USDA Hardiness Zone 5a
Frost Dates for 50401 — Mason City, Iowa
NOAA 1991–2020 normals from Mason City, IA (1,105 ft), under a mile north.
Updated
In Mason City, the last spring frost (32°F) is around April 29 in half of years and has passed by May 9 in nine years out of ten. The first fall frost comes around October 7, as early as September 25 one year in ten, leaving a typical freeze-free season of 161 days (NOAA, Mason City, IA).
- Last spring frost
- April 29
- First fall frost
- October 7
- Growing season
- 161 days
- Hardiness zone
- 5a
50% probability
50% probability
freeze-free, 50%
-20 to -15°F (-28.9 to -26.1°C) coldest night
- First snow
- November 27
- Last snow
- March 31
- Winters with snow
- 100%
- Snow per year
- —
median, 1991–2020
median, 1991–2020
30 of 30 observed
NOAA normal
Frost risk by date in Mason City
Half of springs at Mason City, IA see their last 32°F night by April 29; only one in ten is past it by April 14, and one in ten still has one after May 9. Planting tender crops after May 3 leaves a 30% risk of a frost on them, after May 6 20%, and after May 9 10%. The 10 days between the even-odds date and the one-in-ten date is the narrowest of the 132 Iowa stations behind this site's ZIP pages, so the end of frost is fairly predictable.
In fall the first 32°F night comes by October 7 in half of years, as early as September 25 one year in ten, and holds off past October 21 one year in ten. A hard freeze of 28°F follows 11 days later, around October 18; in spring the one-in-ten date for 28°F is April 30, 9 days before the 32°F one. The freeze-free season lasts 161 days in a typical year, shorter than at 67% of the 132 Iowa stations; it reaches 180 days one year in ten and falls short of 143 one year in ten. On average 161 nights a year drop to 32°F or below. Nights of 36°F, when frost can still settle on leaves in low, open spots, continue 8 days longer, to about May 7, and return around September 30. Severe cold of 16°F or below comes every winter, usually between November 15 and March 22.
| Low | 90% | 80% | 70% | 60% | 50% | 40% | 30% | 20% | 10% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Apr 25 | Apr 30 | May 3 | May 5 | May 7 | May 10 | May 12 | May 15 | May 19 |
| 32°F | Apr 14 | Apr 19 | Apr 22 | Apr 25 | Apr 29 | Apr 30 | May 3 | May 6 | May 9 |
| 28°F | Apr 3 | Apr 8 | Apr 11 | Apr 14 | Apr 18 | Apr 21 | Apr 23 | Apr 27 | Apr 30 |
| 24°F | Mar 24 | Mar 30 | Apr 2 | Apr 5 | Apr 8 | Apr 10 | Apr 14 | Apr 18 | Apr 22 |
| 20°F | Mar 14 | Mar 19 | Mar 25 | Mar 28 | Mar 31 | Apr 2 | Apr 4 | Apr 8 | Apr 14 |
| 16°F | Mar 6 | Mar 11 | Mar 14 | Mar 18 | Mar 22 | Mar 26 | Mar 29 | Apr 1 | Apr 5 |
| Low | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 17 | Sep 22 | Sep 25 | Sep 28 | Sep 30 | Oct 1 | Oct 4 | Oct 7 | Oct 12 |
| 32°F | Sep 25 | Sep 29 | Oct 1 | Oct 4 | Oct 7 | Oct 10 | Oct 13 | Oct 17 | Oct 21 |
| 28°F | Oct 1 | Oct 8 | Oct 12 | Oct 15 | Oct 18 | Oct 20 | Oct 23 | Oct 27 | Oct 31 |
| 24°F | Oct 13 | Oct 19 | Oct 23 | Oct 26 | Oct 29 | Oct 31 | Nov 2 | Nov 5 | Nov 11 |
| 20°F | Oct 25 | Oct 30 | Nov 1 | Nov 3 | Nov 6 | Nov 9 | Nov 12 | Nov 16 | Nov 22 |
| 16°F | Nov 2 | Nov 6 | Nov 9 | Nov 12 | Nov 15 | Nov 18 | Nov 22 | Nov 26 | Dec 1 |
Freeze probabilities 1991–2020 at Mason City, IA, from NOAA NCEI U.S. Climate Normals. Where a temperature is reached in fewer than 99.5% of years, the dates describe only the years that reach it. Not a forecast.
