USDA Hardiness Zone 5a
Frost Dates for 51239 — Hull, Iowa
NOAA 1991–2020 normals from Sioux Ctr 2 SE, IA (1,360 ft), 9 miles south.
Updated
In Hull, the last spring frost (32°F) is around April 30 in half of years and has passed by May 12 in nine years out of ten. The first fall frost comes around October 3, as early as September 19 one year in ten, leaving a typical freeze-free season of 154 days (NOAA, Sioux Ctr 2 SE, IA).
- Last spring frost
- April 30
- First fall frost
- October 3
- Growing season
- 154 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 7
- Last snow
- April 8
- Winters with snow
- 100%
- Snow per year
- 34.8"
median, 1991–2020
median, 1991–2020
29 of 29 observed
NOAA normal
Frost risk by date in Hull
Half of springs at Sioux Ctr 2 SE, IA see their last 32°F night by April 30; only one in ten is past it by April 17, and one in ten still has one after May 12. Planting tender crops after May 5 leaves a 30% risk of a frost on them, after May 8 20%, and after May 12 10%. The 12 days between the even-odds date and the one-in-ten date is narrower than at 67% of the 132 Iowa stations behind this site's ZIP pages.
In fall the first 32°F night comes by October 3 in half of years, as early as September 19 one year in ten, and holds off past October 18 one year in ten. A hard freeze of 28°F follows 11 days later, around October 14; in spring the one-in-ten date for 28°F is May 4, 8 days before the 32°F one. The freeze-free season lasts 154 days in a typical year, shorter than at 84% of the 132 Iowa stations; it reaches 174 days one year in ten and falls short of 135 one year in ten. On average 154 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 9 days longer, to about May 9, and return around September 24. Severe cold of 16°F or below comes every winter, usually between November 10 and March 23.
| Low | 90% | 80% | 70% | 60% | 50% | 40% | 30% | 20% | 10% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Apr 28 | May 1 | May 4 | May 7 | May 9 | May 11 | May 13 | May 17 | May 21 |
| 32°F | Apr 17 | Apr 22 | Apr 25 | Apr 28 | Apr 30 | May 2 | May 5 | May 8 | May 12 |
| 28°F | Apr 7 | Apr 11 | Apr 15 | Apr 18 | Apr 21 | Apr 23 | Apr 27 | Apr 30 | May 4 |
| 24°F | Mar 29 | Apr 2 | Apr 5 | Apr 8 | Apr 11 | Apr 14 | Apr 17 | Apr 21 | Apr 25 |
| 20°F | Mar 16 | Mar 23 | Mar 29 | Mar 31 | Apr 2 | Apr 5 | Apr 8 | Apr 12 | Apr 18 |
| 16°F | Mar 7 | Mar 12 | Mar 16 | Mar 20 | Mar 23 | Mar 28 | Mar 31 | Apr 4 | Apr 10 |
| Low | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 11 | Sep 16 | Sep 19 | Sep 22 | Sep 24 | Sep 28 | Oct 1 | Oct 4 | Oct 7 |
| 32°F | Sep 19 | Sep 24 | Sep 28 | Oct 1 | Oct 3 | Oct 5 | Oct 9 | Oct 13 | Oct 18 |
| 28°F | Sep 29 | Oct 3 | Oct 7 | Oct 11 | Oct 14 | Oct 17 | Oct 20 | Oct 23 | Oct 27 |
| 24°F | Oct 8 | Oct 14 | Oct 19 | Oct 22 | Oct 24 | Oct 27 | Oct 30 | Nov 1 | Nov 6 |
| 20°F | Oct 20 | Oct 25 | Oct 29 | Oct 31 | Nov 2 | Nov 5 | Nov 7 | Nov 11 | Nov 16 |
| 16°F | Oct 29 | Nov 1 | Nov 4 | Nov 7 | Nov 10 | Nov 13 | Nov 17 | Nov 21 | Nov 26 |
Freeze probabilities 1991–2020 at Sioux Ctr 2 SE, 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 51239 →Compare Hull with another place →
Weather by month in Hull
July is the warmest month at Sioux Ctr 2 SE, IA, averaging 85°F by day and 62°F at night; January is the coldest at 27°F and 9°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 47% of years.
Warmth for growing adds up to 3,058 growing degree days a year (base 50°F), more than at 56% of the 132 Iowa stations. 36% of them (1,097) have built up by the end of June, and July adds the most, 722. Precipitation totals 30.0 inches a year, most in June (4.5") and least in January (0.7"); May to September bring 63% of it.
| Month | High °F | Low °F | Precip. in | 32°F night | Nights ≤ 32°F | GDD 50°F |
|---|---|---|---|---|---|---|
| Jan | 27 | 9 | 0.7 | 100% | 31 | 0 |
| Feb | 33 | 14 | 0.9 | 100% | 27 | 1 |
| Mar | 46 | 25 | 1.7 | 100% | 24 | 21 |
| Apr | 61 | 36 | 3.0 | 100% | 11 | 107 |
| May | 73 | 48 | 4.2 | 47% | 1 | 350 |
| Jun | 82 | 59 | 4.5 | 0% | 0 | 619 |
| Jul | 85 | 62 | 3.5 | 0% | 0 | 722 |
| Aug | 82 | 60 | 3.4 | 0% | 0 | 648 |
| Sep | 77 | 51 | 3.2 | 38% | 1 | 430 |
| Oct | 64 | 39 | 2.5 | 100% | 9 | 147 |
| Nov | 46 | 25 | 1.4 | 100% | 22 | 14 |
| Dec | 32 | 15 | 1.0 | 100% | 30 | 0 |
Monthly normals 1991–2020 at Sioux Ctr 2 SE, IA (1,360 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, Sheldon, IA, 15 miles east of Hull, has a last frost 5 days later (May 5), a first frost 5 days earlier (September 28), a season 9 days shorter.
When it snows in Hull, Iowa
The first measurable snow (0.1 inch or more) in Hull usually arrives around November 7, and the last of the season around April 8. Those are the middle dates across the winters of 1991–2020 at Sioux Ctr 2 SE, IA, 9.4 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 22 days after the last snow.
Planting calendar for 51239
| When | What to do |
|---|---|
| February 19 – March 5 | Start peppers and aubergines indoorsThey need eight to ten weeks under lights before they can go out. |
| March 5 – March 19 | Start tomatoes indoorsSix to eight weeks gives a transplant big enough to set fruit early. |
| April 2 – April 16 | Sow peas, spinach, lettuce, radish outdoorsThese tolerate light frost and prefer to germinate in cool soil. |
| April 30 onward | Transplant tomatoes, peppers, basil, squash, cucumbersThe average last freeze has passed — still watch the actual forecast. |
| by July 11 | Sow the autumn crop of carrots, beets and brassicasThey need about twelve weeks to size up before growth stops. |
| by September 19 | Harvest tender crops, or be ready to cover themA first freeze two weeks early is well within normal variation. |
| October 3 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 51239 is 9 miles from Sioux Ctr 2 SE, IA, which stands at 1,360 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 3 other ZIP codes share the station; its figures are presented in full for 51250 (Sioux Center). The 2023 USDA Plant Hardiness Zone Map lists ZIP 51239 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.