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Frost dates and planting calendar for Decatur, Illinois

Updated October 30, 2026. Based on USDA, NOAA and Colorado State University Extension data. Published by Humblebee.

USDA hardiness zone
6a[1]
Median last spring frost
Apr 14[2]
Median first fall frost
Oct 24[3]
Growing season, median
192 days[4]

The center of Decatur, Illinois is in USDA hardiness zone 6a[1]. The median last spring frost at the nearby NOAA station is April 14[2].

Frost dates

JanFebMarAprMayJunJulAugSepOctNovDecLast spring frost: 9 in 10 years after Apr 1, half of years Apr 14, 1 in 10 years after May 1Last frostmedian Apr 14First fall frost: 9 in 10 years before Nov 5, half of years Oct 24, 1 in 10 years before Oct 11First frostmedian Oct 24Frost riskHardyHardy vegetables: plant Mar 17 to Mar 31Mar 17 to Mar 31Semi-hardySemi-hardy vegetables: plant Mar 31 to Apr 14Mar 31 to Apr 14TenderTender vegetables: plant around Apr 14around Apr 14Very tenderVery tender vegetables: plant from Apr 28from Apr 28
Shaded: the span between the dates frost passes in 9 of 10 years and 1 of 10 years[2][3]. Green: planting windows from the CSU rules applied to the median last frost (GardenFacts calculation)[6]. Exact dates are in the tables below.

These frost dates come from NOAA climate normals for the Decatur Wtp weather station, using records from 1991 to 2020[5]. In half of years the last spring frost comes after April 14, and in 1 of 10 years it comes after May 1[2]. In half of years the first fall frost comes before October 24, and in 1 of 10 years it comes before October 11[3]. These are chances from past records and not a forecast, so any year can be earlier or later[2].

Frost Likely (9 in 10 years) Even odds (5 in 10) Unlikely (1 in 10)
Last spring frost (32°F)[2] after April 1 after April 14 after May 1
First fall frost (32°F)[3] before October 11 before October 24 before November 5
  • Hard freeze (28°F): median last spring date April 3, median first fall date November 3[7].
  • Frost-free growing season: at least 171 days in 9 of 10 years, 192 days in half of years and 211 days in 1 of 10 years[4].
  • The low reached 32°F or colder in 100% of years and 28°F or colder in 100% of years[8].
  • Weather station: Decatur Wtp (USC00112193), 2.1 miles from the city center, 639 feet, 24 years of 1991 to 2020 records[5].

When to plant vegetables

Colorado State University Extension's planting guide was written for Colorado, so treat these dates as a starting point[6]. We took the NOAA median last frost of April 14 as the average last frost[6]. Hardy vegetables such as lettuce, peas and spinach can go in as early as 2 to 4 weeks before it, which is March 17 to March 31[6]. Semi-hardy vegetables such as carrots and potatoes can go in as early as 2 weeks before it, which is March 31[9]. Tender vegetables such as snap beans and sweet corn can be seeded around April 14[10]. Very tender vegetables such as tomatoes and peppers typically start 2 or more weeks after it, so the earliest start is April 28[11].

