U.S. Frost Dates by Weather Station

NOAA station USC00395870 · South Dakota

Frost Dates for Mount Rushmore National Memorial, SD

NOAA weather station Mount Rushmore National Memorial (USC00395870), South Dakota · elevation 5311 ft (1619 m) · hardiness zone 5b*

  • Lower-risk planting dateafter May 29only 1 spring in 10 still has a frost later
  • Typical last frostMay 1550% chance of a frost after it
  • Typical first fall frostOctober 150% chance of a frost before it
  • Frost-free season138 daystypical year, 32°F
  • Hardiness zone5bcalculated*

The year at a glance

Frost season at Mount Rushmore National Memorial, SDSpring: a frost after May 1 in 9 years out of 10, after May 15 in half of the years, after May 29 in 1 year out of 10. Fall: the first frost comes before September 16 in 1 year out of 10, before October 1 in half of the years, before October 19 in 9 years out of 10. Typical frost-free season: 138 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 15First frost ~Oct 1Frost season at Mount Rushmore National Memorial, SDSpring: a frost after May 1 in 9 years out of 10, after May 15 in half of the years, after May 29 in 1 year out of 10. Fall: the first frost comes before September 16 in 1 year out of 10, before October 1 in half of the years, before October 19 in 9 years out of 10. Typical frost-free season: 138 days.JFMAMJJASONDLast frost ~May 15First frost ~Oct 1
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 29 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 29September 16
50% (typical)May 15October 1
90% (likely)May 1October 19
Frost-free season (32°F, 50% chance)
138 days
Hard frost (28°F), last in spring, 50% chance after
May 4
Hard frost (28°F), first in fall, 50% chance before
October 10

The last 15 years (2006–2020 normals)

Typical last frost May 13 (2 days earlier than the 30-year normal), typical first fall frost October 3 (2 days later). Frost-free season 142 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 Mount Rushmore National Memorial: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 29, 20% May 25, 30% May 21, 40% May 18, 50% May 15, 60% May 11, 70% May 8, 80% May 5, 90% May 1. Observed last spring frost in recent years: 2021 May 28, 2022 May 22, 2023 May 18, 2024 May 4, 2025 May 20, 2026 May 21.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 29May 15May 12021: May 282022: May 222023: May 182024: May 42025: May 202026: May 21Fall at Mount Rushmore National Memorial: chance of a frost before this dateFall, chance of a frost before the date: 10% September 16, 20% September 22, 30% September 26, 40% September 29, 50% October 1, 60% October 4, 70% October 8, 80% October 12, 90% October 19. Observed first fall frost in recent years: 2021 October 12, 2022 October 6, 2023 October 6, 2024 October 29, 2025 October 13.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 16Oct 1Oct 192021: October 122022: October 62023: October 62024: October 292025: October 13
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 28October 12
2022May 22October 6
2023May 18October 6
2024May 4October 29
2025May 20October 13
2026May 21season not over yet

What these dates mean

May 29 is the cautious spring date for Mount Rushmore National Memorial, carrying only a 10 % chance of a light frost later and allowing gardeners to set out tender crops with minimal risk. By contrast, May 1 carries a 90 % chance of a later frost, making an early planting a gamble.

The 50% dates are May 15 and October 1; at each of these points the chance of a light frost is even, meaning a 50 % chance of frost later than May 15 in spring and a 50 % chance of frost earlier than October 1 in fall. The median hard-frost dates are May 4 and October 10, indicating when a 28 °F frost becomes equally likely as not.

September 16 is the cautious fall date, with only a 10 % chance that a light frost has arrived earlier, so it marks a reliable deadline for finishing tender-crop harvests. A hard frost at 28 °F can injure or kill plants that would survive a light frost at 32 °F.

How to read the probabilities

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

  • Spring: "10% after May 29" 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 16" 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 Mount Rushmore National Memorial, SDNumbered nearest stations: 1 — Hill City, SD — 6 mi; 2 — Custer, SD — 10 mi; 3 — near Colonial Pine Hills, SD — 13 mi; 4 — near Blackhawk, SD — 19 mi; 5 — Wind Cave, SD — 22 mi.SDWY25 mi · 40 km1. Hill City, SD — 6 mi12. Custer, SD — 10 mi23. near Colonial Pine Hills, SD — 13 mi34. near Blackhawk, SD — 19 mi45. Wind Cave, SD — 22 mi5
  1. Hill City, SD 6 mi (10 km)
  2. Custer, SD 10 mi (17 km)
  3. near Colonial Pine Hills, SD 13 mi (21 km)
  4. near Blackhawk, SD 19 mi (30 km)
  5. Wind Cave, SD 22 mi (36 km)

Sources

* Hardiness zone 5b: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 43.8769, -103.4578.