USDA Hardiness Zone 5b
Frost Dates for 80109 — Castle Rock, Colorado
NOAA 1991–2020 normals from Castle Rock, CO (6,185 ft), 2 miles north.
Updated
In Castle Rock, the last spring frost (32°F) is around May 9 in half of years and has passed by May 24 in nine years out of ten. The first fall frost comes around October 2, as early as September 17 one year in ten, leaving a typical freeze-free season of 143 days (NOAA, Castle Rock, CO).
- Last spring frost
- May 9
- First fall frost
- October 2
- Growing season
- 143 days
- Hardiness zone
- 5b
50% probability
50% probability
freeze-free, 50%
-15 to -10°F (-26.1 to -23.3°C) coldest night
- First snow
- October 21
- Last snow
- April 29
- Winters with snow
- 100%
- Snow per year
- 57.5"
median, 1991–2020
median, 1991–2020
29 of 29 observed
NOAA normal
Frost risk by date in Castle Rock
Half of springs at Castle Rock, CO see their last 32°F night by May 9; only one in ten is past it by April 30, and one in ten still has one after May 24. Planting tender crops after May 15 leaves a 30% risk of a frost on them, after May 18 20%, and after May 24 10%. The 15 days between the even-odds date and the one-in-ten date is wider than at 56% of the 110 Colorado stations behind this site's ZIP pages.
In fall the first 32°F night comes by October 2 in half of years, as early as September 17 one year in ten, and holds off past October 16 one year in ten. A hard freeze of 28°F follows 10 days later, around October 12; in spring the one-in-ten date for 28°F is May 14, 10 days before the 32°F one. The freeze-free season lasts 143 days in a typical year, longer than at 51% of the 110 Colorado stations; it reaches 161 days one year in ten and falls short of 124 one year in ten. On average 167 nights a year drop to 32°F or below. Nights of 36°F, when frost can still settle on leaves in low, open spots, continue 12 days longer, to about May 21, and return around September 25. Severe cold of 16°F or below comes every winter, usually between November 6 and April 1.
| Low | 90% | 80% | 70% | 60% | 50% | 40% | 30% | 20% | 10% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | May 9 | May 12 | May 15 | May 18 | May 21 | May 24 | May 28 | May 31 | Jun 6 |
| 32°F | Apr 30 | May 3 | May 5 | May 7 | May 9 | May 12 | May 15 | May 18 | May 24 |
| 28°F | Apr 17 | Apr 21 | Apr 25 | Apr 29 | May 1 | May 4 | May 6 | May 10 | May 14 |
| 24°F | Apr 4 | Apr 9 | Apr 14 | Apr 17 | Apr 21 | Apr 24 | Apr 28 | May 2 | May 8 |
| 20°F | Mar 22 | Mar 30 | Apr 2 | Apr 5 | Apr 9 | Apr 13 | Apr 17 | Apr 21 | Apr 27 |
| 16°F | Mar 7 | Mar 15 | Mar 22 | Mar 27 | Apr 1 | Apr 3 | Apr 7 | Apr 12 | Apr 18 |
| Low | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 11 | Sep 16 | Sep 20 | Sep 23 | Sep 25 | Sep 27 | Sep 30 | Oct 2 | Oct 7 |
| 32°F | Sep 17 | Sep 23 | Sep 27 | Sep 30 | Oct 2 | Oct 5 | Oct 8 | Oct 12 | Oct 16 |
| 28°F | Sep 26 | Oct 2 | Oct 5 | Oct 9 | Oct 12 | Oct 14 | Oct 17 | Oct 21 | Oct 25 |
| 24°F | Oct 6 | Oct 12 | Oct 15 | Oct 18 | Oct 21 | Oct 24 | Oct 27 | Oct 30 | Nov 4 |
| 20°F | Oct 15 | Oct 20 | Oct 24 | Oct 27 | Oct 30 | Nov 2 | Nov 5 | Nov 9 | Nov 14 |
| 16°F | Oct 21 | Oct 26 | Nov 1 | Nov 3 | Nov 6 | Nov 10 | Nov 14 | Nov 17 | Nov 22 |
Freeze probabilities 1991–2020 at Castle Rock, CO, from NOAA NCEI U.S. Climate Normals. Where a temperature is reached in fewer than 99.5% of years, the dates describe only the years that reach it. Not a forecast.
