Boise, ID Frost Dates: Last Spring and First Fall Frost

The last spring frost in Boise, ID comes on April 24 in a typical year, and the first fall frost follows on October 21, leaving a frost-free season of 179 days.

Put May 9 and October 5 on the calendar as well. A spring freeze after the first of them, or a fall freeze before the second, happens in only one year out of ten, so those dates cover nine years in ten. Among the 100 cities covered here, whose median season is 229 days, Boise ranks 88th for length and falls 50 days below the middle.

These dates come from the NOAA weather station Boise Air Terminal (ID USW00024131), 2.3 miles south of the Census Bureau's internal point for Boise, at about 2,800 feet above sea level.

When is the last frost in Boise?

Frost is still possible on April 4 in nine springs out of ten and on April 24 in five; only one spring in ten sees a freeze as late as May 9.

From the earliest date to the latest is 35 days, a middling band against the other 100 cities.

Boise's first fall frost, by chance

Expect the first freeze of fall by October 21 in a typical year. One year in ten brings it by October 5, and nine in ten have had it by November 5.

The early and late fall dates are 31 days apart, a typical spread for the cities covered here.

The growing season in Boise

That is shorter than most places on these pages, so days to maturity on the seed packet matter more than usual. A typical year gives 179 days; nine years in ten give at least 156 days, and one in ten reaches 203 days. The swing is 23 days between typical and short, about average for these pages.

NOAA counts these days from its own simulations, so they will not equal the number of days between the two typical dates exactly. At Boise Air Terminal a night at or below 32°F turns up in 100% of years, which means these dates apply every winter.

Hard freeze dates for Boise

Light frost and hard freeze are well separated in Boise. In fall the typical 28°F night comes on October 31, 10 days after the first 32°F frost on October 21; in spring the last one comes on April 2, 22 days before the last 32°F frost. Crops that tolerate light frost have that stretch to themselves. One spring in ten still has a 28°F night on or after April 25, and one fall in ten sees the first by October 17.

What this means for planting in Boise

University of New Hampshire Extension sorts vegetables by how they cope with cold. Cool-season crops go out before the last frost: counting from April 24, cabbage transplants go in on March 27 and lettuce from March 27 to April 3. Tender crops wait until it has passed, with peppers going out on May 8 and beans sown on or after April 24. The University of New Hampshire Extension sheet gives peas no frost count at all; sow them as soon as the soil thaws.

The late date comes 15 days after the typical one; choose it for plants you cannot cover.

Boise's first frost leaves room for only 25 of the 34 fall crops, counting back NC State Extension's days to harvest and University of Georgia Extension's harvest allowance of 18 days; for the other 9 the last planting day falls before July. Of the crops that fit, cabbage is the slowest: 93 days from planting to the early frost, which makes July 4 its last day.

Worked example: setting out tomatoes in Boise

A gardener in Boise who follows University of New Hampshire Extension sets tomato transplants out one to two weeks after the last frost: May 1 to May 8 from the typical date of April 24, or May 16 to May 23 from the late date of May 9, 15 days later.

Seeds start six to eight weeks before the set-out day, so counting back from the first day of the late window gives March 21 to April 4 for sowing indoors.

From the last set-out day to the early first frost on October 5 is 135 days. NC State Extension gives 75 to 85 days, and the longer figure still fits with 50 days to spare.

Where Boise's numbers are measured

The station is 2.3 miles south of the Census Bureau's internal point for Boise, near enough to be the city's reference record, though not in the same yard.

Its elevation is about 2,800 feet. Of the 100 stations behind these pages, 9 sit higher and 90 lower. NOAA flags this station's record as standard, the code S, for the data behind its 50% last-frost value.

Because the Census file has no elevation for Boise, we cannot say how far your yard is above or below this station. When yours is clearly higher or lower, open the nearby stations in the frost dates tool and favor the colder one.

How Boise's frost dates compare

Boise is far from its neighbors on these pages: the closest, Spokane, WA, is 287 miles away, so expect its dates to belong to a different climate. Note that the Spokane station stands 850 feet lower than Boise's.

• Spokane, WA, 287 miles to the north: last frost April 26, first frost October 15, 170 days frost-free; its last frost is nearly the same as Boise's, and it has 9 days less without frost.
• Salt Lake City, UT (294 miles, south-east): spring frost ends 24 days sooner than in Boise, and it has 45 days more without frost. Its typical dates are March 31 and November 8, with 224 days frost-free.
• Reno, NV, 337 miles to the south-west: last frost April 28, first frost October 27, 182 days frost-free; the last frost is 4 days later than in Boise, and the season is within about a week of Boise's.

Put the frost dates to work

Open the frost dates tool with this station already chosen to see the whole ladder of probability dates, or try the nearby stations in the same list. The spring planting planner and the fall planting planner turn the same dates into a full crop list, and the hardiness zone lookup tells you which perennials will survive the winter.

Every city covered is on the city frost dates index, sorted by state.

Where these numbers come from

Every frost and freeze date comes from the station's entry in NOAA's U.S. Climate Normals for 1991–2020 (doi:10.25921/4ek7-fk11), with no adjustment, and NOAA's documentation explains what each probability means. The Census Bureau's 2024 Gazetteer places Boise on the map, and its Vintage 2024 estimates, 237,963 people here, decide which cities get a page.

The planting dates apply the count rules in University of New Hampshire Extension, NC State Extension and University of Georgia Extension to this station's dates, with nothing added. The methodology page explains how each figure is checked.

These dates describe odds over many years, not next week's weather. Look at the forecast before you plant, and keep a sheet or row cover ready for a night that does not follow the odds.