Seed Starting Calendar for Wyoming

Q: When should I start seeds indoors in Wyoming?

Count back from the median last frost where you garden. At Cheyenne (Cheyenne) that date is May 12, so for spring 2027 onions start indoors Feb 3 to Feb 17, peppers Mar 17 to Mar 31 and tomatoes Mar 24 to Apr 7. Across Wyoming's 124 NOAA stations the median last frost runs from May 7 to Jun 9 for the middle 90 percent, so a garden near Moose 1 NNE starts later than one near Basin.

Q: When is the last frost in Wyoming?

It depends on the station. The earliest median last spring freeze in Wyoming is April 28 at Basin; the latest is June 24 at Moose 1 NNE. A median is a coin flip: at Cheyenne the last 32°F freeze comes after May 12 in half of years, and has passed by May 26 in 9 of 10 years (NOAA 1991 to 2020 normals).

Q: When can I plant tomatoes outside in Wyoming?

UNH Extension sets tomatoes out 1 to 2 weeks after the last frost, which at Cheyenne is May 19 to May 26 in 2027. Tomatoes are frost tender, so the date to watch is May 26, when 9 of 10 springs at Cheyenne have had their last 32°F night. Hardening off starts two weeks before setting out (May 5 to May 19).

Q: How many weeks before the last frost do I start peppers and eggplant?

UNH Extension gives 8 to 10 weeks from seed to transplant for both, and sets peppers out 2 weeks after the last frost and eggplant 2 to 3 weeks after, so the sowing date is 6 to 8 weeks before the median last frost. At Cheyenne that is Mar 17 to Mar 31 for peppers.

Q: How long do seedlings need to harden off?

About two weeks. University of Missouri Extension: "Hardening should be started about two weeks before planting in the garden." University of Minnesota Extension adds that seedlings should go out on a cloudy day or in the late afternoon.

Median last frost from May 7 to Jun 9 across Wyoming

NOAA stations

124

Median last frost, Cheyenne

May 12

Passed in 9 of 10 years

May 26

Last frost

Last spring frost dates in Wyoming

Greybull in the Bighorn Basin reaches its median last frost around May 4, Casper and Cheyenne around May 10 to May 12, Rock Springs around May 22, and Moose in Jackson Hole around Jun 24.

Three dates per station from the NOAA 1991 to 2020 normals. The median is the date half of springs run past. The last column is the date the last 32°F night has passed by in 9 years out of 10, the one to hold tender transplants for.

Station	Elevation	Early (1 in 10 years later than this)	Median last frost	Passed in 9 of 10 years	Median first fall frost
Greybull S Big Horn AirportGreybull S Big Horn Airport, USW00024048	3,933 ft	Apr 22	May 4	May 18	Oct 1
CasperCasper Wwtp, USC00481569	5,097 ft	Apr 30	May 10	May 24	Sep 29
CheyenneCheyenne, USW00024018	6,128 ft	May 1	May 12	May 26	Oct 1
Rock Springs AirportRock Springs Airport, USW00024027	6,741 ft	May 5	May 22	Jun 9	Sep 28
Moose, Jackson HoleMoose 1 NNE, USW00004131	6,466 ft	Jun 8	Jun 24	Jul 11	Aug 28

Crop chart

When to start seeds in Wyoming, spring 2027

Counted back from the median last frost at Cheyenne, May 12. Pick another station to redraw the chart for it.

Start indoorsHarden offSet outSow outdoorsMedian last frostLast freeze passed in 9 of 10 years

