Last spring frost
Apr 14
Frost Dates for Prosser, WA
First fall frost
Oct 22
Growing season
189 days
Hardiness zone
7b
In an average year around Prosser, WA, the last spring frost (32°F) is about Apr 14 and the first fall frost is about Oct 22, giving roughly a 189-day growing season. To be safe most years, wait until Mar 24 before setting out tender plants.
Last spring frost
Dates the last frost of spring typically ends. Plant tender crops after the risk has passed for your comfort level.
| Frost intensity | Early (10%) | Average (50%) | Late (90%) |
|---|---|---|---|
| Light (36°F) | May 24 | May 2 | Apr 16 |
| Freeze (32°F) | May 7 | Apr 14 | Mar 24 |
| Hard (28°F) | Apr 22 | Mar 23 | Mar 2 |
| Severe (24°F) | Mar 31 | Mar 3 | Feb 13 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | May 24 | May 15 | May 9 | May 6 | May 2 | Apr 30 | Apr 26 | Apr 22 | Apr 16 |
| 32°F | May 7 | May 1 | Apr 26 | Apr 19 | Apr 14 | Apr 9 | Apr 4 | Mar 31 | Mar 24 |
| 28°F | Apr 22 | Apr 9 | Apr 2 | Mar 28 | Mar 23 | Mar 17 | Mar 12 | Mar 7 | Mar 2 |
| 24°F | Mar 31 | Mar 20 | Mar 12 | Mar 7 | Mar 3 | Mar 1 | Feb 26 | Feb 21 | Feb 13 |
| 20°F | Mar 12 | Mar 3 | Feb 28 | Feb 25 | Feb 21 | Feb 16 | Feb 10 | Feb 3 | Jan 21 |
| 16°F | Feb 28 | Feb 23 | Feb 16 | Feb 9 | Feb 3 | Jan 27 | Jan 17 | Jan 7 | Dec 26 |
First fall frost
Dates the first frost of fall typically arrives. Count back by a crop's days-to-maturity to know your planting cutoff.
| Frost intensity | Early (10%) | Average (50%) | Late (90%) |
|---|---|---|---|
| Light (36°F) | Sep 26 | Oct 9 | Oct 25 |
| Freeze (32°F) | Oct 5 | Oct 22 | Nov 7 |
| Hard (28°F) | Oct 18 | Nov 2 | Nov 20 |
| Severe (24°F) | Oct 27 | Nov 13 | Dec 1 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 26 | Oct 1 | Oct 4 | Oct 6 | Oct 9 | Oct 12 | Oct 16 | Oct 19 | Oct 25 |
| 32°F | Oct 5 | Oct 11 | Oct 15 | Oct 19 | Oct 22 | Oct 25 | Oct 29 | Nov 1 | Nov 7 |
| 28°F | Oct 18 | Oct 22 | Oct 27 | Oct 30 | Nov 2 | Nov 6 | Nov 10 | Nov 13 | Nov 20 |
| 24°F | Oct 27 | Nov 1 | Nov 7 | Nov 10 | Nov 13 | Nov 17 | Nov 21 | Nov 26 | Dec 1 |
| 20°F | Nov 4 | Nov 11 | Nov 16 | Nov 21 | Nov 25 | Nov 29 | Dec 3 | Dec 10 | Dec 19 |
| 16°F | Nov 13 | Nov 23 | Nov 28 | Dec 3 | Dec 8 | Dec 13 | Dec 19 | Dec 26 | Jan 5 |
Growing season is measured at 32°F with about 50% probability. Hardiness zone from the 2023 USDA map; frost normals from NOAA 1991-2020.
Where this data comes from
@endif These dates are computed from the nearest official weather station with long-term records: Prosser, Wa, about 4.2 miles away@if(!empty($station['elevation_ft'])), at 830 ft elevation. The further the station and the greater the elevation difference from your yard, the more your local frost timing can vary.
How we calculate these dates
Frost and freeze dates come from the NOAA 1991–2020 U.S. Climate Normals, the National Oceanic and Atmospheric Administration's 30-year averages for each weather station. A date shown at the 50% level is the median: in half of years the frost falls on or before it. The 10% and 90% columns show the early and late ends of the range, so you can plant on the odds you're comfortable with. These are long-term averages, not a forecast for this year - always check your local outlook before planting.
Source: NOAA National Centers for Environmental Information, 1991–2020 Climate Normals (public domain). Harman is not affiliated with or endorsed by NOAA.
Frequently asked questions
When is the last frost in Prosser, WA?
On average, the last spring frost (32°F) around Prosser, WA is about Apr 14. In cooler years it can run later, so for a safer bet many gardeners wait until Mar 24.
When is the first frost in the fall?
The first fall frost around Prosser, WA typically arrives about Oct 22. Count back by a crop's days to maturity to find the latest date you can plant it.
When can I plant tomatoes and other tender crops?
Tender crops like tomatoes, peppers and basil are damaged by frost, so wait until the last spring frost has passed - around Apr 14 on average, or Mar 24 to be safe most years.
How long is the growing season here?
The frost-free growing season (between the last spring and first fall 32°F frost) is about 189 days in a typical year.
Are these dates a forecast for this year?
No. They are 30-year averages from the NOAA 1991–2020 Climate Normals, not a prediction for the current year. Use them for planning and always check your local weather forecast before planting or protecting crops.
Local weather for 99350 Planting calendar → Washington frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026