USDA Hardiness Zone 5b
Frost Dates for 52361 — Williamsburg, Iowa
NOAA 1991–2020 normals from Williamsburg 1E, IA (780 ft), 1 miles northeast.
Updated
In Williamsburg, the last spring frost (32°F) is around April 28 in half of years and has passed by May 12 in nine years out of ten. The first fall frost comes around October 9, as early as September 24 one year in ten, leaving a typical freeze-free season of 164 days (NOAA, Williamsburg 1E, IA).
- Last spring frost
- April 28
- First fall frost
- October 9
- Growing season
- 164 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 22
- Last snow
- March 25
- Winters with snow
- 100%
- Snow per year
- 28.1"
median, 1991–2020
median, 1991–2020
29 of 29 observed
NOAA normal
Frost risk by date in Williamsburg
Half of springs at Williamsburg 1E, IA see their last 32°F night by April 28; only one in ten is past it by April 14, and one in ten still has one after May 12. Planting tender crops after May 3 leaves a 30% risk of a frost on them, after May 7 20%, and after May 12 10%. The 14 days between the even-odds date and the one-in-ten date is wider than at 58% of the 132 Iowa stations behind this site's ZIP pages.
In fall the first 32°F night comes by October 9 in half of years, as early as September 24 one year in ten, and holds off past October 21 one year in ten. A hard freeze of 28°F follows 10 days later, around October 19; in spring the one-in-ten date for 28°F is May 2, 10 days before the 32°F one. The freeze-free season lasts 164 days in a typical year, shorter than at 52% of the 132 Iowa stations; it reaches 181 days one year in ten and falls short of 145 one year in ten. On average 151 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 7, and return around September 30. Severe cold of 16°F or below comes every winter, usually between November 17 and March 18.
| Low | 90% | 80% | 70% | 60% | 50% | 40% | 30% | 20% | 10% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Apr 24 | Apr 29 | May 2 | May 4 | May 7 | May 9 | May 12 | May 16 | May 21 |
| 32°F | Apr 14 | Apr 18 | Apr 22 | Apr 25 | Apr 28 | May 1 | May 3 | May 7 | May 12 |
| 28°F | Apr 4 | Apr 8 | Apr 12 | Apr 15 | Apr 18 | Apr 21 | Apr 24 | Apr 28 | May 2 |
| 24°F | Mar 23 | Mar 29 | Apr 1 | Apr 4 | Apr 7 | Apr 10 | Apr 13 | Apr 18 | Apr 24 |
| 20°F | Mar 10 | Mar 17 | Mar 21 | Mar 25 | Mar 29 | Mar 31 | Apr 3 | Apr 7 | Apr 13 |
| 16°F | Mar 2 | Mar 7 | Mar 11 | Mar 14 | Mar 18 | Mar 21 | Mar 25 | Mar 29 | Apr 2 |
| Low | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 17 | Sep 21 | Sep 24 | Sep 27 | Sep 30 | Oct 2 | Oct 4 | Oct 7 | Oct 11 |
| 32°F | Sep 24 | Sep 29 | Oct 2 | Oct 6 | Oct 9 | Oct 11 | Oct 14 | Oct 17 | Oct 21 |
| 28°F | Oct 4 | Oct 10 | Oct 14 | Oct 17 | Oct 19 | Oct 21 | Oct 24 | Oct 27 | Oct 31 |
| 24°F | Oct 15 | Oct 20 | Oct 23 | Oct 26 | Oct 28 | Oct 30 | Nov 1 | Nov 4 | Nov 10 |
| 20°F | Oct 26 | Oct 30 | Nov 1 | Nov 4 | Nov 6 | Nov 10 | Nov 13 | Nov 17 | Nov 23 |
| 16°F | Nov 2 | Nov 7 | Nov 11 | Nov 13 | Nov 17 | Nov 20 | Nov 24 | Nov 28 | Dec 4 |
Freeze probabilities 1991–2020 at Williamsburg 1E, 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 52361 →Compare Williamsburg with another place →
Weather by month in Williamsburg
July is the warmest month at Williamsburg 1E, IA, averaging 84°F by day and 63°F at night; January is the coldest at 28°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 40% of years.
Warmth for growing adds up to 2,981 growing degree days a year (base 50°F), fewer than at 54% of the 132 Iowa stations. 35% of them (1,044) have built up by the end of June, and July adds the most, 722. Precipitation totals 37.8 inches a year, most in May (4.9") and least in January (1.2"); May to September bring 59% of it.
| Month | High °F | Low °F | Precip. in | 32°F night | Nights ≤ 32°F | GDD 50°F |
|---|---|---|---|---|---|---|
| Jan | 28 | 10 | 1.2 | 100% | 30 | 0 |
| Feb | 33 | 14 | 1.5 | 100% | 26 | 1 |
| Mar | 47 | 26 | 2.2 | 100% | 23 | 22 |
| Apr | 60 | 36 | 3.9 | 99% | 10 | 94 |
| May | 71 | 49 | 4.9 | 40% | 1 | 329 |
| Jun | 81 | 59 | 4.8 | 0% | 0 | 598 |
| Jul | 84 | 63 | 4.3 | 0% | 0 | 722 |
| Aug | 82 | 60 | 4.5 | 0% | 0 | 648 |
| Sep | 76 | 51 | 3.8 | 24% | <1 | 411 |
| Oct | 63 | 39 | 2.8 | 99% | 9 | 137 |
| Nov | 47 | 27 | 2.2 | 100% | 22 | 18 |
| Dec | 34 | 16 | 1.7 | 100% | 29 | 1 |
Monthly normals 1991–2020 at Williamsburg 1E, IA (780 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, Belle Plaine, IA, 21 miles northwest of Williamsburg, has the same 50% last-frost date, a first frost 1 day later (October 10), a season 1 day longer.
When it snows in Williamsburg, Iowa
The first measurable snow (0.1 inch or more) in Williamsburg usually arrives around November 22, and the last of the season around March 25. Those are the middle dates across the winters of 1991–2020 at Williamsburg 1E, IA, 1.2 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 34 days after the last snow.
Planting calendar for 52361
| When | What to do |
|---|---|
| February 17 – March 3 | Start peppers and aubergines indoorsThey need eight to ten weeks under lights before they can go out. |
| March 3 – March 17 | Start tomatoes indoorsSix to eight weeks gives a transplant big enough to set fruit early. |
| March 31 – April 14 | Sow peas, spinach, lettuce, radish outdoorsThese tolerate light frost and prefer to germinate in cool soil. |
| April 28 onward | Transplant tomatoes, peppers, basil, squash, cucumbersThe average last freeze has passed — still watch the actual forecast. |
| by July 17 | Sow the autumn crop of carrots, beets and brassicasThey need about twelve weeks to size up before growth stops. |
| by September 25 | Harvest tender crops, or be ready to cover themA first freeze two weeks early is well within normal variation. |
| October 9 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 52361 is 1 mile from Williamsburg 1E, IA, which stands at 780 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 52361 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.