U.S. Frost Dates by Weather Station

NOAA station USC00050848 · Colorado

Frost Dates for Boulder, CO

NOAA weather station Boulder (USC00050848), Colorado · elevation 5484 ft (1672 m) · hardiness zone 6a*

  • Lower-risk planting dateafter May 18only 1 spring in 10 still has a frost later
  • Typical last frostMay 650% chance of a frost after it
  • Typical first fall frostOctober 550% chance of a frost before it
  • Frost-free season151 daystypical year, 32°F
  • Hardiness zone6acalculated*

The year at a glance

Frost season at Boulder, COSpring: a frost after April 24 in 9 years out of 10, after May 6 in half of the years, after May 18 in 1 year out of 10. Fall: the first frost comes before September 21 in 1 year out of 10, before October 5 in half of the years, before October 18 in 9 years out of 10. Typical frost-free season: 151 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 6First frost ~Oct 5Frost season at Boulder, COSpring: a frost after April 24 in 9 years out of 10, after May 6 in half of the years, after May 18 in 1 year out of 10. Fall: the first frost comes before September 21 in 1 year out of 10, before October 5 in half of the years, before October 18 in 9 years out of 10. Typical frost-free season: 151 days.JFMAMJJASONDLast frost ~May 6First frost ~Oct 5
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 26 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 18September 21
50% (typical)May 6October 5
90% (likely)April 24October 18
Frost-free season (32°F, 50% chance)
151 days
Hard frost (28°F), last in spring, 50% chance after
April 27
Hard frost (28°F), first in fall, 50% chance before
October 15

The last 15 years (2006–2020 normals)

Typical last frost May 6 (same as the 30-year normal), typical first fall frost October 10 (5 days later). Frost-free season 155 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 Boulder: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 18, 20% May 14, 30% May 11, 40% May 9, 50% May 6, 60% May 4, 70% May 1, 80% April 29, 90% April 24. Observed last spring frost in recent years: 2021 April 24, 2022 May 22, 2023 April 28, 2024 April 27, 2025 April 20, 2026 May 7.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 18May 6Apr 242021: April 242022: May 222023: April 282024: April 272025: April 202026: May 7Fall at Boulder: chance of a frost before this dateFall, chance of a frost before the date: 10% September 21, 20% September 27, 30% September 30, 40% October 3, 50% October 5, 60% October 8, 70% October 11, 80% October 14, 90% October 18. Observed first fall frost in recent years: 2021 October 14, 2022 October 17, 2023 October 13, 2024 October 25, 2025 October 19.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 21Oct 5Oct 182021: October 142022: October 172023: October 132024: 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
2021April 24October 14
2022May 22October 17
2023April 28October 13
2024April 27October 25
2025April 20October 19
2026May 7season not over yet

What these dates mean

May 18 is the cautious spring date, with only a 10 % chance of a light frost later, allowing a gardener to set out tender crops with minimal risk. The 50 % date is May 6, indicating an even chance of a light frost occurring. The 50 % hard-frost date in spring is April 27, marking an even chance of a 28 °F frost.

In fall, September 21 serves as the cautious date, offering only a 10 % chance of a light frost earlier, so tender crops can be finished harvesting safely. The 50 % date is October 5, indicating an even chance of a light frost occurring, and the 50 % hard-frost date is October 15. Hard frost at 28 °F can damage plants that survive a light frost at 32 °F, making it a more critical threshold for gardeners.

How to read the probabilities

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

  • Spring: "10% after May 18" 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 21" 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 Boulder, CONumbered nearest stations: 1 — near Coal Creek, CO — 9 mi; 2 — Northglenn, CO — 15 mi; 3 — Wheat Ridge, CO — 17 mi; 4 — Brighton, CO — 23 mi; 5 — Evergreen, CO — 25 mi.25 mi · 40 km1. near Coal Creek, CO — 9 mi12. Northglenn, CO — 15 mi23. Wheat Ridge, CO — 17 mi34. Brighton, CO — 23 mi45. Evergreen, CO — 25 mi5
  1. near Coal Creek, CO 9 mi (15 km)
  2. Northglenn, CO 15 mi (24 km)
  3. Wheat Ridge, CO 17 mi (27 km)
  4. Brighton, CO 23 mi (37 km)
  5. Evergreen, CO 25 mi (40 km)

Sources

* Hardiness zone 6a: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 39.9919, -105.2667.