Casper, Wyoming: planting zone & frost dates
Casper is in USDA plant hardiness zone 5a, where the average coldest winter night reaches -20 to -15°F (-28.9 to -26.1°C). The typical last spring frost is around May 16 and the first fall frost around Sep 23.
Frost dates
| Probability | Last spring frost (32°F) | First fall frost (32°F) |
|---|---|---|
| Safer (10% risk) | May 24 | Sep 14 |
| Typical planning date (30% risk) | May 16 | Sep 23 |
| Median (50%) | May 10 | Sep 29 |
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 CASPER WWTP, WY (USC00481569), 2.0 mi (3.2 km) from the city center and 56 ft lower than the city center (-17 m). Record used for the 1991–2020 normals: 18 years.
Why your yard may differ from the station: microclimates.
Hardiness zone across Casper
All 3 ZIP codes covering Casper are in zone 5a.
Vegetable planting calendar for Casper
Dates are counted from the 30%-risk frost dates above (last frost May 16, first frost Sep 23). 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 21 – Apr 4 | May 23 – Jun 6 | — | — |
| Pepper | Mar 7 – Mar 21 | May 30 – Jun 13 | — | — |
| Lettuce | Apr 4 – Apr 18 | May 2 – May 23 | Apr 18 – May 30 | Jul 15 – Aug 12 |
| Green Beans | — | — | May 23 – Jun 27 | Jul 1 – Jul 15 |
| Sweet Corn | — | — | May 23 – Jun 20 | — |
| Zucchini & Summer Squash | Apr 18 – Apr 25 | May 23 – Jun 6 | May 23 – Jun 13 | Jul 1 – Jul 15 |
| Cucumber | Apr 18 – Apr 25 | May 30 – Jun 6 | May 23 – Jun 13 | Jul 1 – Jul 15 |
| Carrot | — | — | Apr 25 – May 30 | Jul 1 – Jul 15 |
| Onion | Feb 21 – Mar 7 | Apr 18 – May 2 | — | — |
| Potato | — | Apr 18 – May 9 | — | — |
| Basil | Apr 4 – Apr 18 | May 30 – Jun 13 | May 30 – Jun 20 | — |
| Spinach | — | — | Apr 4 – May 2 | Jul 29 – Aug 26 |
| Kale | Mar 21 – Apr 4 | Apr 18 – May 9 | Apr 18 – May 9 | Jul 15 – Aug 12 |
| Broccoli | Mar 21 – Apr 4 | May 2 – May 23 | — | Jul 1 – Jul 15 |
| Peas | — | — | Apr 4 – Apr 25 | Jul 15 – Jul 29 |
| Eggplant | Mar 7 – Mar 21 | May 30 – Jun 13 | — | — |
| Cabbage | Mar 21 – Apr 4 | Apr 18 – May 2 | — | Jul 1 – Jul 15 |
| Radish | — | — | Apr 11 – May 30 | Aug 12 – Sep 9 |
| Strawberry | — | Apr 18 – May 9 | — | — |
| Beets | — | — | Apr 25 – Jun 6 | Jul 15 – Jul 29 |
Nearby cities in Wyoming
- Cheyenne — zone 5b, last frost May 20, first frost Sep 26
More: all Wyoming cities · other zone 5a cities: Minneapolis, MN, St. Paul, MN, Anchorage, AK, Sioux Falls, SD, Cedar Rapids, IA, Billings, MT, Bloomington, MN, Sioux City, IA
Casper gardening questions
When is the last frost in Casper, WY?
Plan around May 16. At NOAA's Casper Wwtp, WY station, 2.0 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 10, 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 Casper, WY?
Start seeds indoors Mar 21 – Apr 4 and set transplants outside May 23 – Jun 6, once nights stay above 50°F and the soil is near 60°F. That leaves 130 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 Casper?
About 138 days without a 32°F frost in a typical (median) year. In 9 years out of 10 the season lasts at least 118 days, and in 1 year in 10 it runs 155 days or more. In fall there is a 10% chance of frost by Sep 14 and a 30% chance by Sep 23.
When should I plant a fall garden in Casper?
Count back from the first fall frost (Sep 23): transplant broccoli Jul 1 – Jul 15; sow kale Jul 15 – Aug 12; sow lettuce Jul 15 – Aug 12; sow radishes Aug 12 – Sep 9. Hardy crops keep growing through light frosts; the median 28°F hard freeze here is Oct 6. How the math works: succession and fall planting.
Did Casper's hardiness zone change on the 2023 map?
No. Going by the zone that covers most of the city's land, Casper is zone 5a 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.