U.S. Frost Dates by Weather Station

NOAA station USC00051401 · Colorado

Frost Dates for Castle Rock, CO

NOAA weather station Castle Rock (USC00051401), Colorado · elevation 6185 ft (1885 m) · hardiness zone 5b*

  • Lower-risk planting dateafter May 24only 1 spring in 10 still has a frost later
  • Typical last frostMay 950% chance of a frost after it
  • Typical first fall frostOctober 250% chance of a frost before it
  • Frost-free season143 daystypical year, 32°F
  • Hardiness zone5bcalculated*

The year at a glance

Frost season at Castle Rock, COSpring: a frost after April 30 in 9 years out of 10, after May 9 in half of the years, after May 24 in 1 year out of 10. Fall: the first frost comes before September 17 in 1 year out of 10, before October 2 in half of the years, before October 16 in 9 years out of 10. Typical frost-free season: 143 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 9First frost ~Oct 2Frost season at Castle Rock, COSpring: a frost after April 30 in 9 years out of 10, after May 9 in half of the years, after May 24 in 1 year out of 10. Fall: the first frost comes before September 17 in 1 year out of 10, before October 2 in half of the years, before October 16 in 9 years out of 10. Typical frost-free season: 143 days.JFMAMJJASONDLast frost ~May 9First frost ~Oct 2
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.Today

Frost dates (32°F)

Probability of frost, based on 25 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 24September 17
50% (typical)May 9October 2
90% (likely)April 30October 16
Frost-free season (32°F, 50% chance)
143 days
Hard frost (28°F), last in spring, 50% chance after
May 1
Hard frost (28°F), first in fall, 50% chance before
October 12

The last 15 years (2006–2020 normals)

Typical last frost May 11 (2 days later than the 30-year normal), typical first fall frost October 6 (4 days later). Frost-free season 146 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 Castle Rock: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 24, 20% May 18, 30% May 15, 40% May 12, 50% May 9, 60% May 7, 70% May 5, 80% May 3, 90% April 30. Observed last spring frost in recent years: 2021 April 22, 2023 April 29, 2024 April 28, 2025 May 20.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 24May 9Apr 302021: April 222023: April 292024: April 282025: May 20Fall at Castle Rock: chance of a frost before this dateFall, chance of a frost before the date: 10% September 17, 20% September 23, 30% September 27, 40% September 30, 50% October 2, 60% October 5, 70% October 8, 80% October 12, 90% October 16. Observed first fall frost in recent years: 2021 October 20, 2023 October 14, 2024 October 31.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 17Oct 2Oct 162021: October 202023: October 142024: October 31
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 22October 20
2022incomplete recordincomplete record
2023April 29October 14
2024April 28October 31
2025May 20incomplete record
2026incomplete recordseason not over yet

What these dates mean

The 143 days between the 32°F 50% dates in Castle Rock frame the window in which tender crops can be grown; the median light-frost dates are May 9 and October 2, while the median hard-frost dates are May 1 and October 12. For a gardener these 50% dates simply mark the point where the odds of a frost are even, not the moment a frost occurs.

In spring the cautious date for setting out tender crops is May 24, when there is only a 10 % chance of a light frost later; planting earlier on April 30 carries a 90 % chance of a later frost and is a gamble. In fall the cautious date to finish harvesting tender crops is September 17, with only a 10 % chance of a light frost earlier, whereas waiting until October 16 involves a 90 % chance of an earlier frost. A hard frost at 28 °F can damage plants more severely than a light frost at 32 °F, so its timing is especially important for protecting crops.

How to read the probabilities

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

  • Spring: "10% after May 24" 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 17" 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 Castle Rock, CONumbered nearest stations: 1 — near Aurora, CO — 24 mi; 2 — Evergreen, CO — 27 mi; 3 — Wheat Ridge, CO — 28 mi; 4 — Bailey, CO — 30 mi; 5 — Northglenn, CO — 34 mi.25 mi · 40 km1. near Aurora, CO — 24 mi12. Evergreen, CO — 27 mi23. Wheat Ridge, CO — 28 mi34. Bailey, CO — 30 mi45. Northglenn, CO — 34 mi5
  1. near Aurora, CO 24 mi (39 km)
  2. Evergreen, CO 27 mi (43 km)
  3. Wheat Ridge, CO 28 mi (44 km)
  4. Bailey, CO 30 mi (49 km)
  5. Northglenn, CO 34 mi (55 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: 39.4106, -104.9058.