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Frost dates and planting calendar for Caldwell, Idaho

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

USDA hardiness zone
7a[1]
Median last spring frost
Apr 29[2]
Median first fall frost
Oct 13[3]
Growing season, median
164 days[4]

Caldwell, Idaho is in USDA hardiness zone 7a at the city center[1]. The median last spring frost is April 29[2].

Frost dates

JanFebMarAprMayJunJulAugSepOctNovDecLast spring frost: 9 in 10 years after Apr 15, half of years Apr 29, 1 in 10 years after May 18Last frostmedian Apr 29First fall frost: 9 in 10 years before Oct 28, half of years Oct 13, 1 in 10 years before Sep 29First frostmedian Oct 13Frost riskHardyHardy vegetables: plant Apr 1 to Apr 15Apr 1 to Apr 15Semi-hardySemi-hardy vegetables: plant Apr 15 to Apr 29Apr 15 to Apr 29TenderTender vegetables: plant around Apr 29around Apr 29Very tenderVery tender vegetables: plant from May 13from May 13
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.

Frost dates here come from NOAA climate normals for the Nampa Sugar Factory weather station near Caldwell[5]. In half of years the last spring frost comes after April 29, and in 1 of 10 years it comes after May 18[2]. In half of years the first fall frost comes before October 13, and in 1 of 10 years it comes before September 29[3]. These are odds from 1991 to 2020 records, 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 15 after April 29 after May 18
First fall frost (32°F)[3] before September 29 before October 13 before October 28
  • Hard freeze (28°F): median last spring date April 11, median first fall date October 25[7].
  • Frost-free growing season: at least 142 days in 9 of 10 years, 164 days in half of years and 186 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: Nampa Sugar Factory (USC00106305), 5.1 miles from the city center, 2,470 feet, 22 years of 1991 to 2020 records[5].

When to plant vegetables

Colorado State University Extension groups vegetables by how well they handle cold. Its guide was written for Colorado[6]. Hardy vegetables such as broccoli, lettuce, peas and spinach can go in as early as 2 to 4 weeks before the median last frost, which works out to April 1 to April 15 in Caldwell[6]. Semi-hardy vegetables such as carrots, beets and potatoes can go in as early as 2 weeks before the median last frost, which is April 15[9]. Tender vegetables such as snap beans and sweet corn can be seeded around April 29[10]. Very tender vegetables such as tomatoes and peppers typically wait 2 or more weeks after the median last frost, so the earliest start is May 13[11].

  • Hardy (broccoli, cabbage, kohlrabi, lettuce, onions, peas, radishes, spinach, turnips): as early as 2 to 4 weeks before the median last frost (April 29) is April 1 to April 15[6]
  • Semi-hardy (beets, carrots, cauliflower, parsnips, potatoes, Swiss chard): as early as 2 weeks before the median last frost (April 29) is April 15, up to the median date itself[9]
  • Tender (snap beans, sweet corn, cucumbers, summer squash): from seed around the median last frost date, April 29[10]
  • Very tender (cantaloupe, eggplant, peppers, tomatoes, winter squash, watermelon): the earliest start, two weeks after the median last frost (April 29), is May 13[11]
Vegetable (CSU group) Plant from First harvest around
Broccoli (hardy)[12] Apr 1 Jun 5 (from transplants)
Cabbage (hardy)[12] Apr 1 Jun 25 (from transplants)
Kohlrabi (hardy)[12] Apr 1 May 21
Lettuce (hardy)[12] Apr 1 May 31
Onions (hardy)[12] Apr 1 Jul 20
Peas (hardy)[12] Apr 1 Jun 5
Radishes (hardy)[12] Apr 1 May 1
Spinach (hardy)[12] Apr 1 May 11
Turnips (hardy)[12] Apr 1 May 21
Beets (semi-hardy)[12] Apr 15 Jun 14
Carrots (semi-hardy)[12] Apr 15 Jun 24
Cauliflower (semi-hardy)[12] Apr 15 not listed
Parsnips (semi-hardy)[12] Apr 15 Jun 24
Potatoes (semi-hardy)[12] Apr 15 Aug 18
Swiss chard (semi-hardy)[12] Apr 15 Jun 14
Snap beans (tender)[12] Apr 29 Jun 28
Sweet corn (tender)[12] Apr 29 Jun 28
Cucumbers (tender)[12] Apr 29 Jun 23
Summer squash (tender)[12] Apr 29 Jun 18
Cantaloupe (very tender)[12] May 13 Aug 6
Eggplant (very tender)[12] May 13 Jul 12 (from transplants)
Peppers (very tender)[12] May 13 Jul 22 (from transplants)
Tomatoes (very tender)[12] May 13 Jul 17 (from transplants)
Winter squash (very tender)[12] May 13 Aug 21
Watermelon (very tender)[12] May 13 Aug 6
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

The 2023 USDA map puts the center of Caldwell in zone 7a, which means an average coldest night of 0°F to 5°F over 1991 to 2020[1]. A zone describes the average coldest winter night, not summer heat or frost dates[14]. Caldwell overlaps 4 ZIP code areas, and by land area 100% lies in zone 7a[15].

Zone 7a: average annual extreme minimum temperature 0°F to 5°F (1991 to 2020)[1]. Modeled value at the city center: 4.2°F[1].

Share of the city's land area by ZIP code zone: zone 7a 100%[15].

All towns in zone 7a and what zone 7a means

Zone by ZIP code
ZIP code Zone Coldest night range
83605[15] 7a 0 to 5°F
83607[15] 7a 0 to 5°F
83687[15] 7a 0 to 5°F
83651[15] 7a 0 to 5°F

Questions people ask

What growing zone is Caldwell, Idaho?
The center of Caldwell is in USDA zone 7a on the 2023 map[1]. All of the city's land area is in ZIP code areas rated 7a[15].
When is the last frost in Caldwell?
The median last spring frost is April 29 at the Nampa Sugar Factory station[2]. In 1 of 10 years the last frost comes after May 18[2].
When to plant tomatoes in Caldwell?
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 May 13[11].
When is the first frost in Caldwell?
The median first fall frost is October 13[3]. In 1 of 10 years it comes before September 29[3].
How long is the growing season in Caldwell?
The growing season is at least 164 days in half of years[4]. It is at least 142 days in 9 of 10 years[4].

Facts and sources

  1. The 2023 USDA Plant Hardiness Zone Map puts the center of Caldwell, Idaho in zone 7a. Zone 7a means an average annual extreme minimum temperature of 0°F to 5°F over 1991 to 2020. The map's modeled value at the city center is 4.2°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 15, in half of years after April 29, and in 1 of 10 years after May 18 (NOAA 1991 to 2020 normals). The median last frost date is April 29. 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 29, in half of years before October 13, and in 9 of 10 years before October 28 (NOAA 1991 to 2020 normals). The median first frost date is October 13. NOAA Climate Normals 1991-2020
  4. Growing season (days between the last spring and first fall 32°F frost) at this station: at least 142 days in 9 of 10 years, at least 164 days in half of years, and at least 186 days in 1 of 10 years. NOAA Climate Normals 1991-2020
  5. Frost dates on this page come from NOAA climate normals for the Nampa Sugar Factory weather station (USC00106305), 5.1 miles from the center of Caldwell at 2,470 feet, using 22 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 11 and the median first fall date is October 25. 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. Caldwell, Idaho overlaps 4 ZIP code areas. By land area, 100% lies in ZIP code areas rated zone 7a. 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.