Castle Rock, Colorado Frost Dates, Growing Season & Hardiness Zone
Frost odds and planting zone for Castle Rock for the 2027 season, based on 1991–2020 normals.
Last frost date
May 9
typical (50% chance of a later 32°F night)
Safer date: May 24 (10% risk)
earlyMay 9
typicalMay 24
safer
First fall frost
Oct 2
typical (50% chance it has happened by then)
Early-risk date: Sep 17 (10%)
Growing season
143 days
typical frost-free (32°F) season
Short year: 124 days (1 year in 10 is shorter)
USDA hardiness zone
5b
2023 map: -15 to -10°F average coldest night
2012 map: 5b
Across the metro, the typical last frost runs from May 5 (Denver Centennial Airport) to May 16 (Elizabeth 2 NW), depending on where you live.
Half of years have their last 32°F night by May 9. In 1 year out of 10, a freeze still comes after May 24.
Based on NOAA station Castle Rock (USC00051401), 3.7 mi NW, elevation 6,185 ft (−96 ft vs. city center). Confidence: High.
When is the last frost in Castle Rock?
Expect the last 32°F night by May 9 in a typical year; 1 year in 10 still freezes after May 24. Cold 36°F nights (frost possible on plants) typically last longer, until around May 21. Hard freezes (28°F) typically end sooner, around May 1.
Castle Rock has a long late-frost tail: 15 days separate the typical date (May 9) from the safer 1-in-10 date (May 24). If tender plants go out early, be ready to protect them on cold nights.
Cool nights hang on after the last freeze: the typical last 36°F night (frost possible on plants in low spots) is around May 21, 12 days later than the typical last 32°F night. Frost can form on leaves on clear, calm nights even with the thermometer above freezing.
| Threshold | 10% (safer) | 30% | 50% (typical) | 90% (early) |
|---|---|---|---|---|
| 36°F (frost possible on plants) | Jun 6 | May 28 | May 21 | May 9 |
| 32°F freeze | May 24 | May 15 | May 9 | Apr 30 |
| 28°F hard freeze | May 14 | May 6 | May 1 | Apr 17 |
All probabilities (10–90%)
| Threshold | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F (frost possible on plants) | Jun 6 | May 31 | May 28 | May 24 | May 21 | May 18 | May 15 | May 12 | May 9 |
| 32°F freeze | May 24 | May 18 | May 15 | May 12 | May 9 | May 7 | May 5 | May 3 | Apr 30 |
| 28°F hard freeze | May 14 | May 10 | May 6 | May 4 | May 1 | Apr 29 | Apr 25 | Apr 21 | Apr 17 |
Is it safe to plant after Mother's Day in Castle Rock?
Many Castle Rock-area gardeners wait for Mother's Day to plant (CSU PlantTalk). In 2027, it falls on May 9. As of that date, the chance of another 32°F night is about 50%, and the chance of another 36°F night (frost possible on plants) is about 90% (NOAA 1991–2020, Castle Rock).
Is it too early to plant in Castle Rock?
Before May 9 the odds of another 32°F night are better than even; after May 24 they are about 1 in 10 or less.
Today's risk is computed in your browser from the same NOAA probability table, shown as the range between the two nearest NOAA probability levels.
When is the first frost in Castle Rock?
By Oct 2, half of years have had their first 32°F night. In 1 year out of 10, it comes as early as Sep 17. In a typical year, look for the first 36°F night (frost possible on plants) around Sep 25 and the first 28°F hard freeze around Oct 12.
Fall plantings are timed backward from first frost: Oct 2 typically, or Sep 17 if you want a cushion. See the planting calendar for each crop's date.
| Threshold | 10% (early) | 30% | 50% (typical) | 90% (late) |
|---|---|---|---|---|
| 36°F (frost possible on plants) | Sep 11 | Sep 20 | Sep 25 | Oct 7 |
| 32°F freeze | Sep 17 | Sep 27 | Oct 2 | Oct 16 |
| 28°F hard freeze | Sep 26 | Oct 5 | Oct 12 | Oct 25 |
All probabilities (10–90%)
| Threshold | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F (frost possible on plants) | Sep 11 | Sep 16 | Sep 20 | Sep 23 | Sep 25 | Sep 27 | Sep 30 | Oct 2 | Oct 7 |
| 32°F freeze | Sep 17 | Sep 23 | Sep 27 | Sep 30 | Oct 2 | Oct 5 | Oct 8 | Oct 12 | Oct 16 |
| 28°F hard freeze | Sep 26 | Oct 2 | Oct 5 | Oct 9 | Oct 12 | Oct 14 | Oct 17 | Oct 21 | Oct 25 |
How long is Castle Rock's growing season?
