FrostCal

When to Plant Cosmos in Idaho

In a typical Idaho location, start seeds indoors Apr 2 – Apr 9, then sow outdoors May 7 – May 21, then transplant outdoors May 7 – May 14. Across the state, the outdoor planting window starts as early as Apr 13 in Lewiston and as late as May 24 in Eagle. 2 cities in Idaho are essentially frost-free and can plant on a year-round schedule.

Method Direct sow
Days to maturity 60–90 days

Easy from direct seed 1-3 weeks after last frost; can also start indoors 3-4 weeks early.

Cosmos Planting Dates by City

CityStart indoorsDirect sowTransplant
BoiseMar 27 – Apr 3May 1 – May 15May 1 – May 8
MeridianMar 27 – Apr 3May 1 – May 15May 1 – May 8
NampaApr 1 – Apr 8May 6 – May 20May 6 – May 13
Idaho FallsApr 11 – Apr 18May 16 – May 30May 16 – May 23
PocatelloApr 4 – Apr 11May 9 – May 23May 9 – May 16
CaldwellMar 26 – Apr 2Apr 30 – May 14Apr 30 – May 7
Coeur d'AleneMar 25 – Apr 1Apr 29 – May 13Apr 29 – May 6
Twin FallsApr 2 – Apr 9May 7 – May 21May 7 – May 14
LewistonMar 9 – Mar 16Apr 13 – Apr 27Apr 13 – Apr 20
Lewiston OrchardsMar 9 – Mar 16Apr 13 – Apr 27Apr 13 – Apr 20
Post FallsMar 25 – Apr 1Apr 29 – May 13Apr 29 – May 6
RexburgApr 17 – Apr 24May 22 – Jun 5May 22 – May 29
MoscowFrost-free: year-round planting
EagleApr 19 – Apr 26May 24 – Jun 7May 24 – May 31
CondaFrost-free: year-round planting
KunaMar 27 – Apr 3May 1 – May 15May 1 – May 8
AmmonApr 11 – Apr 18May 16 – May 30May 16 – May 23
ChubbuckApr 4 – Apr 11May 9 – May 23May 9 – May 16
HaydenMar 25 – Apr 1Apr 29 – May 13Apr 29 – May 6
Mountain HomeApr 4 – Apr 11May 9 – May 23May 9 – May 16
BlackfootApr 7 – Apr 14May 12 – May 26May 12 – May 19
Garden CityMar 27 – Apr 3May 1 – May 15May 1 – May 8
JeromeApr 4 – Apr 11May 9 – May 23May 9 – May 16
BurleyApr 9 – Apr 16May 14 – May 28May 14 – May 21

Showing the 24 most populous of 24 Idaho cities. Dates are estimates from each city's NOAA frost normals and university extension timing guidelines. See all Idaho cities.

Frequently Asked Questions

When should I plant cosmos in Idaho?

In a typical Idaho location, start seeds indoors Apr 2 – Apr 9, then sow outdoors May 7 – May 21, then transplant outdoors May 7 – May 14. Across the state, the outdoor planting window starts as early as Apr 13 in Lewiston and as late as May 24 in Eagle. 2 cities in Idaho are essentially frost-free and can plant on a year-round schedule.

When is the last frost in Idaho?

The average last spring frost in Idaho ranges from April 6 in Lewiston to May 17 in Eagle, based on NOAA 1991–2020 climate normals.

How long do cosmos take to grow?

Cosmos typically take 60–90 days from planting to harvest. Easy from direct seed 1-3 weeks after last frost; can also start indoors 3-4 weeks early.

Other Crops in Idaho

When to plant cosmos in other states

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