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Frost dates and planting calendar for Waterloo, Iowa

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

USDA hardiness zone
5a[1]
Median last spring frost
Apr 27[2]
Median first fall frost
Oct 10[3]
Growing season, median
165 days[4]

Waterloo, Iowa is in USDA hardiness zone 5a on the 2023 map[1]. The median last spring frost is April 27, so in half of years the last frost comes after that date[2].

Frost dates

JanFebMarAprMayJunJulAugSepOctNovDecLast spring frost: 9 in 10 years after Apr 11, half of years Apr 27, 1 in 10 years after May 10Last frostmedian Apr 27First fall frost: 9 in 10 years before Oct 21, half of years Oct 10, 1 in 10 years before Sep 26First frostmedian Oct 10Frost riskHardyHardy vegetables: plant Mar 30 to Apr 13Mar 30 to Apr 13Semi-hardySemi-hardy vegetables: plant Apr 13 to Apr 27Apr 13 to Apr 27TenderTender vegetables: plant around Apr 27around Apr 27Very tenderVery tender vegetables: plant from May 11from May 11
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 Waterloo Municipal Airport station, 5 miles from the city center[5]. They use records from 1991 to 2020[5]. In half of years the last spring frost comes after April 27, and in 1 of 10 years it comes after May 10[2]. In half of years the first fall frost comes before October 10, and in 1 of 10 years it comes before September 26[3]. These are chances, not promises, and any year can be earlier or later[2][3].

Frost Likely (9 in 10 years) Even odds (5 in 10) Unlikely (1 in 10)
Last spring frost (32°F)[2] after April 11 after April 27 after May 10
First fall frost (32°F)[3] before September 26 before October 10 before October 21
  • Hard freeze (28°F): median last spring date April 16, median first fall date October 20[7].
  • Frost-free growing season: at least 146 days in 9 of 10 years, 165 days in half of years and 184 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: Waterloo Municipal Airport (USW00094910), 5 miles from the city center, 868 feet, 30 years of 1991 to 2020 records[5].

When to plant vegetables

Colorado State University Extension groups vegetables by how well they handle frost, and its guide was written for Colorado[6]. Hardy vegetables such as lettuce, peas and spinach can go in as early as 2 to 4 weeks before the median last frost, which is March 30 to April 13 in Waterloo[6]. Semi-hardy vegetables such as carrots and potatoes can go in as early as 2 weeks before the median last frost, from April 13 up to the median date itself[9]. Tender vegetables such as snap beans and sweet corn go in from seed around April 27[10]. Very tender vegetables such as tomatoes and peppers typically start 2 or more weeks after the median last frost, so the earliest start is May 11[11]. These dates are GardenFacts calculations that apply the CSU rule to the NOAA median last frost[6][9][10][11].

