Frost dates for Idaho Falls, Idaho
In Idaho Falls, half of springs have had their last frost by May 23 (and 1 spring in 10 still has one after June 13), and half of falls have had their first frost by September 20, leaving about 114 frost-free days in a typical year. Here is how sure those dates are, what they mean for planting, and where they come from.
Frost dates for
Idaho Falls, Idaho
Last spring frost
May 23
Half of springs have their last frost by this date. For a safer bet, wait until June 13: only 1 spring in 10 has a frost after that.
First fall frost
September 20
Half of falls have their first frost by this date. In 1 fall in 10 it comes by September 1.
Growing season
114 days
Frost-free days in a typical year. 9 years in 10 get at least 88.
Hardiness zone
5b
2023 USDA map: the coldest night of winter averages −15°F to −10°F. It was 5b on the 2012 map too. All 2 ZIP codes within about 6 miles of the city's center are in zone 5b.
Stations near you disagree. NOAA's Idaho Falls Fanning Field (2 miles away, at 4,730 feet above sea level), the one we used, gives a typical last frost of May 23 and a typical first frost of September 20. NOAA's Idaho Falls - Kifi (2 miles away, at 4,740 feet above sea level) gives a typical last frost of May 9 and a typical first frost of October 1. So the typical last frost could fall anywhere from May 9 to 23 and the typical first frost from September 20 to October 1, depending on whether your garden is more like one or the other. Local terrain matters: hillsides, valley floors, water and city heat can change frost dates a lot over a few miles. Treat the dates here as a starting point, and compare with a nearby gardener or your county extension office.
Print a year planner for Idaho Falls
Frost risk through the year
How sure are these dates?
A frost date is a probability, not a promise. NOAA worked out, from 30 years of nightly lows, how often the last spring frost and the first fall frost fall on each date. The middle column is the typical year. The outer columns are the early and late years that come about once a decade.
| Early year 1 in 10 | Typical year 1 in 2 | Late year 1 in 10 | |
|---|---|---|---|
| Last frost in spring | May 7 | May 23 | June 13 |
| First frost in fall | September 1 | September 20 | October 4 |
The frost-free season between them runs 114 days in a typical year. In 1 year in 10 it is 139 days or longer, and in 1 year in 10 it is 88 days or shorter.
- Hard freeze (28°F): typically last on May 7 and first on October 2. It is a harder freeze than the 32°F frost line, cold enough to damage many garden plants; how hardy a crop is decides whether it comes through. (The National Weather Service's own hard-freeze threshold varies from office to office.)
- Frost possible (36°F at the station): typically last on June 14 and first on September 5. On clear, still nights the ground can be several degrees colder than the thermometer, so frost can form on plants when the official reading is in the mid-30s.
In 8 springs out of 10, the last frost in Idaho Falls comes between May 7 and June 13, a window of 37 days. That is wider than at most NOAA stations (the typical window is 31 days), so spring here is less predictable than average: lean toward the later date.
Among Idaho's 127 NOAA stations with a frost every year, 30 percent have their typical last frost later than Idaho Falls does.
Planting calendar for Idaho Falls
These dates count from the typical last and first frosts, using the planting rules published by Colorado State University Extension (spring), Virginia Cooperative Extension (fall crops) and University of Minnesota Extension (garlic, where winters are hard). Weather in any one year can run early or late, so check the forecast before you plant tender crops.
Your season is too short for a separate fall planting of cabbage and broccoli: sown late enough to be a fall crop, it wouldn't be ready before the typical first frost. Your spring planting is your harvest.
| When | What to do | Why then |
|---|---|---|
| March | ||
| March 7 to April 4 | Broccoli and cabbage Start seeds indoors | 5 to 7 weeks before planting them out |
| March 21 to April 18 | Cauliflower Start seeds indoors | 5 to 7 weeks before planting out |
| April | ||
| April 4 to 25 | Eggplant Start seeds indoors | 6 to 9 weeks before planting out |
| April 11 to 25 | Peppers Start seeds indoors | 6 to 8 weeks before planting out |
| April 18 to May 2 | Tomatoes Start seeds indoors | 5 to 7 weeks before planting out |
| April 25 to May 9 | Peas, spinach, lettuce, radishes, onion sets, turnips Sow outdoors, as early as | hardy crops: 2 to 4 weeks before the typical last frost |
| April 25 to May 9 | Broccoli and cabbage Plant out, as early as | hardy crops: 2 to 4 weeks before the typical last frost |
| May | ||
| May 9 to 23 | Beets, carrots, potatoes, Swiss chard Plant outdoors, as early as | semi-hardy crops: up to 2 weeks before the typical last frost |
| May 9 to 23 | Cauliflower Plant out, as early as | semi-hardy: up to 2 weeks before the typical last frost |
| May 23 | Snap beans, sweet corn, cucumbers, summer squash Sow outdoors around | tender crops: around the typical last frost date |
| June | ||
| June 6 | Tomatoes, peppers, eggplant Plant out from | very tender: 2 weeks or more after the typical last frost |
| June 6 | Melons, winter squash, pumpkins, lima beans Sow outdoors from | very tender: 2 weeks or more after the typical last frost |
| June 24 | Snap beans for fall Last date to sow | 60 days to harvest, plus 2 weeks for slower fall growth, plus 2 weeks because frost kills it |
| June 28 | Carrots for fall Last date to sow | 70 days to harvest, plus 2 weeks for slower fall growth |
| June 29 | Cucumbers for fall Last date to sow | 55 days to harvest, plus 2 weeks for slower fall growth, plus 2 weeks because frost kills it |
| July | ||
| July 3 | Peas for fall Last date to sow | 65 days to harvest, plus 2 weeks for slower fall growth |
| July 4 | Summer squash for fall Last date to sow | 50 days to harvest, plus 2 weeks for slower fall growth, plus 2 weeks because frost kills it |
| July 8 | Leaf lettuce and beets for fall Last date to sow | 60 days to harvest, plus 2 weeks for slower fall growth |
| July 18 | Turnips and kohlrabi for fall Last date to sow | 50 days to harvest, plus 2 weeks for slower fall growth |
| July 28 | Spinach for fall Last date to sow | 40 days to harvest, plus 2 weeks for slower fall growth |
| August | ||
| August 7 | Radishes for fall Last date to sow | 30 days to harvest, plus 2 weeks for slower fall growth |
| September | ||
| September 1 | Tomatoes, peppers, squash, melons, cucumbers Watch the forecast from | in 1 fall in 10 the first frost comes by this date; pick tender crops before a frost |
| October | ||
| October 9 to 16 | Garlic Plant cloves | one or two weeks after the typical first hard freeze (28°F), our reading of the first killing frost |
Want more insurance? Shift every spring date 21 days later. That plans around June 13, the date after which only 1 spring in 10 has a frost.
For spring bulbs such as tulips and daffodils, plant once the soil has cooled to about 50 to 55°F and keep going until a couple of weeks before the ground freezes. Our guide to fall planting explains how to tell.
Where these numbers come from
These dates come from NOAA's weather station Idaho Falls Fanning Field (station USW00024145), 2 miles from the center of Idaho Falls. It sits at about the same elevation, 4,730 feet above sea level. NOAA rates this station's record as representative: at least 10 of the 30 years, with the gaps filled from nearby stations.
The same station also gives the frost dates for Ammon.
Our ZIP code lookup matches these 2 ZIP codes to the same station, so they get the same dates: 83274, 83402. Hardiness zones are worked out separately for each ZIP code.
See frost dates across Idaho, or look up your own ZIP code for the station nearest you. Want the background? Read what a last frost date really means.