Ottawa climate

Ottawa Frost Dates and Growing Season

NOAA 1991-2020 normals for Ottawa, 1.2 km from Ottawa, elevation 905 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Ottawa. 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 October 20, a growing season of about 183 days.

Median at 36°F
last April 29, first October 9
Median at 32°F
last April 18, first October 20
Median at 28°F
last April 8, first October 30

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 10May 6May 3Apr 30Apr 29Apr 26Apr 22Apr 19Apr 15
32°FMay 1Apr 27Apr 24Apr 21Apr 18Apr 16Apr 13Apr 10Apr 5
28°FApr 22Apr 18Apr 14Apr 11Apr 8Apr 5Apr 2Mar 30Mar 24
24°FApr 16Apr 8Apr 5Apr 2Mar 31Mar 28Mar 24Mar 19Mar 11
20°FApr 5Mar 31Mar 28Mar 23Mar 19Mar 16Mar 11Mar 6Feb 28
16°FMar 27Mar 20Mar 15Mar 11Mar 7Mar 4Feb 28Feb 25Feb 18

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 26Sep 30Oct 3Oct 6Oct 9Oct 12Oct 15Oct 18Oct 22
32°FOct 3Oct 9Oct 14Oct 17Oct 20Oct 23Oct 26Oct 29Nov 1
28°FOct 16Oct 21Oct 24Oct 27Oct 30Nov 1Nov 3Nov 6Nov 11
24°FOct 24Oct 29Nov 1Nov 4Nov 6Nov 9Nov 13Nov 17Nov 23
20°FNov 1Nov 5Nov 9Nov 13Nov 16Nov 20Nov 24Nov 29Dec 4
16°FNov 8Nov 14Nov 19Nov 23Nov 27Dec 1Dec 4Dec 9Dec 15

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°F181175170166162159155151144
32°F202196191186183179175170164
28°F223215211207203199196190184
24°F246236231226221218213208201
20°F268260253247242238232226218
16°F289280274268264258253247238

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
5,130
Cooling degree days, base 65°F
1,337
Growing degree days, base 40°F
6,373
Growing degree days, base 45°F
5,099
Growing degree days, base 50°F
3,963
Growing degree days, base 55°F
2,954
Growing degree days, base 60°F
2,080

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,11402
February87509
March661261
April35113184
May11592447
June7271714
July1417882
August2362824
September58153546
October29827243
November654149
December99505

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 1.2 km from Ottawa. 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 USC00146128 (Ottawa). Public domain.