USDA Hardiness Zone 5a
Frost Dates for 50616 — Charles City, Iowa
NOAA 1991–2020 normals from Charles City, IA (993 ft), under a mile south.
Updated
In Charles City, the last spring frost (32°F) is around April 30 in half of years and has passed by May 14 in nine years out of ten. The first fall frost comes around October 6, as early as September 21 one year in ten, leaving a typical freeze-free season of 157 days (NOAA, Charles City, IA).
- Last spring frost
- April 30
- First fall frost
- October 6
- Growing season
- 157 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 13
- Last snow
- April 10
- Winters with snow
- 100%
- Snow per year
- 37.5"
median, 1991–2020
median, 1991–2020
30 of 30 observed
NOAA normal
Frost risk by date in Charles City
Half of springs at Charles City, IA see their last 32°F night by April 30; only one in ten is past it by April 15, and one in ten still has one after May 14. Planting tender crops after May 6 leaves a 30% risk of a frost on them, after May 10 20%, and after May 14 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 6 in half of years, as early as September 21 one year in ten, and holds off past October 19 one year in ten. A hard freeze of 28°F follows 11 days later, around October 17; in spring the one-in-ten date for 28°F is May 6, 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 140 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 27. Severe cold of 16°F or below comes every winter, usually between November 14 and March 21.
| Low | 90% | 80% | 70% | 60% | 50% | 40% | 30% | 20% | 10% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Apr 27 | May 1 | May 4 | May 7 | May 9 | May 12 | May 15 | May 18 | May 22 |
| 32°F | Apr 15 | Apr 20 | Apr 24 | Apr 28 | Apr 30 | May 3 | May 6 | May 10 | May 14 |
| 28°F | Apr 5 | Apr 10 | Apr 14 | Apr 17 | Apr 21 | Apr 24 | Apr 28 | May 1 | May 6 |
| 24°F | Mar 26 | Mar 31 | Apr 3 | Apr 7 | Apr 9 | Apr 13 | Apr 17 | Apr 22 | Apr 29 |
| 20°F | Mar 13 | Mar 20 | Mar 24 | Mar 28 | Mar 31 | Apr 2 | Apr 5 | Apr 10 | Apr 16 |
| 16°F | Mar 5 | Mar 10 | Mar 13 | Mar 18 | Mar 21 | Mar 25 | Mar 29 | Apr 2 | Apr 8 |
| Low | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 15 | Sep 19 | Sep 22 | Sep 25 | Sep 27 | Sep 30 | Oct 3 | Oct 5 | Oct 9 |
| 32°F | Sep 21 | Sep 26 | Oct 1 | Oct 3 | Oct 6 | Oct 8 | Oct 11 | Oct 15 | Oct 19 |
| 28°F | Oct 3 | Oct 7 | Oct 10 | Oct 14 | Oct 17 | Oct 19 | Oct 22 | Oct 25 | Oct 29 |
| 24°F | Oct 12 | Oct 19 | Oct 21 | Oct 24 | Oct 27 | Oct 29 | Oct 31 | Nov 4 | Nov 9 |
| 20°F | Oct 24 | Oct 29 | Oct 31 | Nov 2 | Nov 5 | Nov 7 | Nov 10 | Nov 13 | Nov 18 |
| 16°F | Nov 2 | Nov 5 | Nov 8 | Nov 11 | Nov 14 | Nov 16 | Nov 20 | Nov 24 | Nov 29 |
Freeze probabilities 1991–2020 at Charles 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 50616 →Compare Charles City with another place →
Frost risk through the year in Charles City
In Charles City, the spring curve drops under 10% on May 14 and the fall curve passes 10% on September 21, 130 days later; that gap is shorter than at 82% of the 132 Iowa stations with frost dates. The spring risk falls from 90% to 10% over 29 days (April 15 to May 14) and the fall risk climbs from 10% to 90% over 28 days (September 21 to October 19): both ends of the season vary about as much. At Osage, IA, 16 miles northwest, the same gap runs from May 12 to September 25, 6 days longer.
| Planted on | Another 32°F night | 28°F night |
|---|---|---|
| Apr 16 | 88% | 63% |
| Apr 23 | 73% | 43% |
| Apr 3050% date | 50% | 23% |
| May 7 | 28% | under 10% |
| May 14 | under 10% | under 10% |
| May 21 | under 10% | under 10% |
Weather by month in Charles City
July is the warmest month at Charles City, IA, averaging 82°F by day and 62°F at night; January is the coldest at 25°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 50% of years.
Warmth for growing adds up to 2,754 growing degree days a year (base 50°F), fewer than at 81% of the 132 Iowa stations. 35% of them (956) have built up by the end of June, and July adds the most, 683. Precipitation totals 37.6 inches a year, most in June (6.1") and least in January (1.0"); May to September bring 64% of it.
| Month | High °F | Low °F | Precip. in | 32°F night | Nights ≤ 32°F | GDD 50°F |
|---|---|---|---|---|---|---|
| Jan | 25 | 9 | 1.0 | 100% | 31 | 0 |
| Feb | 30 | 13 | 1.1 | 100% | 27 | 0 |
| Mar | 43 | 25 | 1.9 | 100% | 24 | 15 |
| Apr | 58 | 37 | 3.7 | 100% | 10 | 82 |
| May | 70 | 49 | 5.1 | 50% | 1 | 298 |
| Jun | 79 | 59 | 6.1 | 0% | 0 | 561 |
| Jul | 82 | 62 | 4.9 | 0% | 0 | 683 |
| Aug | 80 | 60 | 4.3 | 0% | 0 | 609 |
| Sep | 73 | 51 | 3.8 | 30% | 1 | 380 |
| Oct | 60 | 39 | 2.6 | 99% | 9 | 114 |
| Nov | 44 | 26 | 1.7 | 100% | 22 | 12 |
| Dec | 30 | 15 | 1.4 | 100% | 30 | 0 |
Monthly normals 1991–2020 at Charles City, IA (993 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, Osage, IA, 16 miles northwest of Charles City and 177 feet higher, has the same 50% last-frost date, a first frost 3 days later (October 9), a season 6 days longer.
When it snows in Charles City, Iowa
The first measurable snow (0.1 inch or more) in Charles City usually arrives around November 13, and the last of the season around April 10. Those are the middle dates across the winters of 1991–2020 at Charles City, IA, 0.9 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 20 days after the last snow.
Planting calendar for 50616
| 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 14 | Sow the autumn crop of carrots, beets and brassicasThey need about twelve weeks to size up before growth stops. |
| by September 22 | Harvest tender crops, or be ready to cover themA first freeze two weeks early is well within normal variation. |
| October 6 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 50616 is under a mile from Charles City, IA, which stands at 993 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 50616 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.