 20only 1 spring in 10 still has a frost later
  • Typical last frostMay 550% chance of a frost after it
  • Typical first fall frostOctober 450% chance of a frost before it
  • Frost-free season151 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at De Smet, SDSpring: a frost after April 22 in 9 years out of 10, after May 5 in half of the years, after May 20 in 1 year out of 10. Fall: the first frost comes before September 23 in 1 year out of 10, before October 4 in half of the years, before October 17 in 9 years out of 10. Typical frost-free season: 151 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 5First frost ~Oct 4Frost season at De Smet, SDSpring: a frost after April 22 in 9 years out of 10, after May 5 in half of the years, after May 20 in 1 year out of 10. Fall: the first frost comes before September 23 in 1 year out of 10, before October 4 in half of the years, before October 17 in 9 years out of 10. Typical frost-free season: 151 days.JFMAMJJASONDLast frost ~May 5First frost ~Oct 4
frost likely chance fading 90% → 10% frost-free in a typical yearThe year at a glance. Blue: frost is likely; the gradient fades as the chance drops from 90% to 10%; green: the frost-free part of a typical year.

Frost dates (32°F)

Probability of frost, based on 27 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 20September 23
50% (typical)May 5October 4
90% (likely)April 22October 17
Frost-free season (32°F, 50% chance)
151 days
Hard frost (28°F), last in spring, 50% chance after
April 28
Hard frost (28°F), first in fall, 50% chance before
October 14

The last 15 years (2006–2020 normals)

Typical last frost May 6 (1 day later than the 30-year normal), typical first fall frost October 6 (2 days later). Frost-free season 152 days.

A shorter, more recent NOAA period. The 30-year normals above remain the main reference: 15 years carry more year-to-year noise.

Add these dates to your calendar Yearly reminders for the typical last frost, the lower-risk planting date and the typical first fall frost (.ics).

How the chance of frost changes with the date

Spring at De Smet: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 20, 20% May 14, 30% May 11, 40% May 8, 50% May 5, 60% May 3, 70% April 30, 80% April 27, 90% April 22. Observed last spring frost in recent years: 2021 May 11, 2022 April 28, 2023 May 3, 2024 April 24, 2025 April 29, 2026 May 20.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 20May 5Apr 222021: May 112022: April 282023: May 32024: April 242025: April 292026: May 20Fall at De Smet: chance of a frost before this dateFall, chance of a frost before the date: 10% September 23, 20% September 27, 30% September 30, 40% October 2, 50% October 4, 60% October 7, 70% October 10, 80% October 14, 90% October 17. Observed first fall frost in recent years: 2021 October 15, 2022 October 7, 2023 October 7, 2024 October 14, 2025 October 22.Fall: first frostChance of a frost before this date10%50%90%Oct 1Sep 23Oct 4Oct 172021: October 152022: October 72023: October 72024: October 142025: October 22
Two charts, because spring and fall answer different questions: in spring, the chance that a frost still comes after a date; in fall, the chance that the first frost has already come before it. NOAA publishes these odds from 10% to 90%, so the lines stop there. ◆ marks the actual date in each recent year.

What actually happened in recent years

Observed dates of the last spring and first fall frost (32°F) at this station; ◆ marks them on the charts above
YearLast spring frostFirst fall frost
2021May 11October 15
2022April 28October 7
2023May 3October 7
2024April 24October 14
2025April 29October 22
2026May 20season not over yet

What these dates mean

May 20 is the cautious spring light-frost date for De Smet; with only a 10 % chance of a frost later, a gardener can plan to set out tender crops on that day without taking a high risk. The median light-frost date, May 5, marks the point where the chance of a frost later is even, while April 22 carries a 90 % chance and would be a gamble for early planting.

In the fall, September 23 is the cautious light-frost date, offering a 10 % chance of a frost earlier and therefore the latest reliable day to finish harvesting tender crops. The median fall light-frost date is October 4, indicating an even odds situation, whereas October 17 has a 90 % chance of a frost earlier and would be risky for extending the season.

Hard frosts at 28 °F are more damaging than light frosts, and the 50 % hard-frost dates—April 28 in spring and October 14 in fall—show when the probability of a hard freeze is balanced, underscoring the need to protect vulnerable plants around those times.

How to read the probabilities

The dates are probabilities from 30 years of weather, not a forecast for this year.

  • Spring: "10% after May 20" means that in about one spring out of ten a frost still happens after that date. The 90% date is the earliest: a frost after it happens in most years.
  • Fall: "10% before September 23" means that in about one fall out of ten the first frost arrives before that date.
  • 50% is the typical date: half of the years are earlier, half are later.
  • A light frost (32°F) damages tender plants; a hard frost (28°F) also stops most hardy vegetables.

Nearby weather stations

Map of weather stations near De Smet, SDNumbered nearest stations: 1 — Huron, SD — 33 mi; 2 — Madison, SD — 35 mi; 3 — Brookings, SD — 39 mi; 4 — Watertown, SD — 41 mi; 5 — Clear Lake, SD — 50 mi.MN25 mi · 40 km1. Huron, SD — 33 mi12. Madison, SD — 35 mi23. Brookings, SD — 39 mi34. Watertown, SD — 41 mi45. Clear Lake, SD — 50 mi5
  1. Huron, SD 33 mi (54 km)
  2. Madison, SD 35 mi (57 km)
  3. Brookings, SD 39 mi (62 km)
  4. Watertown, SD 41 mi (66 km)
  5. Clear Lake, SD 50 mi (80 km)

Sources

* Hardiness zone 4b: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 44.3825, -97.5497.