When is the last frost in Decatur, IL?

The average last spring frost (50% probability of 32°F) in Decatur is around April 14, based on NOAA/NCEI 1991–2020 normals at the Decatur Wtp, IL station (1.8 mi away). Half of years it falls outside that date, so wait 1–2 weeks past it before setting out frost-tender plants.

When is the first fall frost in Decatur?

The average first fall frost in Decatur, IL is around October 24, giving roughly a 192-day frost-free growing season.

What planting zone is Decatur, IL?

Decatur is in USDA plant hardiness zone 6a (average annual extreme minimum temperature -10 to -5°F), from the 2023 USDA Plant Hardiness Zone Map.

What is the weather like in Decatur?

In Decatur, IL the warmest month is July (avg high 84.7°F) and the coldest is January (avg low 19.7°F). It averages about 40.4″ of precipitation a year and 13.6″ of snow.

Home › Illinois › Decatur

Decatur, IL Planting Calendar & Frost Dates

USDA hardiness zone 6a · ZIP 62521 · elevation 195′ · NOAA & USDA data

Decatur is in USDA hardiness zone 6a (winters can dip to -10 to -5°F), with a growing season of about 192 days between the average last spring frost (April 14) and first fall frost (October 24). The warmest month is July (avg high 84.7°F); the coldest is January (avg low 19.7°F). It averages 40.4″ of precipitation a year and 13.6″ of snow, mostly mid-winter. Start cool-season crops several weeks before the last frost; wait until after it for tomatoes, peppers and other tender crops.

Hardiness zone

min temp -10 to -5°F

Last spring frost

April 14

avg (50% prob, 32°F)

First fall frost

October 24

avg (50% prob, 32°F)

Growing season

192 days

frost-free window

Warmest month

July

avg high 84.7°F

Month-by-month typical weather in Decatur

Average conditions from the NOAA/NCEI 1991–2020 U.S. Climate Normals at Decatur Wtp, IL (1.8 mi away). Temperatures °F, precipitation & snow in inches.

Month	Avg high	Avg low	Mean	Precip (in)	Snow (in)
January	34	19.7	26.9	2.4	5.9
February	39.1	23.5	31.3	2	2.5
March	50.8	32.8	41.8	2.6	0.9
April	63.4	43.3	53.3	4.1	0.4
May	73.5	53.8	63.7	5	0
June	82.2	63.1	72.6	4.7	0
July	84.7	66.5	75.6	4	0
August	83.5	64.9	74.2	3.5	0
September	77.7	57	67.3	3.1	0
October	65.3	45.7	55.5	3.4	0
November	50.3	34.3	42.3	3.2	0.4
December	38.6	25	31.8	2.4	3.5

What to plant when in Decatur

Crop	Season	What to do & when
Tomatoes	warm	Start seeds indoors: March 3 · Transplant out: April 21
Peppers	warm	Start seeds indoors: February 17 · Transplant out: April 28
Cucumbers	warm	Direct sow: April 21
Zucchini / squash	warm	Direct sow: April 21
Bush beans	warm	Direct sow: April 21
Sweet corn	warm	Direct sow: April 21
Basil	warm	Transplant out: April 21
Peas	cool	Sow (spring): March 10 · Sow (fall): August 29
Spinach	cool	Sow (spring): March 3 · Sow (fall): August 29
Lettuce	cool	Sow (spring): March 17 · Sow (fall): August 29
Kale	cool	Transplant (spring): March 24 · Sow (fall): August 15
Broccoli	cool	Transplant (spring): March 31
Carrots	cool	Sow (spring): March 24 · Sow (fall): August 15
Onions	cool	Transplant (spring): March 17
Potatoes	cool	Plant seed potatoes: March 31
Radishes	cool	Sow (spring): March 17 · Sow (fall): September 12

Dates are computed from Decatur's average frost dates using standard cool-/warm-season timing. They're a starting point — a cold spring, a warm microclimate, or a fast-maturing variety can shift them by a week or two.

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Gear that helps you plant on time

Some links below are Amazon affiliate links (tag theboringthin-20). If you buy through them we may earn a small commission at no extra cost to you — it keeps this almanac free. We only list generic, genuinely-useful categories.

Soil thermometer →Know when the ground is warm enough to sow — the honest signal, not the calendar.Seed-starting trays + dome →Start tomatoes & peppers indoors on the schedule below.Seedling heat mat →Warms the soil for faster, more even germination indoors.Floating row cover / frost cloth →Buys you 4–8°F to beat an early or late frost.Raised garden bed kit →Warms and drains faster in spring than in-ground soil.Garden gloves →Basic kit for transplant day.

Nearby cities in Illinois

Springfield, IL Champaign, IL Urbana, IL Bloomington, IL Normal, IL Peoria, IL Belleville, IL Joliet, IL

Sources & method. Hardiness zone: 2023 USDA Plant Hardiness Zone Map (PRISM Climate Group, Oregon State University). Frost dates & growing season: NOAA/NCEI 1991–2020 U.S. Climate Normals, nearest station with published freeze normals — Decatur Wtp, IL, 1.8 mi (confidence: high). Monthly weather normals: Decatur Wtp, IL, 1.8 mi (confidence: high). ZIP centroid from U.S. Census 2023 ZCTA data. Frost dates are the 50%-probability date — half of years frost occurs outside the stated date. Nothing here is invented; where the nearest station is far, we say so. Last updated 2026-07-22.