Knox County climate

Knox County Frost Dates and Growing Season

NOAA 1991-2020 normals for Knoxville, 7.2 km from Knox County, elevation 728 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 21 and the first fall freeze around October 20, a growing season of about 181 days.

Median at 36°F
last April 30, first October 11
Median at 32°F
last April 21, first October 20
Median at 28°F
last April 9, 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 15May 10May 7May 4Apr 30Apr 28Apr 25Apr 22Apr 17
32°FMay 6May 1Apr 27Apr 24Apr 21Apr 18Apr 15Apr 12Apr 7
28°FApr 24Apr 19Apr 15Apr 12Apr 9Apr 6Apr 3Apr 1Mar 26
24°FApr 11Apr 6Apr 3Apr 1Mar 31Mar 27Mar 23Mar 18Mar 12
20°FApr 2Mar 30Mar 26Mar 23Mar 19Mar 16Mar 12Mar 9Mar 3
16°FMar 27Mar 21Mar 17Mar 13Mar 11Mar 7Mar 4Mar 1Feb 23

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 29Oct 3Oct 6Oct 9Oct 11Oct 13Oct 16Oct 19Oct 22
32°FOct 7Oct 12Oct 15Oct 18Oct 20Oct 22Oct 24Oct 27Oct 31
28°FOct 17Oct 21Oct 24Oct 27Oct 29Oct 31Nov 2Nov 5Nov 10
24°FOct 28Oct 31Nov 2Nov 5Nov 7Nov 10Nov 13Nov 17Nov 23
20°FNov 4Nov 8Nov 12Nov 15Nov 19Nov 22Nov 25Nov 29Dec 4
16°FNov 10Nov 16Nov 21Nov 24Nov 27Nov 30Dec 4Dec 8Dec 14

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°F179173168164161157153149143
32°F199192188184181177173168161
28°F220214209206202198194189182
24°F245239233228223219215210205
20°F267259254249244241236231224
16°F282275271266262257253247240

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
6,191
Cooling degree days, base 65°F
899
Growing degree days, base 40°F
5,443
Growing degree days, base 45°F
4,286
Growing degree days, base 50°F
3,254
Growing degree days, base 55°F
2,345
Growing degree days, base 60°F
1,555

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,30301
February1,06104
March818133
April4436126
May17361365
June17196628
July3287748
August6232691
September92102463
October39214166
November750028
December1,13302

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 7.2 km from Knox 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 USC00114780 (Knoxville). Public domain.