USDA Hardiness Zone 5b
Frost Dates for 52242 — Iowa City, Iowa
NOAA 1991–2020 normals from Iowa City Municipal Airport, IA (650 ft), 2 miles south.
Updated
In Iowa City, the last spring frost (32°F) is around April 21 in half of years and has passed by May 6 in nine years out of ten. The first fall frost comes around October 18, as early as October 4 one year in ten, leaving a typical freeze-free season of 180 days (NOAA, Iowa City Municipal Airport, IA).
- Last spring frost
- April 21
- First fall frost
- October 18
- Growing season
- 180 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
- December 1
- Last snow
- March 13
- Winters with snow
- 100%
- Snow per year
- 27.5"
median, 1991–2020
median, 1991–2020
29 of 29 observed
NOAA normal
Frost risk by date in Iowa City
Half of springs at Iowa City Municipal Airport, IA see their last 32°F night by April 21; only one in ten is past it by April 5, and one in ten still has one after May 6. Planting tender crops after April 28 leaves a 30% risk of a frost on them, after May 1 20%, and after May 6 10%. The 15 days between the even-odds date and the one-in-ten date is wider than at 82% of the 132 Iowa stations behind this site's ZIP pages.
In fall the first 32°F night comes by October 18 in half of years, as early as October 4 one year in ten, and holds off past October 31 one year in ten. A hard freeze of 28°F follows 10 days later, around October 28; in spring the one-in-ten date for 28°F is April 27, 9 days before the 32°F one. The freeze-free season lasts 180 days in a typical year, longer than at 87% of the 132 Iowa stations; it reaches 201 days one year in ten and falls short of 159 one year in ten. On average 135 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 1, and return around October 9. Severe cold of 16°F or below comes every winter, usually between November 27 and March 9.
| Low | 90% | 80% | 70% | 60% | 50% | 40% | 30% | 20% | 10% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Apr 17 | Apr 22 | Apr 25 | Apr 28 | May 1 | May 3 | May 6 | May 10 | May 15 |
| 32°F | Apr 5 | Apr 10 | Apr 14 | Apr 18 | Apr 21 | Apr 24 | Apr 28 | May 1 | May 6 |
| 28°F | Mar 25 | Mar 31 | Apr 3 | Apr 6 | Apr 9 | Apr 12 | Apr 16 | Apr 21 | Apr 27 |
| 24°F | Mar 11 | Mar 18 | Mar 22 | Mar 27 | Mar 31 | Apr 2 | Apr 5 | Apr 9 | Apr 15 |
| 20°F | Mar 2 | Mar 7 | Mar 10 | Mar 14 | Mar 18 | Mar 21 | Mar 25 | Mar 30 | Apr 3 |
| 16°F | Feb 24 | Feb 28 | Mar 3 | Mar 6 | Mar 9 | Mar 12 | Mar 15 | Mar 19 | Mar 24 |
| Low | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 26 | Oct 1 | Oct 4 | Oct 7 | Oct 9 | Oct 11 | Oct 13 | Oct 17 | Oct 21 |
| 32°F | Oct 4 | Oct 10 | Oct 13 | Oct 16 | Oct 18 | Oct 20 | Oct 23 | Oct 26 | Oct 31 |
| 28°F | Oct 15 | Oct 19 | Oct 23 | Oct 25 | Oct 28 | Oct 30 | Nov 1 | Nov 4 | Nov 9 |
| 24°F | Oct 26 | Oct 30 | Nov 1 | Nov 3 | Nov 6 | Nov 9 | Nov 12 | Nov 16 | Nov 20 |
| 20°F | Nov 3 | Nov 7 | Nov 10 | Nov 14 | Nov 17 | Nov 21 | Nov 24 | Nov 27 | Dec 2 |
| 16°F | Nov 9 | Nov 15 | Nov 20 | Nov 24 | Nov 27 | Nov 30 | Dec 3 | Dec 6 | Dec 13 |
Freeze probabilities 1991–2020 at Iowa City Municipal Airport, 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 52242 →Compare Iowa City with another place →
Weather by month in Iowa City
July is the warmest month at Iowa City Municipal Airport, IA, averaging 86°F by day and 66°F at night; January is the coldest at 31°F and 15°F. A freezing night is all but certain in every month from October to April (a 90% chance or more). From June to September the chance of a 32°F night stays under 10% each month. In spring, May still brings one in 23% of years.
Warmth for growing adds up to 3,441 growing degree days a year (base 50°F), more than at 91% of the 132 Iowa stations. 36% of them (1,235) have built up by the end of June, and July adds the most, 800. Precipitation totals 35.7 inches a year, most in June (5.3") and least in January (1.0"); May to September bring 58% of it.
| Month | High °F | Low °F | Precip. in | 32°F night | Nights ≤ 32°F | GDD 50°F |
|---|---|---|---|---|---|---|
| Jan | 31 | 15 | 1.0 | 100% | 29 | 0 |
| Feb | 36 | 19 | 1.2 | 100% | 26 | 2 |
| Mar | 50 | 30 | 2.2 | 100% | 21 | 34 |
| Apr | 63 | 40 | 3.8 | 96% | 7 | 134 |
| May | 73 | 52 | 4.5 | 23% | 1 | 395 |
| Jun | 83 | 62 | 5.3 | 0% | 0 | 670 |
| Jul | 86 | 66 | 3.9 | 0% | 0 | 800 |
| Aug | 84 | 63 | 3.9 | 0% | 0 | 730 |
| Sep | 78 | 54 | 3.2 | 4% | <1 | 477 |
| Oct | 64 | 42 | 3.0 | 91% | 6 | 172 |
| Nov | 49 | 30 | 2.2 | 100% | 19 | 25 |
| Dec | 36 | 20 | 1.5 | 100% | 28 | 2 |
Monthly normals 1991–2020 at Iowa City Municipal Airport, IA (650 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, Iowa City, IA, 4 miles southeast of Iowa City, has a last frost 8 days later (April 29), a first frost 9 days earlier (October 9), a season 18 days shorter.
When it snows in Iowa City, Iowa
The first measurable snow (0.1 inch or more) in Iowa City usually arrives around December 1, and the last of the season around March 13. Those are the middle dates across the winters of 1991–2020 at Iowa City, IA, 4.1 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 39 days after the last snow.
Planting calendar for 52242
| When | What to do |
|---|---|
| February 10 – February 24 | Start peppers and aubergines indoorsThey need eight to ten weeks under lights before they can go out. |
| February 24 – March 10 | Start tomatoes indoorsSix to eight weeks gives a transplant big enough to set fruit early. |
| March 24 – April 7 | Sow peas, spinach, lettuce, radish outdoorsThese tolerate light frost and prefer to germinate in cool soil. |
| April 21 onward | Transplant tomatoes, peppers, basil, squash, cucumbersThe average last freeze has passed — still watch the actual forecast. |
| by July 26 | Sow the autumn crop of carrots, beets and brassicasThey need about twelve weeks to size up before growth stops. |
| by October 4 | Harvest tender crops, or be ready to cover themA first freeze two weeks early is well within normal variation. |
| October 18 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 52242 is 2 miles from Iowa City Municipal Airport, IA, which stands at 650 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 7 other ZIP codes share the station; its figures are presented in full for 52240 (Iowa City). The 2023 USDA Plant Hardiness Zone Map lists ZIP 52242 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.