Frost Dates for 83420 — Ashton, Idaho

Half of springs at Ashton 1N, ID see their last 32°F night by May 21; only one in ten is past it by May 4, and one in ten still has one after June 13. Planting tender crops after June 1 leaves a 30% risk of a frost on them, after June 7 20%, and after June 13 10%. The 23 days between the even-odds date and the one-in-ten date is wider than at 87% of the 67 Idaho stations behind this site's ZIP pages, so a fixed calendar date is a gamble here.

In fall the first 32°F night comes by September 21 in half of years, as early as September 4 one year in ten, and holds off past October 7 one year in ten. A hard freeze of 28°F follows 13 days later, around October 4; in spring the one-in-ten date for 28°F is May 26, 18 days before the 32°F one. The freeze-free season lasts 121 days in a typical year, shorter than at 78% of the 67 Idaho stations; it reaches 145 days one year in ten and falls short of 93 one year in ten. On average 190 nights a year drop to 32°F or below. Nights of 36°F, when frost can still settle on leaves in low, open spots, continue 20 days longer, to about June 10, and return around September 7. Severe cold of 16°F or below comes every winter, usually between November 6 and March 28.

July is the warmest month at Ashton 1N, ID, averaging 83°F by day and 51°F at night; January is the coldest at 29°F and 13°F. A freezing night is all but certain in every month from October to May (a 90% chance or more). From July to August the chance of a 32°F night stays under 10% each month. In spring, June still brings one in 30% of years.

Warmth for growing adds up to 1,730 growing degree days a year (base 50°F), fewer than at 82% of the 67 Idaho stations. 26% of them (451) have built up by the end of June, and July adds the most, 519. Precipitation totals 19.7 inches a year, most in January (2.3") and least in July (0.6"); May to September bring 36% of it.

The nearest other station, Tetonia Exp Stn, ID, 16 miles southeast of Ashton and 958 feet higher, has a last frost 25 days later (June 15), a first frost 14 days earlier (September 7), a season 36 days shorter.

The first measurable snow (0.1 inch or more) in Ashton usually arrives around November 8, and the last of the season around April 1. Those are the middle dates across the winters of 1991–2020 at Ashton 1N, ID, 2.6 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 50 days after the last snow.

ZIP 83420 is 3 miles from Ashton 1N, ID, which stands at 5,212 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 83420 in zone 5a, the band where the coldest night of the year averaged -20 to -15°F (-28.9 to -26.1°C) over 1991–2020; a zone describes winter lows, not frost dates.