Casper, WY planting zone & frost dates

Casper is in USDA hardiness zone 5a. The last spring frost usually comes around May 10 and the first fall frost around September 29, for a growing season of about 138 days.

USDA hardiness zone5aavg. extreme winter low -20°F to -15°F

Average last spring frostMay 1090% chance of no frost after May 24

Average first fall frostSeptember 2910% chance of frost by September 14

Growing season138 daysfrost-free days in an average year

Planting calendar for Casper

Dates are counted from the average last spring frost (May 10) using the typical timing recommended by U.S. extension services. Warm-season crops go out only after the danger of frost has passed and the soil has warmed.

Crop	When
Tomatoes	Start seeds indoors: Mar 15 – Mar 29 Transplant outdoors: May 17 – May 24
Peppers	Start seeds indoors: Mar 1 – Mar 15 Transplant outdoors: May 24 – May 31
Eggplant	Start seeds indoors: Mar 1 – Mar 15 Transplant outdoors: May 24 – May 31
Broccoli	Start seeds indoors: Mar 1 – Mar 15 Transplant outdoors: Apr 12 – Apr 26
Cabbage	Start seeds indoors: Mar 1 – Mar 15 Transplant outdoors: Apr 12 – Apr 26
Onions (sets)	Plant outdoors: Apr 12 – Apr 26
Peas	Sow outdoors: Mar 29 – Apr 12
Spinach	Sow outdoors: Mar 29 – Apr 12
Lettuce	Sow outdoors: Apr 12 – Apr 26
Carrots	Sow outdoors: Apr 12 – Apr 26
Potatoes	Plant outdoors: Apr 12 – Apr 26
Beans	Sow outdoors: May 17 – May 24
Sweet corn	Sow outdoors: May 17 – May 24
Cucumbers	Sow outdoors: May 17 – May 24
Squash & zucchini	Sow outdoors: May 17 – May 24
Basil	Start seeds indoors: Mar 29 – Apr 12 Transplant outdoors: May 17 – May 24

Fall garden: cool-season crops like lettuce, spinach and kale can be sown again around Aug 4 (8 weeks before the average first fall frost on Sep 29).

Useful for zone 5a gardens

Seed starting kit with dome — to start seeds indoors before the last frost
LED grow light for seedlings — stops leggy seedlings
Frost cloth / plant covers — for late spring and early fall frosts
Soil thermometer — warm-season crops need soil above 60°F
Cold frame / mini greenhouse — adds weeks to a short season

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Frost risk in detail

	90% chance	50% chance	10% chance
Last 32°F frost in spring is after	Apr 30	May 10	May 24
First 32°F frost in fall is before	Oct 12	Sep 29	Sep 14

How to read it: in spring there is a 10% chance of a frost after May 24, so that is a safe date for tender plants. In fall there is a 10% chance of frost before September 14. A hard freeze (28°F) usually ends around May 1.

Hardiness zone by ZIP code in Casper

82601 zone 5a
82602 zone 5a
82604 zone 5a
82605 zone 5a
82609 zone 5a

Nearby cities

Gillette zone 5a, last frost May 15
Laramie zone 4a, last frost Jun 7
Sheridan zone 5a, last frost May 21
Riverton zone 4b, last frost May 12
Cheyenne zone 5b, last frost May 14
Rock Springs zone 5a, last frost May 24
Jackson zone 4b, last frost Jun 20
Evanston zone 5a, last frost Jun 12

All cities in Wyoming → · Other zone 5a cities →

Questions

What growing zone is Casper, WY?

Casper is in USDA Plant Hardiness Zone 5a (average extreme winter low -20°F to -15°F), according to the USDA 2023 map.

When is the last frost in Casper?

The average last spring frost (32°F) in Casper is May 10. There is still a 10% chance of frost until May 24.

When can I plant tomatoes in Casper?

Transplant tomatoes outdoors 1–2 weeks after the average last frost: around May 17 – May 24. Start seeds indoors 6–8 weeks before the last frost.

Frost dates: NOAA U.S. Climate Normals 1991–2020, station Casper Wwtp, WY (USC00481569), 1.8 km (1.1 mi) from the city center, elevation 1554 m, based on 18 years of records. Hardiness zone: USDA Plant Hardiness Zone Map 2023 for the ZIP code nearest to the city center. Microclimates (valleys, hills, cities, coasts) can be warmer or colder: check the local forecast before planting. How we calculate the data.