Frost risk on any date in 80109 →Compare Castle Rock with another place →
Weather by month in Castle Rock
July is the warmest month at Castle Rock, CO, averaging 86°F by day and 56°F at night; January is the coldest at 46°F and 18°F. A freezing night is all but certain in every month from October to April (a 90% chance or more). From June to August the chance of a 32°F night stays under 10% each month. In spring, May still brings one in 88% of years.
Warmth for growing adds up to 2,491 growing degree days a year (base 50°F), more than at 50% of the 110 Colorado stations. 30% of them (747) have built up by the end of June, and July adds the most, 652. Precipitation totals 17.7 inches a year, most in August (2.5") and least in January (0.6"); May to September bring 59% of it.
| Month | High °F | Low °F | Precip. in | 32°F night | Nights ≤ 32°F | GDD 50°F |
|---|---|---|---|---|---|---|
| Jan | 46 | 18 | 0.6 | 100% | 30 | 1 |
| Feb | 47 | 20 | 0.7 | 100% | 26 | 3 |
| Mar | 54 | 27 | 1.5 | 100% | 24 | 19 |
| Apr | 59 | 33 | 2.1 | 100% | 17 | 56 |
| May | 68 | 42 | 2.0 | 88% | 4 | 207 |
| Jun | 80 | 51 | 2.2 | 1% | <1 | 462 |
| Jul | 86 | 56 | 2.3 | 0% | 0 | 652 |
| Aug | 84 | 55 | 2.5 | 0% | 0 | 594 |
| Sep | 77 | 47 | 1.4 | 42% | 1 | 363 |
| Oct | 65 | 35 | 0.9 | 100% | 12 | 110 |
| Nov | 54 | 26 | 0.8 | 100% | 24 | 23 |
| Dec | 46 | 18 | 0.7 | 100% | 29 | 2 |
Monthly normals 1991–2020 at Castle Rock, CO (6,185 ft), NOAA NCEI. "32°F night": chance of at least one in the month. GDD: growing degree days, base 50°F.
Compared with the next station
The nearest other station, Roxborough Sp, CO, 10 miles west of Castle Rock, has a last frost 7 days later (May 16), a first frost 1 day earlier (October 1), a season 7 days shorter.
When it snows in Castle Rock, Colorado
The first measurable snow (0.1 inch or more) in Castle Rock usually arrives around October 21, and the last of the season around April 29. Those are the middle dates across the winters of 1991–2020 at Castle Rock, CO, 2.1 miles away: half of years saw snow earlier, half later. Every one of those winters saw snow. The last 32°F night of spring typically comes 10 days after the last snow.
Planting calendar for 80109
| When | What to do |
|---|---|
| February 28 – March 14 | Start peppers and aubergines indoorsThey need eight to ten weeks under lights before they can go out. |
| March 14 – March 28 | Start tomatoes indoorsSix to eight weeks gives a transplant big enough to set fruit early. |
| April 11 – April 25 | Sow peas, spinach, lettuce, radish outdoorsThese tolerate light frost and prefer to germinate in cool soil. |
| May 9 onward | Transplant tomatoes, peppers, basil, squash, cucumbersThe average last freeze has passed — still watch the actual forecast. |
| by July 10 | Sow the autumn crop of carrots, beets and brassicasThey need about twelve weeks to size up before growth stops. |
| by September 18 | Harvest tender crops, or be ready to cover themA first freeze two weeks early is well within normal variation. |
| October 2 onward | Plant garlic, mulch beds, lift tender bulbsGarlic wants a cold spell; bare soil loses structure over winter. |
The weeks before and after the 50% dates are general rules of thumb, not NOAA figures; a seed packet gives the timing for each variety.
Where these figures come from
ZIP 80109 is 2 miles from Castle Rock, CO, which stands at 6,185 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 2 other ZIP codes share the station; its figures are presented in full for 80104 (Castle Rock). The 2023 USDA Plant Hardiness Zone Map lists ZIP 80109 in zone 5b, the band where the coldest night of the year averaged -15 to -10°F (-26.1 to -23.3°C) over 1991–2020; a zone describes winter lows, not frost dates.
Hardiness zone and temperature range: USDA Plant Hardiness Zone Map 2023 (USDA-ARS, Oregon State University), ZIP-code listing, unchanged. °C converted from °F by this site.
Thirty-year averages, not a forecast: check the forecast before planting out. How stations are matched and which pages are offered to search engines: methodology.