USDA Hardiness Zone 5b
Frost Dates for 50273 — Winterset, Iowa
NOAA 1991–2020 normals from Winterset 1S, IA (1,100 ft), 2 miles southeast.
Updated
In Winterset, the last spring frost (32°F) is around April 30 in half of years and has passed by May 11 in nine years out of ten. The first fall frost comes around October 3, as early as September 20 one year in ten, leaving a typical freeze-free season of 157 days (NOAA, Winterset 1S, IA).
- Last spring frost
- April 30
- First fall frost
- October 3
- Growing season
- 157 days
- Hardiness zone
- 5b
50% probability
50% probability
freeze-free, 50%
-15 to -10°F (-26.1 to -23.3°C) coldest night
- First snow
- November 16
- Last snow
- March 20
- Winters with snow
- 100%
- Snow per year
- 30.7"
median, 1991–2020
median, 1991–2020
21 of 21 observed
NOAA normal
Frost risk by date in Winterset
Half of springs at Winterset 1S, 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 11. Planting tender crops after May 4 leaves a 30% risk of a frost on them, after May 7 20%, and after May 11 10%. The 11 days between the even-odds date and the one-in-ten date is narrower than at 88% 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 20 one year in ten, and holds off past October 20 one year in ten. A hard freeze of 28°F follows 12 days later, around October 15; in spring the one-in-ten date for 28°F is May 3, 8 days before the 32°F one. The freeze-free season lasts 157 days in a typical year, shorter than at 77% of the 132 Iowa stations; it reaches 178 days one year in ten and falls short of 138 one year in ten. On average 145 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 26. Severe cold of 16°F or below comes every winter, usually between November 12 and March 23.
| Low | 90% | 80% | 70% | 60% | 50% | 40% | 30% | 20% | 10% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Apr 27 | May 1 | May 4 | May 7 | May 9 | May 11 | May 14 | May 18 | May 22 |
| 32°F | Apr 17 | Apr 21 | Apr 24 | Apr 28 | Apr 30 | May 2 | May 4 | May 7 | May 11 |
| 28°F | Apr 6 | Apr 11 | Apr 15 | Apr 17 | Apr 20 | Apr 23 | Apr 26 | Apr 30 | May 3 |
| 24°F | Mar 27 | Apr 1 | Apr 5 | Apr 8 | Apr 10 | Apr 13 | Apr 17 | Apr 20 | Apr 24 |
| 20°F | Mar 14 | Mar 21 | Mar 26 | Mar 31 | Apr 2 | Apr 4 | Apr 7 | Apr 11 | Apr 17 |
| 16°F | Mar 4 | Mar 10 | Mar 14 | Mar 19 | Mar 23 | Mar 27 | Mar 31 | Apr 2 | Apr 8 |
| Low | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 12 | Sep 17 | Sep 20 | Sep 23 | Sep 26 | Sep 29 | Oct 1 | Oct 4 | Oct 9 |
| 32°F | Sep 20 | Sep 25 | Sep 28 | Oct 1 | Oct 3 | Oct 6 | Oct 11 | Oct 14 | Oct 20 |
| 28°F | Sep 29 | Oct 3 | Oct 8 | Oct 12 | Oct 15 | Oct 18 | Oct 22 | Oct 25 | Oct 28 |
| 24°F | Oct 10 | Oct 16 | Oct 20 | Oct 24 | Oct 26 | Oct 29 | Oct 31 | Nov 2 | Nov 7 |
| 20°F | Oct 20 | Oct 26 | Oct 30 | Nov 1 | Nov 3 | Nov 5 | Nov 9 | Nov 13 | Nov 19 |
| 16°F | Oct 30 | Nov 2 | Nov 6 | Nov 9 | Nov 12 | Nov 15 | Nov 19 | Nov 24 | Dec 1 |
Freeze probabilities 1991–2020 at Winterset 1S, 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 50273 →Compare Winterset with another place →
Weather by month in Winterset
July is the warmest month at Winterset 1S, IA, averaging 84°F by day and 62°F at night; January is the coldest at 30°F and 10°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 48% of years.
Warmth for growing adds up to 2,919 growing degree days a year (base 50°F), fewer than at 59% of the 132 Iowa stations. 34% of them (1,000) have built up by the end of June, and July adds the most, 713. Precipitation totals 34.8 inches a year, most in May (5.0") and least in January (0.8"); May to September bring 60% of it.
| Month | High °F | Low °F | Precip. in | 32°F night | Nights ≤ 32°F | GDD 50°F |
|---|---|---|---|---|---|---|
| Jan | 30 | 10 | 0.8 | 100% | 30 | 0 |
| Feb | 35 | 13 | 1.3 | 100% | 25 | 1 |
| Mar | 48 | 25 | 2.1 | 100% | 22 | 27 |
| Apr | 61 | 35 | 3.6 | 100% | 9 | 90 |
| May | 71 | 48 | 5.0 | 48% | 1 | 309 |
| Jun | 80 | 58 | 4.6 | 0% | 0 | 575 |
| Jul | 84 | 62 | 4.1 | 0% | 0 | 713 |
| Aug | 82 | 59 | 3.6 | 0% | 0 | 646 |
| Sep | 76 | 51 | 3.6 | 38% | 1 | 399 |
| Oct | 63 | 38 | 2.9 | 99% | 8 | 141 |
| Nov | 48 | 26 | 2.0 | 100% | 20 | 19 |
| Dec | 35 | 16 | 1.2 | 100% | 29 | 1 |
Monthly normals 1991–2020 at Winterset 1S, IA (1,100 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, Indianola 2W, IA, 20 miles east of Winterset and 176 feet lower, has a last frost 6 days earlier (April 24), a first frost 9 days later (October 12), a season 11 days longer.
When it snows in Winterset, Iowa
The first measurable snow (0.1 inch or more) in Winterset usually arrives around November 16, and the last of the season around March 20. Those are the middle dates across the winters of 1991–2020 at Winterset 1S, IA, 1.6 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 41 days after the last snow.
Planting calendar for 50273
| 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 50273 is 2 miles from Winterset 1S, IA, which stands at 1,100 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 2 other ZIP codes share the station. The 2023 USDA Plant Hardiness Zone Map lists ZIP 50273 in zone 5b, the band where the coldest night of the year averaged -15 to -10°F (-26.1 to -23.3°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.