Polk County climate

Polk County Frost Dates and Growing Season

NOAA 1991-2020 normals for Falls City 2 Ssw, 8.2 km from Polk County, elevation 690 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Falls City 2 Ssw. A median date has an even chance of a later freeze in spring, or an earlier one in fall.

The last spring freeze typically comes around April 18 and the first fall freeze around November 7, a growing season of about 202 days.

Median at 36°F
last May 9, first October 21
Median at 32°F
last April 18, first November 7
Median at 28°F
last March 13, first November 23

Last freeze in spring

The chance of a later freeze than the date shown. At 10 percent, one spring in ten still freezes after the date; planting after the 10 percent date for 32°F is the cautious choice.

Threshold10%20%30%40%50%60%70%80%90%
36°FMay 29May 21May 17May 13May 9May 6May 3Apr 30Apr 26
32°FMay 6May 1Apr 27Apr 23Apr 18Apr 13Apr 7Apr 2Mar 27
28°FApr 5Mar 29Mar 24Mar 19Mar 13Mar 8Mar 1Feb 25Feb 15
24°FMar 10Mar 2Feb 25Feb 20Feb 13Feb 2Jan 24Jan 10Dec 29
20°FFeb 23Feb 13Feb 2Jan 25Jan 15Jan 7Dec 31Dec 21Dec 8
16°FFeb 15Feb 3Jan 25Jan 13Jan 3Dec 31Dec 24Dec 18Dec 9

First freeze in fall

The chance of an earlier freeze than the date shown. At 10 percent, one fall in ten freezes before the date.

Threshold10%20%30%40%50%60%70%80%90%
36°FOct 2Oct 8Oct 13Oct 18Oct 21Oct 24Oct 27Nov 1Nov 5
32°FOct 21Oct 27Oct 31Nov 4Nov 7Nov 10Nov 13Nov 18Nov 24
28°FNov 3Nov 9Nov 13Nov 18Nov 23Nov 28Dec 4Dec 10Dec 20
24°FNov 13Nov 23Dec 1Dec 7Dec 14Dec 20Dec 29Jan 6Jan 21
20°FNov 26Dec 8Dec 15Dec 23Dec 29Jan 3Jan 10Jan 21Feb 9
16°FDec 7Dec 14Dec 23Dec 27Jan 1Jan 6Jan 16Jan 31Feb 11

Growing season length

Days between the last spring and first fall freeze at each threshold, with the chance of a season at least that long.

Threshold10%20%30%40%50%60%70%80%90%
36°F183176170165160156151145137
32°F230219212207202196190185176
28°F290275265259252246238229218
24°F352338325316308302292283268
20°F365365365359351342333323310
16°F365365365365365365365356339

How often it gets that cold

The share of years with at least one night at or below each temperature.

36°F or colder
100% of years
32°F or colder
100% of years
28°F or colder
100% of years
24°F or colder
94% of years
20°F or colder
65% of years
16°F or colder
25% of years

Degree days

A degree day is one degree of the daily mean temperature beyond a base, for one day. Heating degree days count cold (the fuel a house burns), cooling degree days heat, growing degree days the warmth a crop needs; each crop has its own base.

Heating degree days, base 65°F
5,302
Cooling degree days, base 65°F
193
Growing degree days, base 40°F
4,278
Growing degree days, base 45°F
2,923
Growing degree days, base 50°F
1,872
Growing degree days, base 55°F
1,087
Growing degree days, base 60°F
528

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January80801
February66601
March645012
April522041
May3504147
June21418257
July6870466
August5572482
September14228337
October3912115
November638011
December80402

What this means

NOAA computes these from the daily lows at the station over 1991-2020, fitting the dates of each year's last and first freeze to a distribution, so a date can be stated at any level of risk. 36°F is a frost risk for tender plants on a clear, calm night, since the ground cools below the air above it; 32°F is a freeze; 28°F is a hard freeze that kills most annuals.

The station is 8.2 km from Polk County. Low ground, valleys and open fields freeze earlier and later than a town center or a hillside.

More

Sources

NOAA NCEI U.S. Climate Normals, 1991-2020, annual and seasonal normals for station USC00352800 (Falls City 2 Ssw). Public domain.