In a typical year, Castle Rock gets 143 days between the last and first 32°F nights. About 1 year in 10, the season is shorter than 124 days.
A season this long fits most warm-season vegetables planted on time. Long-season crops do best when started from transplants.
The station averages 2,491 growing degree days (base 50°F) a year. That number measures how much heat crops have to grow on.
| Threshold | 10% (long year) | 50% (typical) | 90% (short year) |
|---|---|---|---|
| 36°F (frost possible on plants) | 142 days | 124 days | 103 days |
| 32°F freeze | 161 days | 143 days | 124 days |
| 28°F hard freeze | 181 days | 161 days | 141 days |
How does Castle Rock's season compare?
Among the 20 Colorado cities with frost dates on this site, the typical frost-free season in Castle Rock (143 days) is the 19th longest.
Compared with the 56 U.S. cities with frost dates on this site, this typical frost-free season is longer than in 1 and shorter than in 54.
Growing degree days (base 50°F)
Annual normal: 2,491 GDD (NOAA flag S, 25 years). Running total by the end of each month (sum of NOAA daily GDD normals, so the year-end total can differ slightly from the annual normal):
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 3 | 22 | 79 | 285 | 747 | 1,399 | 1,993 | 2,356 | 2,465 | 2,488 | 2,491 |
What growing zone is Castle Rock? (USDA 2023)
Castle Rock is USDA zone 5b (-15 to -10°F average coldest winter night), the same as on the 2012 map.
| ZIP | 2023 | 2012 | Change | Share of ZIP in Castle Rock |
|---|---|---|---|---|
| 80104 | 5b | 5b | unchanged | 19% |
| 80108 | 5b | 5b | unchanged | 22% |
| 80109 | 5b | 5b | unchanged | 49% |
| 80116 | 5b | 5b | unchanged | 0% |
| 80134 | 5b | 5b | unchanged | 0% |
Other ZIPs touching Castle Rock
ZIPs outside the 801 prefix that overlap the city boundary:
- none
Zones: PRISM Group, Oregon State University, https://prism.oregonstate.edu, accessed 2026-10-08. Shown as published in the ZIP tables; this page does not show a zone map. Not the official USDA Plant Hardiness Zone Map (official map: planthardiness.ars.usda.gov).
Does the new zone change Castle Rock's frost dates?
No. Hardiness zones are based only on the average coldest winter temperature. They say nothing about when spring and fall frosts happen. Castle Rock's frost dates come from NOAA's 1991–2020 frost probabilities, which did not change with the 2023 zone map. Use your zone to pick perennials that will survive winter, and your frost dates to time planting.
Why do Castle Rock frost dates differ between websites?
If you've seen May 15 listed as Castle Rock's last frost date, that's the 30%-risk date: about 3 years in 10 still get a 32°F night after it. We lead with May 9, the 50% (median) date.
The 'safer' date, May 24, is the 10%-risk date. A 32°F night after it happens in only about 1 year in 10.
You may also see May 21. That's the 50% date for a 36°F night (frost possible on plants) rather than a 32°F freeze, so sites using 36°F as their cutoff list later dates.
The choice of weather station matters too. The 6 stations within 25 miles of the city center that pass our quality rules give typical (50%) last 32°F frost dates from May 5 to May 16; see the table below.
| Date | What it is (NOAA 1991–2020) |
|---|---|
| May 15 | 32°F last frost, 30% risk (a 'balanced' date) |
| May 21 | 36°F last cold night, 50% (frost possible on plants) |
| May 24 | 32°F last frost, 10% risk (the 'safer' date) |
| May 1 | 28°F last hard freeze, 50% |
| Station | Distance | Elevation | Typical last frost |
|---|---|---|---|
| Denver Centennial Airport | 13.4 mi N | 5,883 ft | May 5 |
| Castle Rock used here | 3.7 mi NW | 6,185 ft | May 9 |
| Parker | 11.0 mi NNE | 5,904 ft | May 9 |
| Elizabeth 2 NW | 12.0 mi E | 6,611 ft | May 16 |
| Roxborough State Park | 12.2 mi WNW | 6,117 ft | May 16 |
| Strontia Springs Dam | 14.9 mi WNW | 5,840 ft | May 16 |
Which weather station is this based on?
