Frost dates for Casper, Wyoming
In Casper, half of springs have had their last frost by May 26 (and 1 spring in 10 still has one after June 10), and half of falls have had their first frost by September 22, leaving about 117 frost-free days in a typical year. Here is how sure those dates are, what they mean for planting, and where they come from.
Frost dates for
Casper, Wyoming
Last spring frost
May 26
Half of springs have their last frost by this date. For a safer bet, wait until June 10: only 1 spring in 10 has a frost after that.
First fall frost
September 22
Half of falls have their first frost by this date. In 1 fall in 10 it comes by September 5.
Growing season
117 days
Frost-free days in a typical year. 9 years in 10 get at least 96.
Hardiness zone
5a
2023 USDA map: the coldest night of winter averages −20°F to −15°F. It was 5a on the 2012 map too. All 4 ZIP codes within about 6 miles of the city's center are in zone 5a.
Stations near you disagree. NOAA's Casper Natrona Co Airport (8 miles away, at 5,340 feet above sea level), the one we used, gives a typical last frost of May 26 and a typical first frost of September 22. NOAA's Casper WWTP (2 miles away, at 5,100 feet above sea level) gives a typical last frost of May 10 and a typical first frost of September 29. So the typical last frost could fall anywhere from May 10 to 26 and the typical first frost from September 22 to 29, depending on whether your garden is more like one or the other. Local terrain matters: hillsides, valley floors, water and city heat can change frost dates a lot over a few miles. Treat the dates here as a starting point, and compare with a nearby gardener or your county extension office.
Print a year planner for Casper
Frost risk through the year
How sure are these dates?
A frost date is a probability, not a promise. NOAA worked out, from 30 years of nightly lows, how often the last spring frost and the first fall frost fall on each date. The middle column is the typical year. The outer columns are the early and late years that come about once a decade.
| Early year 1 in 10 | Typical year 1 in 2 | Late year 1 in 10 | |
|---|---|---|---|
| Last frost in spring | May 12 | May 26 | June 10 |
| First frost in fall | September 5 | September 22 | October 4 |
The frost-free season between them runs 117 days in a typical year. In 1 year in 10 it is 136 days or longer, and in 1 year in 10 it is 96 days or shorter.
- Hard freeze (28°F): typically last on May 11 and first on October 2. It is a harder freeze than the 32°F frost line, cold enough to damage many garden plants; how hardy a crop is decides whether it comes through. (The National Weather Service's own hard-freeze threshold varies from office to office.)
- Frost possible (36°F at the station): typically last on June 9 and first on September 9. On clear, still nights the ground can be several degrees colder than the thermometer, so frost can form on plants when the official reading is in the mid-30s.
In 8 springs out of 10, the last frost in Casper comes between May 12 and June 10, a window of 29 days. That is about average for NOAA's stations (the typical window is 31 days).
Among Wyoming's 129 NOAA stations with a frost every year, 22 percent have their typical last frost later than Casper does.
Planting calendar for Casper
These dates count from the typical last and first frosts, using the planting rules published by Colorado State University Extension (spring), Virginia Cooperative Extension (fall crops) and University of Minnesota Extension (garlic, where winters are hard). Weather in any one year can run early or late, so check the forecast before you plant tender crops.
Your season is too short for a separate fall planting of cabbage and broccoli: sown late enough to be a fall crop, it wouldn't be ready before the typical first frost. Your spring planting is your harvest.
| When | What to do | Why then |
|---|---|---|
| March | ||
| March 10 to April 7 | Broccoli and cabbage Start seeds indoors | 5 to 7 weeks before planting them out |
| March 24 to April 21 | Cauliflower Start seeds indoors | 5 to 7 weeks before planting out |
| April | ||
| April 7 to 28 | Eggplant Start seeds indoors | 6 to 9 weeks before planting out |
| April 14 to 28 | Peppers Start seeds indoors | 6 to 8 weeks before planting out |
| April 21 to May 5 | Tomatoes Start seeds indoors | 5 to 7 weeks before planting out |
| April 28 to May 12 | Peas, spinach, lettuce, radishes, onion sets, turnips Sow outdoors, as early as | hardy crops: 2 to 4 weeks before the typical last frost |
| April 28 to May 12 | Broccoli and cabbage Plant out, as early as | hardy crops: 2 to 4 weeks before the typical last frost |
| May | ||
| May 12 to 26 | Beets, carrots, potatoes, Swiss chard Plant outdoors, as early as | semi-hardy crops: up to 2 weeks before the typical last frost |
| May 12 to 26 | Cauliflower Plant out, as early as | semi-hardy: up to 2 weeks before the typical last frost |
| May 26 | Snap beans, sweet corn, cucumbers, summer squash Sow outdoors around | tender crops: around the typical last frost date |
| June | ||
| June 9 | Tomatoes, peppers, eggplant Plant out from | very tender: 2 weeks or more after the typical last frost |
| June 9 | Melons, winter squash, pumpkins, lima beans Sow outdoors from | very tender: 2 weeks or more after the typical last frost |
| June 26 | Snap beans for fall Last date to sow | 60 days to harvest, plus 2 weeks for slower fall growth, plus 2 weeks because frost kills it |
| June 30 | Carrots for fall Last date to sow | 70 days to harvest, plus 2 weeks for slower fall growth |
| July | ||
| July 1 | Cucumbers for fall Last date to sow | 55 days to harvest, plus 2 weeks for slower fall growth, plus 2 weeks because frost kills it |
| July 5 | Peas for fall Last date to sow | 65 days to harvest, plus 2 weeks for slower fall growth |
| July 6 | Summer squash for fall Last date to sow | 50 days to harvest, plus 2 weeks for slower fall growth, plus 2 weeks because frost kills it |
| July 10 | Leaf lettuce and beets for fall Last date to sow | 60 days to harvest, plus 2 weeks for slower fall growth |
| July 20 | Turnips and kohlrabi for fall Last date to sow | 50 days to harvest, plus 2 weeks for slower fall growth |
| July 30 | Spinach for fall Last date to sow | 40 days to harvest, plus 2 weeks for slower fall growth |
| August | ||
| August 9 | Radishes for fall Last date to sow | 30 days to harvest, plus 2 weeks for slower fall growth |
| September | ||
| September 5 | Tomatoes, peppers, squash, melons, cucumbers Watch the forecast from | in 1 fall in 10 the first frost comes by this date; pick tender crops before a frost |
| October | ||
| October 9 to 16 | Garlic Plant cloves | one or two weeks after the typical first hard freeze (28°F), our reading of the first killing frost |
Want more insurance? Shift every spring date 15 days later. That plans around June 10, the date after which only 1 spring in 10 has a frost.
For spring bulbs such as tulips and daffodils, plant once the soil has cooled to about 50 to 55°F and keep going until a couple of weeks before the ground freezes. Our guide to fall planting explains how to tell.
Where these numbers come from
These dates come from NOAA's weather station Casper Natrona Co Airport (station USW00024089), 8 miles from the center of Casper. It sits 140 feet higher, at 5,340 feet above sea level. NOAA rates this station's record as standard: at least 24 of the 30 years, with gaps filled from nearby stations.
The nearest other places with pages
- Gillette, Wyoming: last frost around May 22, first frost around September 25 (105 miles away)
- Cheyenne, Wyoming: last frost around May 12, first frost around October 1 (140 miles away)
See frost dates across Wyoming, or look up your own ZIP code for the station nearest you. Want the background? Read what a last frost date really means.