Frost dates for Boise, Idaho
In Boise, half of springs have had their last frost by April 24 (and 1 spring in 10 still has one after May 9), and half of falls have had their first frost by October 21, leaving about 179 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
Boise, Idaho
Last spring frost
April 24
Half of springs have their last frost by this date. For a safer bet, wait until May 9: only 1 spring in 10 has a frost after that.
First fall frost
October 21
Half of falls have their first frost by this date. In 1 fall in 10 it comes by October 5.
Growing season
179 days
Frost-free days in a typical year. 9 years in 10 get at least 156.
Hardiness zone
7a
2023 USDA map: the coldest night of winter averages 0°F to 5°F. It was 7a on the 2012 map too. All 10 ZIP codes within about 6 miles of the city's center are in zone 7a.
Stations near you disagree. NOAA's Boise Air Terminal (2 miles away, at 2,810 feet above sea level), the one we used, gives a typical last frost of April 24 and a typical first frost of October 21. NOAA's Boise 7 N (10 miles away, at 3,880 feet above sea level) gives a typical last frost of May 17 and a typical first frost of October 12. So the typical last frost could fall anywhere from April 24 to May 17 and the typical first frost from October 12 to 21, 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 Boise
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 | April 4 | April 24 | May 9 |
| First frost in fall | October 5 | October 21 | November 5 |
The frost-free season between them runs 179 days in a typical year. In 1 year in 10 it is 203 days or longer, and in 1 year in 10 it is 156 days or shorter.
- Hard freeze (28°F): typically last on April 2 and first on October 31. 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 May 8 and first on October 8. 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 Boise comes between April 4 and May 9, a window of 35 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, 90 percent have their typical last frost later than Boise does.
Planting calendar for Boise
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.
| When | What to do | Why then |
|---|---|---|
| February | ||
| February 6 to March 6 | Broccoli and cabbage Start seeds indoors | 5 to 7 weeks before planting them out |
| February 20 to March 20 | Cauliflower Start seeds indoors | 5 to 7 weeks before planting out |
| March | ||
| March 6 to 27 | Eggplant Start seeds indoors | 6 to 9 weeks before planting out |
| March 13 to 27 | Peppers Start seeds indoors | 6 to 8 weeks before planting out |
| March 20 to April 3 | Tomatoes Start seeds indoors | 5 to 7 weeks before planting out |
| March 27 to April 10 | Peas, spinach, lettuce, radishes, onion sets, turnips Sow outdoors, as early as | hardy crops: 2 to 4 weeks before the typical last frost |
| March 27 to April 10 | Broccoli and cabbage Plant out, as early as | hardy crops: 2 to 4 weeks before the typical last frost |
| April | ||
| April 10 to 24 | Beets, carrots, potatoes, Swiss chard Plant outdoors, as early as | semi-hardy crops: up to 2 weeks before the typical last frost |
| April 10 to 24 | Cauliflower Plant out, as early as | semi-hardy: up to 2 weeks before the typical last frost |
| April 24 | Snap beans, sweet corn, cucumbers, summer squash Sow outdoors around | tender crops: around the typical last frost date |
| May | ||
| May 8 | Tomatoes, peppers, eggplant Plant out from | very tender: 2 weeks or more after the typical last frost |
| May 8 | Melons, winter squash, pumpkins, lima beans Sow outdoors from | very tender: 2 weeks or more after the typical last frost |
| May 26 to June 9 | Cabbage for fall Start seeds | set the plants out by July 14 |
| June | ||
| June 15 to 29 | Broccoli for fall Start seeds | set the plants out by August 3 |
| July | ||
| July 25 | 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 |
| July 29 | Carrots for fall Last date to sow | 70 days to harvest, plus 2 weeks for slower fall growth |
| July 30 | 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 |
| August | ||
| August 3 | Peas for fall Last date to sow | 65 days to harvest, plus 2 weeks for slower fall growth |
| August 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 |
| August 8 | Leaf lettuce and beets for fall Last date to sow | 60 days to harvest, plus 2 weeks for slower fall growth |
| August 18 | Turnips and kohlrabi for fall Last date to sow | 50 days to harvest, plus 2 weeks for slower fall growth |
| August 28 | Spinach for fall Last date to sow | 40 days to harvest, plus 2 weeks for slower fall growth |
| September | ||
| September 7 | Radishes for fall Last date to sow | 30 days to harvest, plus 2 weeks for slower fall growth |
| October | ||
| October 5 | 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 |
| November | ||
| November 7 to 14 | 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 15 days later. That plans around May 9, 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 Boise Air Terminal (station USW00024131), 2 miles from the center of Boise. It sits at about the same elevation, 2,810 feet above sea level. NOAA rates this station's record as standard: at least 24 of the 30 years, with gaps filled from nearby stations.
The same station also gives the frost dates for Meridian, Kuna and Garden City.
Our ZIP code lookup matches these 13 ZIP codes to the same station, so they get the same dates: 83634, 83642, 83702, 83703, 83704, 83705, 83706, 83709, 83712, 83713, 83714, 83720, 83725. Hardiness zones are worked out separately for each ZIP code.
Nearby places
- Nampa, Idaho: last frost around April 29, first frost around October 13 (15 miles away)
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.