USDA Hardiness Zone 5b
Frost Dates for 83427 — Iona, Idaho
NOAA 1991–2020 normals from Idaho Falls 16 SE, ID (5,828 ft), 5 miles southeast.
Updated
In Iona, the last spring frost (32°F) is around June 9 in half of years and has passed by June 26 in nine years out of ten. The first fall frost comes around September 11, as early as August 27 one year in ten, leaving a typical freeze-free season of 93 days (NOAA, Idaho Falls 16 SE, ID).
- Last spring frost
- June 9
- First fall frost
- September 11
- Growing season
- 93 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
- October 29
- Last snow
- April 30
- Winters with snow
- 100%
- Snow per year
- 65.7"
median, 1991–2020
median, 1991–2020
30 of 30 observed
NOAA normal
Frost risk by date in Iona
Half of springs at Idaho Falls 16 SE, ID see their last 32°F night by June 9; only one in ten is past it by May 19, and one in ten still has one after June 26. Planting tender crops after June 16 leaves a 30% risk of a frost on them, after June 21 20%, and after June 26 10%. The 17 days between the even-odds date and the one-in-ten date is narrower than at 54% of the 67 Idaho stations behind this site's ZIP pages.
In fall the first 32°F night comes by September 11 in half of years, as early as August 27 one year in ten, and holds off past September 27 one year in ten. A hard freeze of 28°F follows 12 days later, around September 23; in spring the one-in-ten date for 28°F is June 10, 16 days before the 32°F one. The freeze-free season lasts 93 days in a typical year, shorter than at 94% of the 67 Idaho stations; it reaches 117 days one year in ten and falls short of 69 one year in ten. On average 199 nights a year drop to 32°F or below. Severe cold of 16°F or below comes every winter, usually between October 24 and April 6.
| Low | 90% | 80% | 70% | 60% | 50% | 40% | 30% | 20% | 10% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | too rare for a date | ||||||||
| 32°F | May 19 | May 27 | Jun 1 | Jun 6 | Jun 9 | Jun 13 | Jun 16 | Jun 21 | Jun 26 |
| 28°F | May 4 | May 9 | May 12 | May 16 | May 19 | May 24 | May 30 | Jun 4 | Jun 10 |
| 24°F | Apr 20 | Apr 25 | Apr 30 | May 2 | May 5 | May 8 | May 11 | May 15 | May 22 |
| 20°F | Apr 3 | Apr 8 | Apr 13 | Apr 17 | Apr 20 | Apr 24 | Apr 28 | May 2 | May 8 |
| 16°F | Mar 16 | Mar 24 | Mar 30 | Apr 3 | Apr 6 | Apr 10 | Apr 14 | Apr 19 | Apr 26 |
| Low | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | too rare for a date | ||||||||
| 32°F | Aug 27 | Sep 1 | Sep 4 | Sep 8 | Sep 11 | Sep 14 | Sep 18 | Sep 22 | Sep 27 |
| 28°F | Sep 4 | Sep 11 | Sep 15 | Sep 19 | Sep 23 | Sep 26 | Sep 29 | Oct 4 | Oct 10 |
| 24°F | Sep 14 | Sep 21 | Sep 26 | Sep 29 | Oct 2 | Oct 6 | Oct 10 | Oct 15 | Oct 20 |
| 20°F | Sep 25 | Oct 1 | Oct 5 | Oct 9 | Oct 13 | Oct 17 | Oct 21 | Oct 25 | Oct 31 |
| 16°F | Oct 4 | Oct 11 | Oct 16 | Oct 20 | Oct 24 | Oct 28 | Nov 1 | Nov 5 | Nov 11 |
Freeze probabilities 1991–2020 at Idaho Falls 16 SE, ID, 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 83427 →Compare Iona with another place →
Weather by month in Iona
July is the warmest month at Idaho Falls 16 SE, ID, averaging 81°F by day and 50°F at night; December is the coldest at 30°F and 13°F. A freezing night is all but certain in every month from September to May (a 90% chance or more). July is the only month with under a 10% chance of a 32°F night. In spring, June still brings one in 71% of years.
Warmth for growing adds up to 1,430 growing degree days a year (base 50°F), fewer than at 97% of the 67 Idaho stations. 22% of them (318) have built up by the end of June, and July adds the most, 472. Precipitation totals 17.5 inches a year, most in May (2.2") and least in July (0.9"); May to September bring 39% of it.
| Month | High °F | Low °F | Precip. in | 32°F night | Nights ≤ 32°F | GDD 50°F |
|---|---|---|---|---|---|---|
| Jan | 31 | 14 | 1.8 | 100% | 30 | 0 |
| Feb | 34 | 15 | 1.2 | 100% | 27 | 0 |
| Mar | 42 | 24 | 1.4 | 100% | 27 | 0 |
| Apr | 51 | 29 | 1.6 | 100% | 21 | 10 |
| May | 62 | 37 | 2.2 | 100% | 10 | 83 |
| Jun | 71 | 43 | 1.4 | 71% | 3 | 225 |
| Jul | 81 | 50 | 0.9 | 6% | <1 | 472 |
| Aug | 79 | 48 | 0.9 | 18% | 1 | 422 |
| Sep | 70 | 40 | 1.4 | 94% | 6 | 187 |
| Oct | 56 | 30 | 1.4 | 100% | 19 | 30 |
| Nov | 41 | 21 | 1.4 | 100% | 27 | 1 |
| Dec | 30 | 13 | 1.9 | 100% | 30 | 0 |
Monthly normals 1991–2020 at Idaho Falls 16 SE, ID (5,828 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, Idaho Falls - Kifi, ID, 12 miles northwest of Iona and 1,086 feet lower, has a last frost 31 days earlier (May 9), a first frost 20 days later (October 1), a season 49 days longer.
When it snows in Iona, Idaho
The first measurable snow (0.1 inch or more) in Iona usually arrives around October 29, and the last of the season around April 30. Those are the middle dates across the winters of 1991–2020 at Idaho Falls 16 SE, ID, 4.9 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 40 days after the last snow.
Planting calendar for 83427
| When | What to do |
|---|---|
| March 31 – April 14 | Start peppers and aubergines indoorsThey need eight to ten weeks under lights before they can go out. |
| April 14 – April 28 | Start tomatoes indoorsSix to eight weeks gives a transplant big enough to set fruit early. |
| May 12 – May 26 | Sow peas, spinach, lettuce, radish outdoorsThese tolerate light frost and prefer to germinate in cool soil. |
| June 9 onward | Transplant tomatoes, peppers, basil, squash, cucumbersThe average last freeze has passed — still watch the actual forecast. |
| by June 19 | Sow the autumn crop of carrots, beets and brassicasThey need about twelve weeks to size up before growth stops. |
| by August 28 | Harvest tender crops, or be ready to cover themA first freeze two weeks early is well within normal variation. |
| September 11 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 83427 is 5 miles from Idaho Falls 16 SE, ID, which stands at 5,828 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 83427 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.