Last spring frost
Apr 20
Frost Dates for Northboro, IA
First fall frost
Oct 17
Growing season
179 days
Hardiness zone
5b
In an average year around Northboro, IA, the last spring frost (32°F) is about Apr 20 and the first fall frost is about Oct 17, giving roughly a 179-day growing season. To be safe most years, wait until Apr 5 before setting out tender plants.
Last spring frost
Dates the last frost of spring typically ends. Plant tender crops after the risk has passed for your comfort level.
| Frost intensity | Early (10%) | Average (50%) | Late (90%) |
|---|---|---|---|
| Light (36°F) | May 11 | Apr 30 | Apr 17 |
| Freeze (32°F) | May 3 | Apr 20 | Apr 5 |
| Hard (28°F) | Apr 23 | Apr 9 | Mar 27 |
| Severe (24°F) | Apr 16 | Apr 1 | Mar 14 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | May 11 | May 7 | May 4 | May 2 | Apr 30 | Apr 28 | Apr 25 | Apr 22 | Apr 17 |
| 32°F | May 3 | Apr 29 | Apr 26 | Apr 23 | Apr 20 | Apr 17 | Apr 14 | Apr 10 | Apr 5 |
| 28°F | Apr 23 | Apr 19 | Apr 16 | Apr 12 | Apr 9 | Apr 6 | Apr 4 | Mar 31 | Mar 27 |
| 24°F | Apr 16 | Apr 11 | Apr 7 | Apr 3 | Apr 1 | Mar 30 | Mar 26 | Mar 21 | Mar 14 |
| 20°F | Apr 8 | Apr 1 | Mar 29 | Mar 25 | Mar 21 | Mar 17 | Mar 13 | Mar 9 | Mar 3 |
| 16°F | Mar 29 | Mar 23 | Mar 18 | Mar 14 | Mar 11 | Mar 7 | Mar 4 | Feb 28 | Feb 23 |
First fall frost
Dates the first frost of fall typically arrives. Count back by a crop's days-to-maturity to know your planting cutoff.
| Frost intensity | Early (10%) | Average (50%) | Late (90%) |
|---|---|---|---|
| Light (36°F) | Sep 23 | Oct 6 | Oct 19 |
| Freeze (32°F) | Oct 1 | Oct 17 | Oct 29 |
| Hard (28°F) | Oct 13 | Oct 26 | Nov 7 |
| Severe (24°F) | Oct 23 | Nov 4 | Nov 18 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 23 | Sep 27 | Oct 1 | Oct 3 | Oct 6 | Oct 9 | Oct 12 | Oct 15 | Oct 19 |
| 32°F | Oct 1 | Oct 7 | Oct 11 | Oct 15 | Oct 17 | Oct 19 | Oct 22 | Oct 25 | Oct 29 |
| 28°F | Oct 13 | Oct 18 | Oct 21 | Oct 24 | Oct 26 | Oct 29 | Oct 31 | Nov 3 | Nov 7 |
| 24°F | Oct 23 | Oct 27 | Oct 31 | Nov 1 | Nov 4 | Nov 6 | Nov 9 | Nov 12 | Nov 18 |
| 20°F | Nov 1 | Nov 3 | Nov 6 | Nov 10 | Nov 13 | Nov 16 | Nov 19 | Nov 23 | Nov 29 |
| 16°F | Nov 6 | Nov 11 | Nov 14 | Nov 18 | Nov 22 | Nov 26 | Nov 29 | Dec 3 | Dec 9 |
Growing season is measured at 32°F with about 50% probability. Hardiness zone from the 2023 USDA map; frost normals from NOAA 1991-2020.
Where this data comes from
@endif These dates are computed from the nearest official weather station with long-term records: Shenandoah, Ia, about 11.5 miles away@if(!empty($station['elevation_ft'])), at 999 ft elevation. The further the station and the greater the elevation difference from your yard, the more your local frost timing can vary.
How we calculate these dates
Frost and freeze dates come from the NOAA 1991–2020 U.S. Climate Normals, the National Oceanic and Atmospheric Administration's 30-year averages for each weather station. A date shown at the 50% level is the median: in half of years the frost falls on or before it. The 10% and 90% columns show the early and late ends of the range, so you can plant on the odds you're comfortable with. These are long-term averages, not a forecast for this year - always check your local outlook before planting.
Source: NOAA National Centers for Environmental Information, 1991–2020 Climate Normals (public domain). Harman is not affiliated with or endorsed by NOAA.
Frequently asked questions
When is the last frost in Northboro, IA?
On average, the last spring frost (32°F) around Northboro, IA is about Apr 20. In cooler years it can run later, so for a safer bet many gardeners wait until Apr 5.
When is the first frost in the fall?
The first fall frost around Northboro, IA typically arrives about Oct 17. Count back by a crop's days to maturity to find the latest date you can plant it.
When can I plant tomatoes and other tender crops?
Tender crops like tomatoes, peppers and basil are damaged by frost, so wait until the last spring frost has passed - around Apr 20 on average, or Apr 5 to be safe most years.
How long is the growing season here?
The frost-free growing season (between the last spring and first fall 32°F frost) is about 179 days in a typical year.
Are these dates a forecast for this year?
No. They are 30-year averages from the NOAA 1991–2020 Climate Normals, not a prediction for the current year. Use them for planning and always check your local weather forecast before planting or protecting crops.
Local weather for 51647 Planting calendar → Iowa frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026