Riverton, WY planting zone & frost dates

Riverton is in USDA hardiness zone 4b. The last spring frost usually comes around May 12 and the first fall frost around September 24, for a growing season of about 132 days.

USDA hardiness zone4bavg. extreme winter low -25°F to -20°F

Average last spring frostMay 1290% chance of no frost after May 26

Average first fall frostSeptember 2410% chance of frost by September 11

Growing season132 daysfrost-free days in an average year

Planting calendar for Riverton

Dates are counted from the average last spring frost (May 12) 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 17 – Mar 31 Transplant outdoors: May 19 – May 26
Peppers	Start seeds indoors: Mar 3 – Mar 17 Transplant outdoors: May 26 – Jun 2
Eggplant	Start seeds indoors: Mar 3 – Mar 17 Transplant outdoors: May 26 – Jun 2
Broccoli	Start seeds indoors: Mar 3 – Mar 17 Transplant outdoors: Apr 14 – Apr 28
Cabbage	Start seeds indoors: Mar 3 – Mar 17 Transplant outdoors: Apr 14 – Apr 28
Onions (sets)	Plant outdoors: Apr 14 – Apr 28
Peas	Sow outdoors: Mar 31 – Apr 14
Spinach	Sow outdoors: Mar 31 – Apr 14
Lettuce	Sow outdoors: Apr 14 – Apr 28
Carrots	Sow outdoors: Apr 14 – Apr 28
Potatoes	Plant outdoors: Apr 14 – Apr 28
Beans	Sow outdoors: May 19 – May 26
Sweet corn	Sow outdoors: May 19 – May 26
Cucumbers	Sow outdoors: May 19 – May 26
Squash & zucchini	Sow outdoors: May 19 – May 26
Basil	Start seeds indoors: Mar 31 – Apr 14 Transplant outdoors: May 19 – May 26

Fall garden: cool-season crops like lettuce, spinach and kale can be sown again around Jul 30 (8 weeks before the average first fall frost on Sep 24).

Useful for zone 4b 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	May 1	May 12	May 26
First 32°F frost in fall is before	Oct 7	Sep 24	Sep 11

How to read it: in spring there is a 10% chance of a frost after May 26, so that is a safe date for tender plants. In fall there is a 10% chance of frost before September 11. A hard freeze (28°F) usually ends around May 1.

Hardiness zone by ZIP code in Riverton

82501 zone 5a

Nearby cities

Rock Springs zone 5a, last frost May 24
Casper zone 5a, last frost May 10
Sheridan zone 5a, last frost May 21
Jackson zone 4b, last frost Jun 20
Evanston zone 5a, last frost Jun 12
Gillette zone 5a, last frost May 15
Laramie zone 4a, last frost Jun 7
Cheyenne zone 5b, last frost May 14

All cities in Wyoming → · Other zone 4b cities →

Questions

What growing zone is Riverton, WY?

Riverton is in USDA Plant Hardiness Zone 4b (average extreme winter low -25°F to -20°F), according to the USDA 2023 map. Some ZIP codes of the city fall in zone 5a.

When is the last frost in Riverton?

The average last spring frost (32°F) in Riverton is May 12. There is still a 10% chance of frost until May 26.

When can I plant tomatoes in Riverton?

Transplant tomatoes outdoors 1–2 weeks after the average last frost: around May 19 – May 26. Start seeds indoors 6–8 weeks before the last frost.

Frost dates: NOAA U.S. Climate Normals 1991–2020, station Riverton, WY (USC00487760), 0.8 km (0.5 mi) from the city center, elevation 1510 m, based on 30 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.