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Frost dates and planting calendar for Portland, Maine

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 26[2]
Median first fall frost
Oct 15[3]
Growing season, median
170 days[4]

The USDA map puts the center of Portland, Maine in zone 6a[1]. The median last spring frost here is April 26, so in half of years the last frost comes after that date[2].

Frost dates

JanFebMarAprMayJunJulAugSepOctNovDecLast spring frost: 9 in 10 years after Apr 16, half of years Apr 26, 1 in 10 years after May 7Last frostmedian Apr 26First fall frost: 9 in 10 years before Oct 30, half of years Oct 15, 1 in 10 years before Oct 2First frostmedian Oct 15Frost riskHardyHardy vegetables: plant Mar 29 to Apr 12Mar 29 to Apr 12Semi-hardySemi-hardy vegetables: plant Apr 12 to Apr 26Apr 12 to Apr 26TenderTender vegetables: plant around Apr 26around Apr 26Very tenderVery tender vegetables: plant from May 10from May 10
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 are odds, not promises. They come from NOAA climate normals for the Portland International Jetport weather station, using records from 1991 to 2020[5]. In 9 of 10 years the last spring frost comes after April 16, and in 1 of 10 years it comes after May 7[2]. In the fall, the median first frost is October 15, and in 1 of 10 years the first frost comes before October 2[3]. Any single 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 16 after April 26 after May 7
First fall frost (32°F)[3] before October 2 before October 15 before October 30
  • Hard freeze (28°F): median last spring date April 14, median first fall date October 27[7].
  • Frost-free growing season: at least 153 days in 9 of 10 years, 170 days in half of years and 188 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: Portland International Jetport (USW00014764), 5.9 miles from the city center, 45 feet, 30 years of 1991 to 2020 records[5].

When to plant vegetables

Colorado State University Extension wrote its planting guide for Colorado, so these dates are simple arithmetic and not advice from CSU[6]. Hardy vegetables such as peas, spinach and lettuce can go in as early as 2 to 4 weeks before the median last frost, which works out to March 29 to April 12[6]. Tender vegetables such as snap beans and sweet corn can be seeded around the median last frost date, April 26[9]. 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 10[10].

  • Hardy (broccoli, cabbage, kohlrabi, lettuce, onions, peas, radishes, spinach, turnips): as early as 2 to 4 weeks before the median last frost (April 26) is March 29 to April 12[6]
  • Semi-hardy (beets, carrots, cauliflower, parsnips, potatoes, Swiss chard): as early as 2 weeks before the median last frost (April 26) is April 12, up to the median date itself[11]
  • Tender (snap beans, sweet corn, cucumbers, summer squash): from seed around the median last frost date, April 26[9]
  • Very tender (cantaloupe, eggplant, peppers, tomatoes, winter squash, watermelon): the earliest start, two weeks after the median last frost (April 26), is May 10[10]
Vegetable (CSU group) Plant from First harvest around
Broccoli (hardy)[12] Mar 29 Jun 2 (from transplants)
Cabbage (hardy)[12] Mar 29 Jun 22 (from transplants)
Kohlrabi (hardy)[12] Mar 29 May 18
Lettuce (hardy)[12] Mar 29 May 28
Onions (hardy)[12] Mar 29 Jul 17
Peas (hardy)[12] Mar 29 Jun 2
Radishes (hardy)[12] Mar 29 Apr 28
Spinach (hardy)[12] Mar 29 May 8
Turnips (hardy)[12] Mar 29 May 18
Beets (semi-hardy)[12] Apr 12 Jun 11
Carrots (semi-hardy)[12] Apr 12 Jun 21
Cauliflower (semi-hardy)[12] Apr 12 not listed
Parsnips (semi-hardy)[12] Apr 12 Jun 21
Potatoes (semi-hardy)[12] Apr 12 Aug 15
Swiss chard (semi-hardy)[12] Apr 12 Jun 11
Snap beans (tender)[12] Apr 26 Jun 25
Sweet corn (tender)[12] Apr 26 Jun 25
Cucumbers (tender)[12] Apr 26 Jun 20
Summer squash (tender)[12] Apr 26 Jun 15
Cantaloupe (very tender)[12] May 10 Aug 3
Eggplant (very tender)[12] May 10 Jul 9 (from transplants)
Peppers (very tender)[12] May 10 Jul 19 (from transplants)
Tomatoes (very tender)[12] May 10 Jul 14 (from transplants)
Winter squash (very tender)[12] May 10 Aug 18
Watermelon (very tender)[12] May 10 Aug 3
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 the winter averages -10°F to -5°F, using 1991 to 2020 records[1]. The zone is not about 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: -5.8°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
04103[15] 6a -10 to -5°F
04102[15] 6a -10 to -5°F
04101[15] 6a -10 to -5°F
04109[15] 6a -10 to -5°F
04108[15] 6a -10 to -5°F
04019[15] 6a -10 to -5°F

Questions people ask

What growing zone is Portland, Maine?
The 2023 USDA map puts the center of Portland in zone 6a[1]. By land area, 100% of the city lies in ZIP code areas rated 6a[15].
When is the last frost in Portland, Maine?
The median last spring frost at the Portland Jetport station is April 26, so in half of years the last frost comes after that date[2]. In 1 of 10 years it comes after May 7[2].
When to plant tomatoes in Portland, Maine?
CSU Extension says very tender vegetables like tomatoes typically wait 2 or more weeks after the average last spring frost[10]. Using the median last frost, the earliest start is May 10[10].
When is the first frost in Portland, Maine?
The median first fall frost is October 15, so in half of years the first frost comes before that date[3]. In 1 of 10 years it comes before October 2[3].
How long is the growing season in Portland, Maine?
The growing season is at least 170 days in half of years[4]. It is at least 153 days in 9 of 10 years[4].

Facts and sources

  1. The 2023 USDA Plant Hardiness Zone Map puts the center of Portland, Maine 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 5.8°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 16, in half of years after April 26, and in 1 of 10 years after May 7 (NOAA 1991 to 2020 normals). The median last frost date is April 26. 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 2, in half of years before October 15, and in 9 of 10 years before October 30 (NOAA 1991 to 2020 normals). The median first frost date is October 15. NOAA Climate Normals 1991-2020
  4. Growing season (days between the last spring and first fall 32°F frost) at this station: at least 153 days in 9 of 10 years, at least 170 days in half of years, and at least 188 days in 1 of 10 years. NOAA Climate Normals 1991-2020
  5. Frost dates on this page come from NOAA climate normals for the Portland International Jetport weather station (USW00014764), 5.9 miles from the center of Portland at 45 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 14 and the median first fall date is October 27. 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 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
  10. 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
  11. 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
  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. Portland, Maine overlaps 6 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.