Frost dates for 99350Prosser, WA
The average last spring frost in Prosser is April 14 and the average first fall frost is October 22, giving a typical growing season of about 189 days. Dates are based on NOAA 1991-2020 climate normals.
- Average last spring frost
- Apr 14
- Average first fall frost
- Oct 22
- Growing season
- 189 days
Safer after May 7 (1-in-10 risk)
Possible by Oct 5 (1-in-10 risk)
Last frost typically in 164 days
Frost risk by date
NOAA reports frost dates as probabilities. The middle date is the average (a 50% chance). For spring, the 10% date is a safer planting date: frost after it happens in about 1 year in 10. Light frost (36°F) can damage tender plants; a hard freeze (28°F) kills most annuals.
| Temperature | Last spring: 90% / 50% / 10% chance | First fall: 10% / 50% / 90% chance | Growing season (50%) | Years with at least one |
|---|---|---|---|---|
| Light frost (36°F) | Apr 16 · May 2 · May 24 | Sep 26 · Oct 9 · Oct 25 | 158 days | 100% |
| Frost / freeze (32°F) | Mar 24 · Apr 14 · May 7 | Oct 5 · Oct 22 · Nov 7 | 189 days | 100% |
| Hard freeze (28°F) | Mar 2 · Mar 23 · Apr 22 | Oct 18 · Nov 2 · Nov 20 | 224 days | 100% |
Planting calendar for Prosser
Planting windows from the University of Maryland Extension calendar, shifted to your average frost dates. Use them as a starting point and adjust for seed packet instructions and local conditions.
| Crop | Task | Window |
|---|---|---|
| Tomatoes | Start seeds indoors | Mar 14 – May 20 |
| Tomatoes | Transplant outdoors | Apr 30 – Jul 2 |
| Peppers | Start seeds indoors | Mar 14 – Apr 14 |
| Peppers | Transplant outdoors | May 14 – Jun 14 |
| Eggplant | Start seeds indoors | Mar 31 – Apr 29 |
| Eggplant | Transplant outdoors | May 14 – Jun 14 |
| Cucumbers | Sow outdoors | May 14 – Jul 12 |
| Summer squash | Sow outdoors | Apr 30 – Jun 14 |
| Winter squash & pumpkins | Sow outdoors | May 14 – Jun 14 |
| Snap beans | Sow outdoors | Apr 30 – Aug 4 |
| Sweet corn | Sow outdoors | Apr 30 – Jul 2 |
| Watermelon & cantaloupe | Sow outdoors | May 14 – Jun 14 |
| Okra | Sow outdoors | May 7 – May 30 |
| Peas | Sow outdoors (spring) | Mar 14 – Apr 29 |
| Potatoes | Plant outdoors (spring) | Mar 14 – May 14 |
| Onions | Start seeds indoors (spring) | Jan 31 – Feb 27 |
| Onions | Transplant outdoors (spring) | Mar 31 – Apr 29 |
| Broccoli | Start seeds indoors (spring) | Feb 24 – Mar 30 |
| Broccoli | Transplant outdoors (spring) | Mar 31 – Apr 29 |
| Broccoli | Start seeds indoors (fall) | Jun 22 – Jul 2 |
| Broccoli | Transplant outdoors (fall) | Jul 22 – Aug 2 |
| Cabbage | Start seeds indoors (spring) | Feb 14 – Mar 14 |
| Cabbage | Transplant outdoors (spring) | Mar 14 – Apr 14 |
| Cabbage | Start seeds indoors (fall) | Jun 12 – Jul 12 |
| Cabbage | Transplant outdoors (fall) | Jul 12 – Aug 12 |
| Leaf lettuce | Sow outdoors (spring) | Mar 14 – May 30 |
| Leaf lettuce | Sow outdoors (fall) | Jul 12 – Aug 28 |
| Spinach | Sow outdoors (spring) | Mar 7 – Apr 20 |
| Spinach | Sow outdoors (fall) | Jul 29 – Sep 2 |
| Radishes | Sow outdoors (spring) | Mar 14 – May 4 |
| Radishes | Sow outdoors (fall) | Jul 12 – Sep 12 |
| Kale | Sow outdoors (spring) | Mar 31 – Apr 20 |
| Kale | Sow outdoors (fall) | Jul 5 – Aug 12 |
| Carrots | Sow outdoors (spring) | Apr 14 – May 30 |
| Carrots | Sow outdoors (fall) | Jun 12 – Jul 28 |
| Garlic | Plant outdoors (fall) | Oct 12 – Nov 12 |
Where these dates come from
ZIP code 99350 is matched to the NOAA station Prosser, WA, 2.6 km away, at 830 ft elevation (135 ft higher than the ZIP code's center). Nearby stations for comparison:
| Station | Elevation | Avg last frost | Avg first frost |
|---|---|---|---|
| Prosser, WA (used) | 830 ft | Apr 14 | Oct 22 |
| Sunnyside, WA | 747 ft | May 1 | Oct 6 |
| Sunnyside #2, WA | 805 ft | Apr 30 | Oct 8 |
| Richland, WA | 373 ft | Apr 6 | Oct 24 |
Frost dates near Prosser
- Grandview, WA 98930 · last May 1, first Oct 6
- Mabton, WA 98935 · last May 1, first Oct 6
- Sunnyside, WA 98944 · last May 1, first Oct 6
- Benton City, WA 99320 · last Apr 6, first Oct 24
- Paterson, WA 99345 · last Apr 14, first Oct 21
- Outlook, WA 98938 · last May 1, first Oct 6
- West Richland, WA 99353 · last Apr 6, first Oct 24
- Granger, WA 98932 · last May 1, first Oct 6
Frequently asked questions
- When is the last frost in Prosser, WA?
- For ZIP code 99350, the average last spring frost (32°F) is April 14, based on NOAA 1991-2020 climate normals for the Prosser, WA weather station. There is a 1-in-10 chance of a frost after May 7.
- When is the first frost in Prosser, WA?
- The average first fall frost (32°F) is October 22. There is a 1-in-10 chance of frost by October 5, so tender plants are at risk from that point on.
- How long is the growing season in 99350?
- In half of years, the frost-free season (between the last and first 32°F frost) is at least 189 days long.
- Are these frost dates a forecast?
- No. They are 30-year averages and probabilities from NOAA weather station records. Use them to plan, then watch your local forecast before planting tender crops or leaving them uncovered.