Marshalltown climate

Marshalltown Frost Dates and Growing Season

NOAA 1991-2020 normals for Marshalltown, 2.2 km from Marshalltown, elevation 870 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Marshalltown. 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 28 and the first fall freeze around October 7, a growing season of about 162 days.

Median at 36°F
last May 7, first September 28
Median at 32°F
last April 28, first October 7
Median at 28°F
last April 17, first October 18

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 20May 16May 12May 9May 7May 4May 2Apr 29Apr 25
32°FMay 11May 6May 3Apr 30Apr 28Apr 25Apr 22Apr 18Apr 13
28°FMay 2Apr 28Apr 24Apr 20Apr 17Apr 15Apr 11Apr 8Apr 3
24°FApr 24Apr 18Apr 14Apr 11Apr 8Apr 5Apr 2Mar 31Mar 24
20°FApr 14Apr 8Apr 4Apr 1Mar 30Mar 26Mar 22Mar 18Mar 12
16°FApr 6Apr 1Mar 28Mar 23Mar 20Mar 16Mar 13Mar 9Mar 4

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°FSep 16Sep 19Sep 22Sep 26Sep 28Oct 1Oct 4Oct 7Oct 11
32°FSep 22Sep 27Oct 1Oct 4Oct 7Oct 10Oct 13Oct 17Oct 21
28°FOct 1Oct 7Oct 12Oct 15Oct 18Oct 20Oct 24Oct 27Oct 31
24°FOct 15Oct 21Oct 23Oct 27Oct 29Oct 31Nov 2Nov 5Nov 10
20°FOct 27Oct 31Nov 1Nov 3Nov 6Nov 9Nov 12Nov 16Nov 22
16°FNov 2Nov 6Nov 9Nov 13Nov 16Nov 19Nov 23Nov 27Dec 2

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°F161155150147144140136131126
32°F180174169166162158154149143
28°F200194189185181177172168161
24°F223215210205201197192187181
20°F244236231225220217212207200
16°F263255251246242237232227219

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
100% of years
20°F or colder
100% of years
16°F or colder
100% 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
7,056
Cooling degree days, base 65°F
753
Growing degree days, base 40°F
4,964
Growing degree days, base 45°F
3,881
Growing degree days, base 50°F
2,915
Growing degree days, base 55°F
2,068
Growing degree days, base 60°F
1,343

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,44900
February1,18601
March926119
April523487
May21346316
June29173594
July5257716
August16190639
September13575397
October4629130
November858016
December1,25401

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 2.2 km from Marshalltown. Low ground, valleys and open fields freeze earlier and later than a town center or a hillside.

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Sources

NOAA NCEI U.S. Climate Normals, 1991-2020, annual and seasonal normals for station USC00135198 (Marshalltown). Public domain.