  • Hardy (broccoli, cabbage, kohlrabi, lettuce, onions, peas, radishes, spinach, turnips): as early as 2 to 4 weeks before the median last frost (April 27) is March 30 to April 13[6]
  • Semi-hardy (beets, carrots, cauliflower, parsnips, potatoes, Swiss chard): as early as 2 weeks before the median last frost (April 27) is April 13, up to the median date itself[9]
  • Tender (snap beans, sweet corn, cucumbers, summer squash): from seed around the median last frost date, April 27[10]
  • Very tender (cantaloupe, eggplant, peppers, tomatoes, winter squash, watermelon): the earliest start, two weeks after the median last frost (April 27), is May 11[11]
Vegetable (CSU group) Plant from First harvest around
Broccoli (hardy)[12] Mar 30 Jun 3 (from transplants)
Cabbage (hardy)[12] Mar 30 Jun 23 (from transplants)
Kohlrabi (hardy)[12] Mar 30 May 19
Lettuce (hardy)[12] Mar 30 May 29
Onions (hardy)[12] Mar 30 Jul 18
Peas (hardy)[12] Mar 30 Jun 3
Radishes (hardy)[12] Mar 30 Apr 29
Spinach (hardy)[12] Mar 30 May 9
Turnips (hardy)[12] Mar 30 May 19
Beets (semi-hardy)[12] Apr 13 Jun 12
Carrots (semi-hardy)[12] Apr 13 Jun 22
Cauliflower (semi-hardy)[12] Apr 13 not listed
Parsnips (semi-hardy)[12] Apr 13 Jun 22
Potatoes (semi-hardy)[12] Apr 13 Aug 16
Swiss chard (semi-hardy)[12] Apr 13 Jun 12
Snap beans (tender)[12] Apr 27 Jun 26
Sweet corn (tender)[12] Apr 27 Jun 26
Cucumbers (tender)[12] Apr 27 Jun 21
Summer squash (tender)[12] Apr 27 Jun 16
Cantaloupe (very tender)[12] May 11 Aug 4
Eggplant (very tender)[12] May 11 Jul 10 (from transplants)
Peppers (very tender)[12] May 11 Jul 20 (from transplants)
Tomatoes (very tender)[12] May 11 Jul 15 (from transplants)
Winter squash (very tender)[12] May 11 Aug 19
Watermelon (very tender)[12] May 11 Aug 4
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 5a means an average annual extreme minimum temperature of -20°F to -15°F over 1991 to 2020[1]. The zone is based on the coldest night of each winter, not on summer heat or frost dates[14]. All of Waterloo's land area lies in ZIP code areas rated zone 5a, so the city sits in one zone[15].

Zone 5a: average annual extreme minimum temperature -20°F to -15°F (1991 to 2020)[1]. Modeled value at the city center: -17.4°F[1].

Share of the city's land area by ZIP code zone: zone 5a 100%[15].

All towns in zone 5a and what zone 5a means

Zone by ZIP code
ZIP code Zone Coldest night range
50701[15] 5a -20 to -15°F
50703[15] 5a -20 to -15°F
50702[15] 5a -20 to -15°F
50707[15] 5a -20 to -15°F
50613[15] 5a -20 to -15°F

Questions people ask

What growing zone is Waterloo, Iowa?
The 2023 USDA map puts the center of Waterloo in zone 5a[1]. By land area, 100% of the city lies in ZIP code areas rated 5a[15].
When is the last frost in Waterloo?
The median last spring frost is April 27, so in half of years the last frost comes after that date[2]. In 1 of 10 years it comes after May 10[2].
When is the first frost in Waterloo?
The median first fall frost is October 10, so in half of years the first frost comes before that date[3]. In 1 of 10 years it comes before September 26[3].
When to plant tomatoes in Waterloo?
CSU Extension lists tomatoes as very tender and says to start typically 2 or more weeks after the average last frost[11]. Using the median last frost, the earliest start is May 11[11].
How long is the growing season in Waterloo?
The season between the last spring and first fall frost is at least 165 days in half of years[4]. It is at least 146 days in 9 of 10 years[4].

Facts and sources

  1. The 2023 USDA Plant Hardiness Zone Map puts the center of Waterloo, Iowa in zone 5a. Zone 5a means an average annual extreme minimum temperature of -20°F to -15°F over 1991 to 2020. The map's modeled value at the city center is minus 17.4°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 11, in half of years after April 27, and in 1 of 10 years after May 10 (NOAA 1991 to 2020 normals). The median last frost date is April 27. NOAA Climate Normals 1991-2020
  3. First fall frost (32°F) at this station: in 1 of 10 years the first frost comes before September 26, in half of years before October 10, and in 9 of 10 years before October 21 (NOAA 1991 to 2020 normals). The median first frost date is October 10. NOAA Climate Normals 1991-2020
  4. Growing season (days between the last spring and first fall 32°F frost) at this station: at least 146 days in 9 of 10 years, at least 165 days in half of years, and at least 184 days in 1 of 10 years. NOAA Climate Normals 1991-2020
  5. Frost dates on this page come from NOAA climate normals for the Waterloo Municipal Airport weather station (USW00094910), 5 miles from the center of Waterloo at 868 feet, using 30 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 16 and the median first fall date is October 20. 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. Waterloo, Iowa overlaps 5 ZIP code areas. By land area, 100% lies in ZIP code areas rated zone 5a. 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.