Castle Rock, Colorado: planting zone & frost dates
Castle Rock is in USDA plant hardiness zone 5b, where the average coldest winter night reaches -15 to -10°F (-26.1 to -23.3°C). The typical last spring frost is around May 15 and the first fall frost around Sep 27.
Frost dates
| Probability | Last spring frost (32°F) | First fall frost (32°F) |
|---|---|---|
| Safer (10% risk) | May 24 | Sep 17 |
| Typical planning date (30% risk) | May 15 | Sep 27 |
| Median (50%) | May 9 | Oct 2 |
How to read this: after the spring "10% risk" date, a 32°F frost happens in only about 1 year in 10. In fall, the "10% risk" date is the early end — a frost before it happens about 1 year in 10.
Frost data comes from NOAA station CASTLE ROCK, CO (USC00051401), 3.7 mi (5.9 km) from the city center and 95 ft lower than the city center (-29 m). Record used for the 1991–2020 normals: 25 years.
Why your yard may differ from the station: microclimates.
Hardiness zone across Castle Rock
All 3 ZIP codes covering Castle Rock are in zone 5b.
Vegetable planting calendar for Castle Rock
Dates are counted from the 30%-risk frost dates above (last frost May 15, first frost Sep 27). Shift a week later in cold spots or low-lying ground.
| Crop | Start indoors | Transplant / plant out | Direct sow | Fall crop sow/plant |
|---|---|---|---|---|
| Tomato | Mar 20 – Apr 3 | May 22 – Jun 5 | — | — |
| Pepper | Mar 6 – Mar 20 | May 29 – Jun 12 | — | — |
| Lettuce | Apr 3 – Apr 17 | May 1 – May 22 | Apr 17 – May 29 | Jul 19 – Aug 16 |
| Green Beans | — | — | May 22 – Jun 26 | Jul 5 – Jul 19 |
| Sweet Corn | — | — | May 22 – Jun 19 | — |
| Zucchini & Summer Squash | Apr 17 – Apr 24 | May 22 – Jun 5 | May 22 – Jun 12 | Jul 5 – Jul 19 |
| Cucumber | Apr 17 – Apr 24 | May 29 – Jun 5 | May 22 – Jun 12 | Jul 5 – Jul 19 |
| Carrot | — | — | Apr 24 – May 29 | Jul 5 – Jul 19 |
| Onion | Feb 20 – Mar 6 | Apr 17 – May 1 | — | — |
| Potato | — | Apr 17 – May 8 | — | — |
| Basil | Apr 3 – Apr 17 | May 29 – Jun 12 | May 29 – Jun 19 | — |
| Spinach | — | — | Apr 3 – May 1 | Aug 2 – Aug 30 |
| Kale | Mar 20 – Apr 3 | Apr 17 – May 8 | Apr 17 – May 8 | Jul 19 – Aug 16 |
| Broccoli | Mar 20 – Apr 3 | May 1 – May 22 | — | Jul 5 – Jul 19 |
| Peas | — | — | Apr 3 – Apr 24 | Jul 19 – Aug 2 |
| Eggplant | Mar 6 – Mar 20 | May 29 – Jun 12 | — | — |
| Cabbage | Mar 20 – Apr 3 | Apr 17 – May 1 | — | Jul 5 – Jul 19 |
| Radish | — | — | Apr 10 – May 29 | Aug 16 – Sep 13 |
| Strawberry | — | Apr 17 – May 8 | — | — |
| Beets | — | — | Apr 24 – Jun 5 | Jul 19 – Aug 2 |
Nearby cities in Colorado
- Centennial — zone 6a, last frost May 10, first frost Oct 3
- Aurora — zone 6a, last frost May 12, first frost Oct 2
- Denver — zone 6a, last frost May 8, first frost Oct 3
- Lakewood — zone 6a, last frost May 8, first frost Oct 2
- Colorado Springs — zone 6a, last frost May 8, first frost Oct 3
- Westminster — zone 6a, last frost May 7, first frost Oct 4
More: all Colorado cities · other zone 5b cities: Madison, WI, Des Moines, IA, Aurora, IL, Fort Collins, CO, Naperville, IL, Rockford, IL, Elgin, IL, Greeley, CO
Castle Rock gardening questions
When is the last frost in Castle Rock, CO?
Plan around May 15. At NOAA's Castle Rock, CO station, 3.7 mi from the city center, there is a 30% chance of a 32°F frost after that date. Half of springs have their last frost by May 9, and after May 24 the risk drops to about 1 in 10. A hard freeze (28°F) usually ends earlier, around May 1. The last frost is fairly predictable here: only 24 days separate the 90% and 10% dates (median for cities on this site: 36 days).
When should I plant tomatoes in Castle Rock, CO?
Start seeds indoors Mar 20 – Apr 3 and set transplants outside May 22 – Jun 5, once nights stay above 50°F and the soil is near 60°F. That leaves 135 days between the 30% frost dates, enough for most varieties including 80–85-day slicers. More: tomato planting dates by zone.
How long is the growing season in Castle Rock?
About 143 days without a 32°F frost in a typical (median) year. In 9 years out of 10 the season lasts at least 124 days, and in 1 year in 10 it runs 161 days or more. In fall there is a 10% chance of frost by Sep 17 and a 30% chance by Sep 27. That ranks 15th longest of 16 Colorado cities on this site.
When should I plant a fall garden in Castle Rock?
Count back from the first fall frost (Sep 27): transplant broccoli Jul 5 – Jul 19; sow kale Jul 19 – Aug 16; sow lettuce Jul 19 – Aug 16; sow radishes Aug 16 – Sep 13. Hardy crops keep growing through light frosts; the median 28°F hard freeze here is Oct 12. How the math works: succession and fall planting.
Did Castle Rock's hardiness zone change on the 2023 map?
No. Going by the zone that covers most of the city's land, Castle Rock is zone 5b on both the 2012 and the 2023 USDA maps (PRISM/OSU ZIP listings). About 42% of U.S. ZIP codes kept their half-zone in 2023. Why the 2023 map is warmer.
How does Castle Rock compare with Centennial, CO?
Castle Rock is zone 5b and Centennial is zone 6a. The typical last spring frost in Castle Rock (May 15) comes about 5 days later than in Centennial, CO (May 10), 15 miles away, and the first fall frost is Sep 27 versus Oct 3. Castle Rock sits about 541 ft higher, which explains part of the gap. The two cities use different NOAA stations.