- Station
- Castle Rock (NOAA USC00051401, COOP)
- Distance
- 3.7 mi NW of the city center
- Elevation
- 6,185 ft; city center 6,281 ft (USGS), difference −96 ft
- Record
- NOAA completeness flag S, 25 of 30 years (1991–2020)
- Confidence
- High (rule)
Confidence: High. At 3.7 miles from the city center and only 96 ft off in elevation, with frost statistics that meet our completeness rule (NOAA flag S, 25 of 30 years), this station is a close match.
| Station | ID | Distance | Elev. (Δ) | Last frost 50% | First frost 50% | Season 50% | NOAA flag / years |
|---|---|---|---|---|---|---|---|
| Castle Rock | USC00051401 | 3.7 mi NW | 6,185 ft (-96) | May 9 | Oct 2 | 143 d | S / 25 |
| Parker | USC00056323 | 11.0 mi NNE | 5,904 ft (-377) | May 9 | Oct 3 | 145 d | R / 16 |
| Elizabeth 2 NW | USC00052631 | 12.0 mi E | 6,611 ft (+330) | May 16 | Oct 2 | 137 d | R / 15 |
| Roxborough State Park | USC00057249 | 12.2 mi WNW | 6,117 ft (-164) | May 16 | Oct 1 | 136 d | R / 24 |
| Denver Centennial Airport | USW00093067 | 13.4 mi N | 5,883 ft (-398) | May 5 | Oct 8 | 154 d | R / 20 |
| Strontia Springs Dam | USC00058022 | 14.9 mi WNW | 5,840 ft (-441) | May 16 | Sep 29 | 132 d | R / 19 |
Other stations within 25 mi (not used)
- Kassler (USC00054452), 15.1 mi WNW, 5,587 ft: typical last frost May 8. Not used: |elev diff| 694 ft > 500 ft.
- Cherry Creek Dam (USC00051547), 17.3 mi N, 5,647 ft: typical last frost May 7. Not used: |elev diff| 634 ft > 500 ft.
- Monument (USC00055734), 18.9 mi S, 7,080 ft: typical last frost May 14. Not used: |elev diff| 799 ft > 500 ft.
- Marston Filter Plant (USC00055402), 20.4 mi NW, 5,610 ft: typical last frost May 2. Not used: |elev diff| 671 ft > 500 ft.
- Monument 3 S (USC00055733), 22.1 mi S, 6,821 ft: typical last frost May 17. Not used: |elev diff| 540 ft > 500 ft.
- Eastonville 2 NNW (USC00052494), 22.9 mi SE, 7,210 ft: typical last frost May 29. Not used: frost normals incomplete or flagged (needs all 81 values with S/R flags, or values missing only because NOAA's probability of occurrence is below 10%); |elev diff| 929 ft > 500 ft.
- Black Forest 6 WNW (USC00050756), 24.1 mi S, 6,957 ft: typical last frost May 11. Not used: |elev diff| 676 ft > 500 ft.
53 other NOAA normals stations within 40 km (mostly rain gauges) have no frost statistics.
Castle Rock planting dates at a glance
Balanced dates use the 30%-risk last-frost date as the anchor. The Castle Rock planting calendar has typical and safe dates, fall planting and a month-by-month list for 27 vegetables.