  • Hardy (broccoli, cabbage, kohlrabi, lettuce, onions, peas, radishes, spinach, turnips): as early as 2 to 4 weeks before the median last frost (April 14) is March 17 to March 31[6]
  • Semi-hardy (beets, carrots, cauliflower, parsnips, potatoes, Swiss chard): as early as 2 weeks before the median last frost (April 14) is March 31, up to the median date itself[9]
  • Tender (snap beans, sweet corn, cucumbers, summer squash): from seed around the median last frost date, April 14[10]
  • Very tender (cantaloupe, eggplant, peppers, tomatoes, winter squash, watermelon): the earliest start, two weeks after the median last frost (April 14), is April 28[11]
Vegetable (CSU group) Plant from First harvest around
Broccoli (hardy)[12] Mar 17 May 21 (from transplants)
Cabbage (hardy)[12] Mar 17 Jun 10 (from transplants)
Kohlrabi (hardy)[12] Mar 17 May 6
Lettuce (hardy)[12] Mar 17 May 16
Onions (hardy)[12] Mar 17 Jul 5
Peas (hardy)[12] Mar 17 May 21
Radishes (hardy)[12] Mar 17 Apr 16
Spinach (hardy)[12] Mar 17 Apr 26
Turnips (hardy)[12] Mar 17 May 6
Beets (semi-hardy)[12] Mar 31 May 30
Carrots (semi-hardy)[12] Mar 31 Jun 9
Cauliflower (semi-hardy)[12] Mar 31 not listed
Parsnips (semi-hardy)[12] Mar 31 Jun 9
Potatoes (semi-hardy)[12] Mar 31 Aug 3
Swiss chard (semi-hardy)[12] Mar 31 May 30
Snap beans (tender)[12] Apr 14 Jun 13
Sweet corn (tender)[12] Apr 14 Jun 13
Cucumbers (tender)[12] Apr 14 Jun 8
Summer squash (tender)[12] Apr 14 Jun 3
Cantaloupe (very tender)[12] Apr 28 Jul 22
Eggplant (very tender)[12] Apr 28 Jun 27 (from transplants)
Peppers (very tender)[12] Apr 28 Jul 7 (from transplants)
Tomatoes (very tender)[12] Apr 28 Jul 2 (from transplants)
Winter squash (very tender)[12] Apr 28 Aug 6
Watermelon (very tender)[12] Apr 28 Jul 22
Spacing, depth and soil temperature (CSU Extension)
Vegetable Min. soil temp Depth Spacing Days to harvest
Broccoli[13] 40°F 1/2 in 18 in 65
Cabbage[13] 40°F 1/2 in 18 in 85
Kohlrabi[13] 40°F 1/2 in 7-9 in 50
Lettuce[13] 35°F 1/4 in 7-9 in 60
Onions[13] 35°F 1/4 in (seed), 1-2 in (sets) 4-6 in 110
Peas[13] 40°F 1 in 4-6 in 65
Radishes[13] 40°F 1/2 in 2-3 in 30
Spinach[13] 40°F 1/2 in 4-6 in 40
Turnips[13] 40°F 1/2 in 4-6 in 50
Beets[13] 40°F 3/4-1 in 4-6 in 60
Carrots[13] 40°F 1/4 in 2-3 in 70
Cauliflower[13] 40°F 1/2 in 18 in –
Parsnips[13] 35°F 1/2 in 5-6 in 70
Potatoes[13] 45°F – – 125
Swiss chard[13] 40°F 1 in 7-9 in 60
Snap beans[13] 55°F 1-1 1/2 in 6 in 60
Sweet corn[13] 50°F 1-1 1/2 in 12 x 30 in or 9 x 36 in 60-90
Cucumbers[13] 60°F 1 in 6 in trellised, 24-36 in untrellised 55
Summer squash[13] 60°F 1-1 1/2 in 36-48 in 50
Cantaloupe[13] 60°F 1-1 1/2 in 36-48 in 85
Eggplant[13] 60°F 1/4 in 18-24 in 60
Peppers[13] 60°F 1/4 in 15-18 in 70
Tomatoes[13] 50°F 1/4 in 24 in trellised 65
Winter squash[13] 60°F 1-1 1/2 in 36-48 in 100
Watermelon[13] 60°F 1-1 1/2 in 36-48 in 85

Hardiness zone

Zone 6a means the coldest night of each winter averages -10°F to -5°F over 1991 to 2020[1]. The zone is based on winter cold, not on summer heat or frost dates[14]. All of the city's land area lies in ZIP code areas rated zone 6a[15].

Zone 6a: average annual extreme minimum temperature -10°F to -5°F (1991 to 2020)[1]. Modeled value at the city center: -5.9°F[1].

Share of the city's land area by ZIP code zone: zone 6a 100%[15].

All towns in zone 6a and what zone 6a means

Zone by ZIP code
ZIP code Zone Coldest night range
62526[15] 6a -10 to -5°F
62521[15] 6a -10 to -5°F
62522[15] 6a -10 to -5°F
62501[15] 6a -10 to -5°F
62523[15] 6a -10 to -5°F
62535[15] 6a -10 to -5°F
62554[15] 6a -10 to -5°F

Questions people ask

What growing zone is Decatur, Illinois?
The center of Decatur is in USDA zone 6a on the 2023 map[1]. By land area, 100% of the city lies in ZIP code areas rated zone 6a[15].
When is the last frost in Decatur?
The median last spring frost is April 14[2]. In 9 of 10 years the last frost comes after April 1, and in 1 of 10 years it comes after May 1[2].
When to plant tomatoes in Decatur?
CSU Extension says tomatoes typically go in 2 or more weeks after the average last spring frost[11]. Using the median last frost, the earliest start is April 28[11].
When is the first frost in Decatur?
The median first fall frost is October 24[3]. In 1 of 10 years it comes before October 11[3].
How long is the growing season in Decatur?
The growing season is at least 192 days in half of years[4]. It is at least 171 days in 9 of 10 years[4].