Frost risk on any date in 50401 →Compare Mason City with another place →
Frost risk through the year in Mason City
In Mason City, the spring curve drops under 10% on May 9 and the fall curve passes 10% on September 25, 139 days later; that gap is longer than at 51% of the 132 Iowa stations with frost dates. The spring risk falls from 90% to 10% over 25 days (April 14 to May 9) and the fall risk climbs from 10% to 90% over 26 days (September 25 to October 21): both ends of the season vary about as much. At Mason City Municipal Airport, IA, 6 miles west, the same gap runs from May 15 to September 18, 13 days shorter.
| Planted on | Another 32°F night | 28°F night |
|---|---|---|
| Apr 15 | 88% | 58% |
| Apr 22 | 70% | 35% |
| Apr 2950% date | 50% | 13% |
| May 6 | 20% | under 10% |
| May 13 | under 10% | under 10% |
| May 20 | under 10% | under 10% |
Weather by month in Mason City
July is the warmest month at Mason City, IA, averaging 82°F by day and 62°F at night; January is the coldest at 24°F and 7°F. A freezing night is all but certain in every month from October to April (a 90% chance or more). From June to August the chance of a 32°F night stays under 10% each month. In spring, May still brings one in 39% of years.
Warmth for growing adds up to 2,638 growing degree days a year (base 50°F), fewer than at 90% of the 132 Iowa stations. 34% of them (888) have built up by the end of June, and July adds the most, 673. Precipitation totals 36.4 inches a year, most in June (5.7") and least in January (0.9"); May to September bring 63% of it.
| Month | High °F | Low °F | Precip. in | 32°F night | Nights ≤ 32°F | GDD 50°F |
|---|---|---|---|---|---|---|
| Jan | 24 | 7 | 0.9 | 100% | 31 | 0 |
| Feb | 29 | 11 | 1.1 | 100% | 27 | 0 |
| Mar | 41 | 23 | 2.0 | 100% | 25 | 11 |
| Apr | 56 | 35 | 3.9 | 100% | 13 | 65 |
| May | 69 | 47 | 5.6 | 39% | 1 | 267 |
| Jun | 79 | 58 | 5.7 | 0% | 0 | 545 |
| Jul | 82 | 62 | 4.6 | 0% | 0 | 673 |
| Aug | 80 | 59 | 3.7 | 0% | 0 | 595 |
| Sep | 74 | 50 | 3.2 | 25% | 1 | 364 |
| Oct | 60 | 38 | 2.6 | 99% | 10 | 108 |
| Nov | 43 | 25 | 1.8 | 100% | 23 | 10 |
| Dec | 30 | 14 | 1.3 | 100% | 30 | 0 |
Monthly normals 1991–2020 at Mason City, IA (1,105 ft), NOAA NCEI. "32°F night": chance of at least one in the month. GDD: growing degree days, base 50°F.
Compared with the next station
The nearest other station, Mason City Municipal Airport, IA, 6 miles west of Mason City, has a last frost 4 days later (May 3), a first frost 6 days earlier (October 1), a season 11 days shorter.
When it snows in Mason City, Iowa
The first measurable snow (0.1 inch or more) in Mason City usually arrives around November 27, and the last of the season around March 31. Those are the middle dates across the winters of 1991–2020 at Mason City, IA, 0.7 miles away: half of years saw snow earlier, half later. Every one of those winters saw snow. The last 32°F night of spring typically comes 29 days after the last snow.
Planting calendar for 50401
| When | What to do |
|---|---|
| February 18 – March 4 | Start peppers and aubergines indoorsThey need eight to ten weeks under lights before they can go out. |
| March 4 – March 18 | Start tomatoes indoorsSix to eight weeks gives a transplant big enough to set fruit early. |
| April 1 – April 15 | Sow peas, spinach, lettuce, radish outdoorsThese tolerate light frost and prefer to germinate in cool soil. |
| April 29 onward | Transplant tomatoes, peppers, basil, squash, cucumbersThe average last freeze has passed — still watch the actual forecast. |
| by July 15 | Sow the autumn crop of carrots, beets and brassicasThey need about twelve weeks to size up before growth stops. |
| by September 23 | Harvest tender crops, or be ready to cover themA first freeze two weeks early is well within normal variation. |
| October 7 onward | Plant garlic, mulch beds, lift tender bulbsGarlic wants a cold spell; bare soil loses structure over winter. |
The weeks before and after the 50% dates are general rules of thumb, not NOAA figures; a seed packet gives the timing for each variety.
Where these figures come from
ZIP 50401 is under a mile from Mason City, IA, which stands at 1,105 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 1 other ZIP code shares the station. The 2023 USDA Plant Hardiness Zone Map lists ZIP 50401 in zone 5a, the band where the coldest night of the year averaged -20 to -15°F (-28.9 to -26.1°C) over 1991–2020; a zone describes winter lows, not frost dates.
Hardiness zone and temperature range: USDA Plant Hardiness Zone Map 2023 (USDA-ARS, Oregon State University), ZIP-code listing, unchanged. °C converted from °F by this site.
Thirty-year averages, not a forecast: check the forecast before planting out. How stations are matched and which pages are offered to search engines: methodology.