| Crop | Start indoors | Plant outdoors (balanced) | Fall |
|---|---|---|---|
| Tomatoes | May 4–18 | Jun 22 | – |
| Peppers | Apr 3–17 | May 29 | – |
| Potatoes | – | May 1–15 | – |
| Onions (dry, from sets) | – | Apr 17 – May 1 | – |
| Peas | – | Apr 17 – May 1 | sow by Jul 15 |
| Lettuce (leaf) | – | Apr 17 – May 1 | sow by Jul 20 |
| Carrots | – | May 1–15 | sow by Jul 10 |
| Beans (snap) | – | May 15 | sow by Jul 6 |
| Summer squash | – | May 15 | sow by Jul 16 |
| Broccoli | Feb 27 – Mar 13 | Apr 17 – May 1 | transplant by Jul 15 |
| Cabbage | Feb 27 – Mar 13 | Apr 17 – May 1 | transplant by Jun 25 |
| Cauliflower | Mar 13–27 | May 1–15 | – |
| Kohlrabi | – | Apr 17 – May 1 | sow by Jul 30 |
| Green onions | – | Apr 17 – May 1 | sow by Jul 20 |
| Radishes | – | Apr 17 – May 1 | sow by Aug 19 |
| Spinach | – | Apr 17 – May 1 | sow by Aug 9 |
| Turnips | – | Apr 17 – May 1 | sow by Jul 30 |
| Beets | – | May 1–15 | sow by Jul 20 |
| Swiss chard | – | May 1–15 | sow by Jul 20 |
| Parsnips | – | May 1–15 | – |
| Sweet corn | – | May 15 | – |
| Cucumbers | – | May 15 | sow by Jul 11 |
| Eggplant | Mar 27 – Apr 17 | May 29 | – |
| Cantaloupe | – | May 29 | – |
| Watermelon | – | May 29 | – |
| Winter squash | – | May 29 | – |
| Garlic | – | – | plant Oct 1–14 |
When to plant tomatoes in Castle Rock · Fall planting dates for Castle Rock
Frost dates near Castle Rock
| City | Distance | Last frost | First frost | Season | Zone 2023 |
|---|---|---|---|---|---|
| Parker frost dates | 10 mi | May 9 | Oct 3 | 145 days | 5b–6a |
| Centennial frost dates | 15 mi | May 5 | Oct 8 | 154 days | 6a |
| Littleton frost dates | 17 mi | May 2 | Oct 11 | 160 days | 5b–6a |
| Aurora frost dates | 24 mi | May 7 | Oct 8 | 152 days | 6a |
| Denver frost dates | 26 mi | Apr 30 | Oct 12 | 165 days | 6a |
| Lakewood frost dates | 26 mi | May 3 | Oct 7 | 155 days | 6a |
Frequently asked questions
What is the average last frost date in Castle Rock?
May 9 is the median (50%) date for Castle Rock's last 32°F night, from NOAA 1991–2020 normals at Castle Rock. To play it safe, use May 24: only about 1 year in 10 has a freeze after it.
When is the first frost in Castle Rock?
Expect the first 32°F night by Oct 2 in a typical year, and as early as Sep 17 in about 1 year out of 10. Cold 36°F nights (frost possible on plants) typically start around Sep 25.
What growing zone is Castle Rock, Colorado?
Zone 5b. That's Castle Rock's USDA hardiness zone on both the 2023 map and the older 2012 map.
Is it too early to plant in Castle Rock?
If it's before May 9, it's too early for frost-tender plants: another 32°F night is more likely than not. From May 9 to May 24, the risk falls from 50% to 10%. Cool-season crops such as peas and lettuce can be planted weeks earlier (see the planting calendar).
What temperature is a killing frost?
It depends on the crop. Tender plants such as tomatoes are killed by a light frost, which CSU Extension describes as 28–32°F; below 28°F is a heavy frost. We show NOAA odds for 36°F (frost possible on plants), 32°F (freeze, our headline) and 28°F (hard freeze).
Is first frost the same as first freeze?
Not quite. A freeze is when the air reaches 32°F. Frost is ice that forms on surfaces, and on calm, clear nights it can form with the air a few degrees above freezing. So in Castle Rock, the first 36°F night (frost possible on plants) typically comes around Sep 25, before the first 32°F night (Oct 2).
Download & sources
- Add Castle Rock frost & planting dates to your calendar (.ics, 2027)
- Printable planting calendar (PDF)
- How these numbers are calculated
Data: NOAA NCEI U.S. Climate Normals 1991–2020 (annual/seasonal and daily normals, station USC00051401); USDA Plant Hardiness Zone ZIP tables 2023 and 2012 by the PRISM Group, Oregon State University; Census 2020 ZIP/place relationship file; USGS elevation. Long-term odds, not a forecast. Figures and sentences are generated from these data by fixed rules; see the methodology. Last updated 2026-10-08.