Facts and sources

  1. The 2023 USDA Plant Hardiness Zone Map puts the center of Decatur, Illinois in zone 6a. Zone 6a means an average annual extreme minimum temperature of -10°F to -5°F over 1991 to 2020. The map's modeled value at the city center is minus 5.9°F. USDA zone map 2023 (PRISM, OSU)
  2. Last spring frost (32°F) at this station: in 9 of 10 years the last frost comes after April 1, in half of years after April 14, and in 1 of 10 years after May 1 (NOAA 1991 to 2020 normals). The median last frost date is April 14. NOAA Climate Normals 1991-2020
  3. First fall frost (32°F) at this station: in 1 of 10 years the first frost comes before October 11, in half of years before October 24, and in 9 of 10 years before November 5 (NOAA 1991 to 2020 normals). The median first frost date is October 24. NOAA Climate Normals 1991-2020
  4. Growing season (days between the last spring and first fall 32°F frost) at this station: at least 171 days in 9 of 10 years, at least 192 days in half of years, and at least 211 days in 1 of 10 years. NOAA Climate Normals 1991-2020
  5. Frost dates on this page come from NOAA climate normals for the Decatur Wtp weather station (USC00112193), 2.1 miles from the center of Decatur at 639 feet, using 24 years of records from 1991 to 2020. NOAA Climate Normals 1991-2020
  6. Colorado State University Extension's vegetable planting guide (written for Colorado) groups broccoli, cabbage, kohlrabi, lettuce, onions, peas, radishes, spinach, turnips as hardy vegetables, with this rule: as early as 2-4 weeks before the average last... CSU Extension GardenNotes #720
  7. Hard freeze (28°F) at this station: the median last spring date is April 3 and the median first fall date is November 3. NOAA Climate Normals 1991-2020
  8. At this station the low reached 32°F or colder in 100% of years from 1991 to 2020, and 28°F or colder in 100% of years. NOAA Climate Normals 1991-2020
  9. Colorado State University Extension's vegetable planting guide (written for Colorado) groups beets, carrots, cauliflower, parsnips, potatoes, Swiss chard as semi-hardy vegetables, with this rule: as early as 0-2 weeks before the average last spring frost.... CSU Extension GardenNotes #720
  10. Colorado State University Extension's vegetable planting guide (written for Colorado) groups snap beans, sweet corn, cucumbers, summer squash as tender vegetables, with this rule: from seed around the average last spring frost; cucumber and summer squash... CSU Extension GardenNotes #720
  11. Colorado State University Extension's vegetable planting guide (written for Colorado) groups cantaloupe, eggplant, peppers, tomatoes, winter squash, watermelon as very tender vegetables, with this rule: typically 2 or more weeks after the average last... CSU Extension GardenNotes #720
  12. GardenFacts calculation: earliest planting date plus CSU days to harvest (table on this page), from the station's median last frost. NOAA Climate Normals 1991-2020
  13. CSU Extension vegetable table: soil temperature, depth, spacing, days to harvest (table on this page). CSU Extension GardenNotes #720
  14. USDA hardiness zones are based on the average annual extreme minimum winter temperature (the coldest night of each winter, averaged over 1991 to 2020 for the 2023 map), not on summer heat or frost dates. Each zone spans 10°F and each half zone (a or b)... USDA Plant Hardiness Zone Map
  15. Decatur, Illinois overlaps 7 ZIP code areas. By land area, 100% lies in ZIP code areas rated zone 6a. USDA zone map 2023 (PRISM, OSU)

Frost dates are probabilities from 1991 to 2020 weather records at one NOAA station, not a forecast. Any year can be earlier or later. The hardiness zone describes the average coldest winter night, not frost dates or summer heat. Spots inside the city can differ (ZIP table). Planting rules come from Colorado State University Extension's guide, written for Colorado. A local extension office may give dates tuned to the area. Planting and harvest dates marked GardenFacts calculation are simple arithmetic from the facts above, not advice from the source.

Hardiness zone data: USDA Plant Hardiness Zone Map (2023), USDA Agricultural Research Service and PRISM Group, Oregon State University, prism.oregonstate.edu, accessed October 30, 2026. This is not the official USDA map. Frost data: NOAA NCEI U.S. Climate Normals 1991-2020. Planting rules: Colorado State University Extension.