Every crop, counted from a median last frost of May 12
Crop	Start indoors	Set out	Sow outdoors	Frost class
Tomato	Mar 24 to Apr 7	May 19 to May 26		Tender, very tender
Pepper	Mar 17 to Mar 31	May 26 to Jun 2		Tender, very tender
Eggplant	Mar 17 to Mar 31	May 26 to Jun 2		Tender, very tender
Basil	Mar 31 to Apr 14	May 26 to Jun 2		Tender
Celery	Feb 17 to Mar 3	May 12 to May 19		Tender
Cucumber	Apr 21 to Apr 28	May 19 to May 26	May 12 to May 19	Tender
Summer squash	Apr 28 to May 5	May 26 to Jun 2	May 12 to May 19	Tender
Winter squash	Apr 28 to May 5	May 26 to Jun 2	May 26 to Jun 2	Tender, very tender
Pumpkin	Apr 28 to May 5	May 26 to Jun 2	May 26 to Jun 2	Tender, very tender
Melon (cantaloupe)	Apr 28 to May 5	May 26 to Jun 2		Tender, very tender
Watermelon	Apr 21 to May 5	May 26 to Jun 2		Tender, very tender
Onion (from seed)	Feb 3 to Feb 17	Apr 14 to Apr 21		Hardy
Leek	Feb 3 to Feb 17	Apr 14 to Apr 21		Hardy
Parsley	Feb 10 to Feb 17	Apr 21 to Apr 28		Half-hardy
Broccoli	Mar 3 to Mar 17	Apr 14 to Apr 28		Hardy
Brussels sprouts	Mar 3 to Mar 17	Apr 14 to Apr 28		Hardy
Cabbage	Mar 3 to Mar 17	Apr 14 to Apr 21		Hardy
Cauliflower	Mar 17 to Mar 31	Apr 28 to May 5		Half-hardy
Kale	Mar 3 to Mar 17	Apr 14 to Apr 21		Hardy
Lettuce	Mar 10 to Mar 17	Apr 14 to Apr 21		Hardy
Swiss chard	Mar 17 to Mar 31	Apr 28 to May 5	Apr 14 to Apr 21	Half-hardy
Spinach	Feb 17 to Mar 3	Mar 31 to Apr 21	Apr 14 to Apr 28	Hardy
Peas			Apr 14 to Apr 28	Hardy
Radish			Apr 14 to Apr 28	Hardy
Carrot			Apr 28 to May 12	Half-hardy
Beet			Apr 28 to May 12	Half-hardy
Beans			May 12 to May 19	Tender
Sweet corn			May 12 to May 19	Tender

Six square pots of dark compost seen from above, each holding a single seedling with two narrow seed leaves — Tomato seedlings at the seed leaf stage, one to a pot. The University of Minnesota Extension holds off on fertilizer until a seedling has several sets of true leaves, then feeds at quarter strength once a week. “Tomato Seedlings (49768696597)” by Amanda Slater from Coventry, West Midlands, UK, CC BY-SA 2.0

Extension guide

What University of Wyoming Extension advises

University of Wyoming Extension publishes the planting guide Wyoming gardeners work from, Wyoming Vegetable & Fruit Growing Guide (B-1340).

“Read all the seed packets and separate them into groups by the recommended weeks before the average last frost that they should be planted indoors (for example, one pile each for those needing eight‑10 weeks, six weeks, four weeks, etc.).”

University of Wyoming Extension, Wyoming Vegetable & Fruit Growing Guide, quoted exactly.

“Starting plants from seed indoors can help growers get a jump on the season.”

University of Wyoming Extension, Starting plants from seed indoors (chapter 1 of B-1340), quoted exactly.

“Start plants from seeds four to six weeks before transplanting outside. Broccoli can be transplanted when soil temperatures reach 40 F (two to three weeks before the average last frost).”

University of Wyoming Extension, on broccoli, quoted exactly.

“Start seeds indoors four to six weeks before transplanting into a field.”

University of Wyoming Extension, on cabbage, quoted exactly.

“Eggplants should be started about six to eight weeks before they moved outside.”

University of Wyoming Extension, on eggplant, quoted exactly.

“They are often started 8 to 12 weeks before transplanting. Transplants can be planted one to two weeks before the last anticipated light frost.”

University of Wyoming Extension, on onion, quoted exactly.

A gardener kneeling beside a raised bed setting a young tomato plant into dark soil, an empty nursery pot beside her — Setting out a tomato in June at a community garden. Tender crops go out after the last frost, and after a week or two of hardening off outdoors. “Volunteer planting heirloom tomato (4704185147)”, Pacific Southwest Region 5, Public domain

Seed starting questions for Wyoming

When should I start seeds indoors in Wyoming?

When is the last frost in Wyoming?

When can I plant tomatoes outside in Wyoming?

What does University of Wyoming Extension say about planting dates in Wyoming?

University of Wyoming Extension's Wyoming Vegetable & Fruit Growing Guide puts it this way: "Read all the seed packets and separate them into groups by the recommended weeks before the average last frost that they should be planted indoors (for example, one pile each for those needing eight‑10 weeks, six weeks, four weeks, etc.)." The guide is linked below with the date it was read (September 21, 2026).

How many weeks before the last frost do I start peppers and eggplant?

How long do seedlings need to harden off?

Why do seed starting dates differ across Wyoming?

Frost dates from NOAA NCEI U.S. Climate Normals 1991-2020, annual/seasonal, by station (v1.0.1). Planting guidance from University of Wyoming Extension, quoted exactly. Seed starting weeks from UNH Cooperative Extension. Compiled 2026-09-21.