U.S. Frost Dates by Weather Station

NOAA station USC00053005 · Colorado

Frost Dates for Fort Collins, CO

NOAA weather station Fort Collins (USC00053005), Colorado · elevation 5004 ft (1525 m) · hardiness zone 5b*

  • Lower-risk planting dateafter May 14only 1 spring in 10 still has a frost later
  • Typical last frostMay 250% chance of a frost after it
  • Typical first fall frostOctober 750% chance of a frost before it
  • Frost-free season157 daystypical year, 32°F
  • Hardiness zone5bcalculated*

The year at a glance

Frost season at Fort Collins, COSpring: a frost after April 18 in 9 years out of 10, after May 2 in half of the years, after May 14 in 1 year out of 10. Fall: the first frost comes before September 24 in 1 year out of 10, before October 7 in half of the years, before October 20 in 9 years out of 10. Typical frost-free season: 157 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 2First frost ~Oct 7Frost season at Fort Collins, COSpring: a frost after April 18 in 9 years out of 10, after May 2 in half of the years, after May 14 in 1 year out of 10. Fall: the first frost comes before September 24 in 1 year out of 10, before October 7 in half of the years, before October 20 in 9 years out of 10. Typical frost-free season: 157 days.JFMAMJJASONDLast frost ~May 2First frost ~Oct 7
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 14September 24
50% (typical)May 2October 7
90% (likely)April 18October 20
Frost-free season (32°F, 50% chance)
157 days
Hard frost (28°F), last in spring, 50% chance after
April 20
Hard frost (28°F), first in fall, 50% chance before
October 17

The last 15 years (2006–2020 normals)

Typical last frost May 3 (1 day later than the 30-year normal), typical first fall frost October 10 (3 days later). Frost-free season 159 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 Fort Collins: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 14, 20% May 10, 30% May 7, 40% May 4, 50% May 2, 60% April 30, 70% April 28, 80% April 24, 90% April 18. Observed last spring frost in recent years: 2021 May 11, 2022 May 22, 2023 April 24, 2024 April 22, 2025 April 20, 2026 May 7.Spring: last frostChance of a frost after this date10%50%90%May 1May 14May 2Apr 182021: May 112022: May 222023: April 242024: April 222025: April 202026: May 7Fall at Fort Collins: chance of a frost before this dateFall, chance of a frost before the date: 10% September 24, 20% September 29, 30% October 2, 40% October 5, 50% October 7, 60% October 10, 70% October 13, 80% October 16, 90% October 20. Observed first fall frost in recent years: 2021 October 14, 2022 October 17, 2023 October 14, 2024 October 25, 2025 October 19.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 24Oct 7Oct 202021: October 142022: October 172023: October 142024: October 252025: October 19
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 14
2022May 22October 17
2023April 24October 14
2024April 22October 25
2025April 20October 19
2026May 7season not over yet

What these dates mean

Setting out tender crops on the early spring date, April 18, is a gamble in Fort Collins, because there is a 90 % chance of a light frost later than that day. The cautious alternative is the later date, May 14, when only a 10 % chance of frost remains.

For a gardener, the 50 % dates represent the point at which the odds of a light frost are equal. The 50 % date is May 2 for spring and the 50 % date is October 7 for fall; they are not frost dates but indicate median risk. The 50 % hard-frost dates are April 20 and October 17, marking the moments when a 28 °F freeze is equally likely to have occurred or not. The cautious fall date to finish harvesting tender crops is September 24, which carries only a 10 % chance that a light frost has arrived.

A hard frost at 28 °F can injure plants that would survive a light frost at 32 °F, making it a more critical threshold.

How to read the probabilities

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

  • Spring: "10% after May 14" 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 24" 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 Fort Collins, CONumbered nearest stations: 1 — Fort Collins 4 E, CO — 3 mi; 2 — Loveland, CO — 10 mi; 3 — near Greeley, CO — 24 mi; 4 — Rustic, CO — 34 mi; 5 — Boulder, CO — 41 mi.WY25 mi · 40 km1. Fort Collins 4 E, CO — 3 mi12. Loveland, CO — 10 mi23. near Greeley, CO — 24 mi34. Rustic, CO — 34 mi45. Boulder, CO — 41 mi5
  1. Fort Collins 4 E, CO 3 mi (5 km)
  2. Loveland, CO 10 mi (16 km)
  3. near Greeley, CO 24 mi (38 km)
  4. Rustic, CO 34 mi (55 km)
  5. Boulder, CO 41 mi (67 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: 40.5764, -105.0858.