USDA Hardiness Zone 5b
Frost Dates for 50240 — Saint Charles, Iowa
NOAA 1991–2020 normals from Indianola 2W, IA (924 ft), 9 miles northeast.
Updated
In Saint Charles, the last spring frost (32°F) is around April 24 in half of years and has passed by May 8 in nine years out of ten. The first fall frost comes around October 12, as early as September 27 one year in ten, leaving a typical freeze-free season of 168 days (NOAA, Indianola 2W, IA).
- Last spring frost
- April 24
- First fall frost
- October 12
- Growing season
- 168 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 20
- Last snow
- March 27
- Winters with snow
- 100%
- Snow per year
- 30.8"
median, 1991–2020
median, 1991–2020
30 of 30 observed
NOAA normal
Frost risk by date in Saint Charles
Half of springs at Indianola 2W, IA see their last 32°F night by April 24; only one in ten is past it by April 11, and one in ten still has one after May 8. Planting tender crops after May 1 leaves a 30% risk of a frost on them, after May 4 20%, and after May 8 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 12 in half of years, as early as September 27 one year in ten, and holds off past October 26 one year in ten. A hard freeze of 28°F follows 9 days later, around October 21; in spring the one-in-ten date for 28°F is April 29, 9 days before the 32°F one. The freeze-free season lasts 168 days in a typical year, longer than at 62% of the 132 Iowa stations; it reaches 188 days one year in ten and falls short of 147 one year in ten. On average 143 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 10 days longer, to about May 4, and return around October 2. Severe cold of 16°F or below comes every winter, usually between November 18 and March 15.
| Low | 90% | 80% | 70% | 60% | 50% | 40% | 30% | 20% | 10% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Apr 20 | Apr 25 | Apr 30 | May 2 | May 4 | May 6 | May 9 | May 13 | May 19 |
| 32°F | Apr 11 | Apr 15 | Apr 18 | Apr 21 | Apr 24 | Apr 28 | May 1 | May 4 | May 8 |
| 28°F | Apr 1 | Apr 5 | Apr 8 | Apr 12 | Apr 14 | Apr 17 | Apr 20 | Apr 23 | Apr 29 |
| 24°F | Mar 19 | Mar 25 | Mar 30 | Apr 1 | Apr 4 | Apr 7 | Apr 10 | Apr 14 | Apr 20 |
| 20°F | Mar 9 | Mar 13 | Mar 18 | Mar 22 | Mar 26 | Mar 30 | Apr 1 | Apr 5 | Apr 11 |
| 16°F | Feb 28 | Mar 5 | Mar 9 | Mar 13 | Mar 15 | Mar 19 | Mar 23 | Mar 28 | Apr 1 |
| Low | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 19 | Sep 23 | Sep 27 | Sep 30 | Oct 2 | Oct 5 | Oct 7 | Oct 11 | Oct 15 |
| 32°F | Sep 27 | Oct 1 | Oct 6 | Oct 9 | Oct 12 | Oct 14 | Oct 18 | Oct 21 | Oct 26 |
| 28°F | Oct 6 | Oct 12 | Oct 16 | Oct 19 | Oct 21 | Oct 24 | Oct 27 | Oct 30 | Nov 4 |
| 24°F | Oct 18 | Oct 23 | Oct 26 | Oct 29 | Nov 1 | Nov 2 | Nov 5 | Nov 9 | Nov 14 |
| 20°F | Oct 28 | Nov 1 | Nov 3 | Nov 6 | Nov 9 | Nov 12 | Nov 16 | Nov 20 | Nov 27 |
| 16°F | Nov 4 | Nov 8 | Nov 11 | Nov 14 | Nov 18 | Nov 21 | Nov 25 | Nov 29 | Dec 4 |
Freeze probabilities 1991–2020 at Indianola 2W, 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 50240 →Compare Saint Charles with another place →
Weather by month in Saint Charles
July is the warmest month at Indianola 2W, IA, averaging 86°F by day and 64°F at night; January is the coldest at 32°F and 12°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 30% of years.
Warmth for growing adds up to 3,252 growing degree days a year (base 50°F), more than at 80% of the 132 Iowa stations. 35% of them (1,129) have built up by the end of June, and July adds the most, 767. Precipitation totals 37.8 inches a year, most in June (5.7") and least in January (0.9"); May to September bring 62% of it.
| Month | High °F | Low °F | Precip. in | 32°F night | Nights ≤ 32°F | GDD 50°F |
|---|---|---|---|---|---|---|
| Jan | 32 | 12 | 0.9 | 100% | 30 | 0 |
| Feb | 36 | 15 | 1.4 | 100% | 26 | 2 |
| Mar | 50 | 27 | 1.9 | 100% | 22 | 34 |
| Apr | 62 | 38 | 3.9 | 99% | 9 | 121 |
| May | 72 | 50 | 5.6 | 30% | 1 | 350 |
| Jun | 81 | 60 | 5.7 | 0% | 0 | 621 |
| Jul | 86 | 64 | 4.3 | 0% | 0 | 767 |
| Aug | 84 | 62 | 3.9 | 0% | 0 | 707 |
| Sep | 78 | 52 | 3.9 | 17% | <1 | 454 |
| Oct | 65 | 40 | 3.0 | 96% | 7 | 168 |
| Nov | 50 | 28 | 1.9 | 100% | 20 | 27 |
| Dec | 37 | 17 | 1.4 | 100% | 29 | 1 |
Monthly normals 1991–2020 at Indianola 2W, IA (924 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, Winterset 1S, IA, 11 miles west of Saint Charles and 176 feet higher, has a last frost 6 days later (April 30), a first frost 9 days earlier (October 3), a season 11 days shorter.
When it snows in Saint Charles, Iowa
The first measurable snow (0.1 inch or more) in Saint Charles usually arrives around November 20, and the last of the season around March 27. Those are the middle dates across the winters of 1991–2020 at Indianola 2W, IA, 8.9 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 28 days after the last snow.
Planting calendar for 50240
| When | What to do |
|---|---|
| February 13 – February 27 | Start peppers and aubergines indoorsThey need eight to ten weeks under lights before they can go out. |
| February 27 – March 13 | Start tomatoes indoorsSix to eight weeks gives a transplant big enough to set fruit early. |
| March 27 – April 10 | Sow peas, spinach, lettuce, radish outdoorsThese tolerate light frost and prefer to germinate in cool soil. |
| April 24 onward | Transplant tomatoes, peppers, basil, squash, cucumbersThe average last freeze has passed — still watch the actual forecast. |
| by July 20 | Sow the autumn crop of carrots, beets and brassicasThey need about twelve weeks to size up before growth stops. |
| by September 28 | Harvest tender crops, or be ready to cover themA first freeze two weeks early is well within normal variation. |
| October 12 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 50240 is 9 miles from Indianola 2W, IA, which stands at 924 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 2 other ZIP codes share the station; its figures are presented in full for 50125 (Indianola). The 2023 USDA Plant Hardiness Zone Map lists ZIP 50240 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.