USDA Hardiness Zone 4b
Frost Dates for 99672 — Sterling, Alaska
NOAA 1991–2020 normals from Sterling 6SW, AK (292 ft), 7 miles southwest.
Updated
In Sterling, the last spring frost (32°F) is around May 20 in half of years and has passed by June 1 in nine years out of ten. The first fall frost comes around September 21, as early as September 6 one year in ten, leaving a typical freeze-free season of 123 days (NOAA, Sterling 6SW, AK).
- Last spring frost
- May 20
- First fall frost
- September 21
- Growing season
- 123 days
- Hardiness zone
- 4b
50% probability
50% probability
freeze-free, 50%
-25 to -20°F (-31.7 to -28.9°C) coldest night
- First snow
- October 22
- Last snow
- April 8
- Winters with snow
- 100%
- Snow per year
- —
median, 1991–2020
median, 1991–2020
15 of 15 observed
NOAA normal
Frost risk by date in Sterling
Half of springs at Sterling 6SW, AK see their last 32°F night by May 20; only one in ten is past it by May 6, and one in ten still has one after June 1. Planting tender crops after May 25 leaves a 30% risk of a frost on them, after May 28 20%, and after June 1 10%. The 12 days between the even-odds date and the one-in-ten date is narrower than at 65% of the 37 Alaska stations behind this site's ZIP pages.
In fall the first 32°F night comes by September 21 in half of years, as early as September 6 one year in ten, and holds off past October 4 one year in ten. A hard freeze of 28°F follows 9 days later, around September 30; in spring the one-in-ten date for 28°F is May 17, 15 days before the 32°F one. The freeze-free season lasts 123 days in a typical year, shorter than at 65% of the 37 Alaska stations; it reaches 141 days one year in ten and falls short of 105 one year in ten. On average 200 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 15 days longer, to about June 4, and return around September 8. Severe cold of 16°F or below comes every winter, usually between October 27 and April 4.
| Low | 90% | 80% | 70% | 60% | 50% | 40% | 30% | 20% | 10% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | May 25 | May 29 | May 31 | Jun 2 | Jun 4 | Jun 6 | Jun 8 | Jun 11 | Jun 15 |
| 32°F | May 6 | May 11 | May 14 | May 17 | May 20 | May 22 | May 25 | May 28 | Jun 1 |
| 28°F | Apr 19 | Apr 24 | Apr 29 | May 1 | May 3 | May 6 | May 9 | May 12 | May 17 |
| 24°F | Apr 7 | Apr 11 | Apr 13 | Apr 16 | Apr 19 | Apr 23 | Apr 26 | Apr 30 | May 2 |
| 20°F | Mar 31 | Apr 3 | Apr 5 | Apr 7 | Apr 9 | Apr 12 | Apr 15 | Apr 19 | Apr 25 |
| 16°F | Mar 23 | Mar 28 | Mar 31 | Apr 2 | Apr 4 | Apr 6 | Apr 8 | Apr 12 | Apr 18 |
| Low | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Aug 26 | Aug 31 | Sep 2 | Sep 5 | Sep 8 | Sep 11 | Sep 14 | Sep 18 | Sep 23 |
| 32°F | Sep 6 | Sep 11 | Sep 15 | Sep 18 | Sep 21 | Sep 24 | Sep 26 | Sep 29 | Oct 4 |
| 28°F | Sep 17 | Sep 23 | Sep 25 | Sep 28 | Sep 30 | Oct 3 | Oct 6 | Oct 9 | Oct 15 |
| 24°F | Sep 29 | Oct 2 | Oct 5 | Oct 8 | Oct 11 | Oct 14 | Oct 16 | Oct 20 | Oct 25 |
| 20°F | Oct 5 | Oct 9 | Oct 14 | Oct 16 | Oct 20 | Oct 22 | Oct 25 | Oct 29 | Nov 5 |
| 16°F | Oct 12 | Oct 18 | Oct 21 | Oct 24 | Oct 27 | Oct 29 | Nov 1 | Nov 5 | Nov 12 |
Freeze probabilities 1991–2020 at Sterling 6SW, AK, 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 99672 →Compare Sterling with another place →
Compared with the next station
The nearest other station, Soldotna 5SSW, AK, 16 miles southwest of Sterling, has a last frost 22 days later (June 11), a first frost 20 days earlier (September 1), a season 48 days shorter.
When it snows in Sterling, Alaska
The first measurable snow (0.1 inch or more) in Sterling usually arrives around October 22, and the last of the season around April 8. Those are the middle dates across the winters of 1991–2020 at Kenai 9N, AK, 21.9 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 42 days after the last snow.
Planting calendar for 99672
| When | What to do |
|---|---|
| March 11 – March 25 | Start peppers and aubergines indoorsThey need eight to ten weeks under lights before they can go out. |
| March 25 – April 8 | Start tomatoes indoorsSix to eight weeks gives a transplant big enough to set fruit early. |
| April 22 – May 6 | Sow peas, spinach, lettuce, radish outdoorsThese tolerate light frost and prefer to germinate in cool soil. |
| May 20 onward | Transplant tomatoes, peppers, basil, squash, cucumbersThe average last freeze has passed — still watch the actual forecast. |
| by June 29 | Sow the autumn crop of carrots, beets and brassicasThey need about twelve weeks to size up before growth stops. |
| by September 7 | Harvest tender crops, or be ready to cover themA first freeze two weeks early is well within normal variation. |
| September 21 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 99672 is 7 miles from Sterling 6SW, AK, which stands at 292 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 1 other ZIP code shares the station; its figures are presented in full for 99669 (Soldotna). The 2023 USDA Plant Hardiness Zone Map lists ZIP 99672 in zone 4b, the band where the coldest night of the year averaged -25 to -20°F (-31.7 to -28.9°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.