Marion County climate

Marion County Frost Dates and Growing Season

NOAA 1991-2020 normals for Knoxville, 1 km from Marion County, elevation 895 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Knoxville. 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 20 and the first fall freeze around October 19, a growing season of about 180 days.

Median at 36°F
last April 30, first October 7
Median at 32°F
last April 20, first October 19
Median at 28°F
last April 10, first October 29

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 12May 8May 6May 3Apr 30Apr 28Apr 24Apr 20Apr 15
32°FMay 4Apr 30Apr 26Apr 23Apr 20Apr 17Apr 14Apr 10Apr 6
28°FApr 24Apr 19Apr 16Apr 12Apr 10Apr 7Apr 5Apr 2Mar 28
24°FApr 16Apr 10Apr 7Apr 4Apr 2Mar 31Mar 27Mar 22Mar 15
20°FApr 7Apr 3Mar 31Mar 28Mar 24Mar 20Mar 16Mar 11Mar 6
16°FMar 31Mar 26Mar 21Mar 17Mar 14Mar 11Mar 8Mar 4Feb 28

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 23Sep 29Oct 1Oct 5Oct 7Oct 10Oct 13Oct 17Oct 21
32°FOct 1Oct 8Oct 13Oct 15Oct 19Oct 22Oct 24Oct 27Nov 1
28°FOct 14Oct 20Oct 23Oct 26Oct 29Nov 1Nov 2Nov 5Nov 9
24°FOct 23Oct 29Nov 1Nov 3Nov 5Nov 8Nov 10Nov 15Nov 20
20°FNov 1Nov 4Nov 7Nov 10Nov 14Nov 18Nov 21Nov 25Dec 1
16°FNov 7Nov 12Nov 17Nov 20Nov 24Nov 27Dec 1Dec 4Dec 10

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°F179172167163159156151147141
32°F199192188184180177172167159
28°F219213208203199195191186179
24°F240231226221217213208203197
20°F259252246241236231226221213
16°F277269263259254249244238231

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
6,415
Cooling degree days, base 65°F
950
Growing degree days, base 40°F
5,450
Growing degree days, base 45°F
4,306
Growing degree days, base 50°F
3,279
Growing degree days, base 55°F
2,374
Growing degree days, base 60°F
1,596

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,35800
February1,09103
March850130
April4607120
May17956353
June20199628
July2315778
August8252710
September99107461
October39813168
November780028
December1,17201

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 km from Marion County. 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 USC00134502 (Knoxville). Public domain.