Frost dates for 99176Thornton, WA
The average last spring frost in Thornton is May 7 and the average first fall frost is September 28, giving a typical growing season of about 144 days. Dates are based on NOAA 1991-2020 climate normals.
- Average last spring frost
- May 7
- Average first fall frost
- Sep 28
- Growing season
- 144 days
Safer after May 25 (1-in-10 risk)
Possible by Sep 11 (1-in-10 risk)
Last frost typically in 185 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) | May 8 · May 31 · Jul 5 | Aug 26 · Sep 13 · Sep 28 | 103 days | 100% |
| Frost / freeze (32°F) | Apr 22 · May 7 · May 25 | Sep 11 · Sep 28 · Oct 16 | 144 days | 100% |
| Hard freeze (28°F) | Mar 25 · Apr 20 · May 8 | Sep 25 · Oct 12 · Nov 2 | 175 days | 100% |
Planting calendar for Thornton
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 | Apr 6 – Jun 12 |
| Tomatoes | Transplant outdoors | May 23 – Jun 8 |
| Peppers | Start seeds indoors | Apr 6 – May 7 |
| Peppers | Transplant outdoors | Jun 6 – Jul 7 |
| Eggplant | Start seeds indoors | Apr 23 – May 22 |
| Eggplant | Transplant outdoors | Jun 6 – Jul 7 |
| Cucumbers | Sow outdoors | Jun 6 – Jun 18 |
| Summer squash | Sow outdoors | May 23 – Jul 7 |
| Winter squash & pumpkins | Sow outdoors | Jun 6 – Jul 7 |
| Snap beans | Sow outdoors | May 23 – Jul 11 |
| Sweet corn | Sow outdoors | May 23 – Jun 8 |
| Watermelon & cantaloupe | Sow outdoors | Jun 6 – Jul 7 |
| Okra | Sow outdoors | May 30 – Jun 22 |
| Peas | Sow outdoors (spring) | Apr 6 – May 22 |
| Potatoes | Plant outdoors (spring) | Apr 6 – Jun 6 |
| Onions | Start seeds indoors (spring) | Feb 23 – Mar 22 |
| Onions | Transplant outdoors (spring) | Apr 23 – May 22 |
| Broccoli | Start seeds indoors (spring) | Mar 19 – Apr 22 |
| Broccoli | Transplant outdoors (spring) | Apr 23 – May 22 |
| Broccoli | Start seeds indoors (fall) | May 29 – Jun 8 |
| Broccoli | Transplant outdoors (fall) | Jun 28 – Jul 9 |
| Cabbage | Start seeds indoors (spring) | Mar 9 – Apr 6 |
| Cabbage | Transplant outdoors (spring) | Apr 6 – May 7 |
| Cabbage | Start seeds indoors (fall) | May 19 – Jun 18 |
| Cabbage | Transplant outdoors (fall) | Jun 18 – Jul 19 |
| Leaf lettuce | Sow outdoors (spring) | Apr 6 – Jun 22 |
| Leaf lettuce | Sow outdoors (fall) | Jun 18 – Aug 4 |
| Spinach | Sow outdoors (spring) | Mar 30 – May 13 |
| Spinach | Sow outdoors (fall) | Jul 5 – Aug 9 |
| Radishes | Sow outdoors (spring) | Apr 6 – May 27 |
| Radishes | Sow outdoors (fall) | Jun 18 – Aug 19 |
| Kale | Sow outdoors (spring) | Apr 23 – May 13 |
| Kale | Sow outdoors (fall) | Jun 11 – Jul 19 |
| Carrots | Sow outdoors (spring) | May 7 – Jun 22 |
| Carrots | Sow outdoors (fall) | May 19 – Jul 4 |
| Garlic | Plant outdoors (fall) | Sep 18 – Oct 19 |
Where these dates come from
ZIP code 99176 is matched to the NOAA station Rosalia, WA, 12.3 km away, at 2,420 ft elevation (28 ft lower than the ZIP code's center). Nearby stations for comparison:
| Station | Elevation | Avg last frost | Avg first frost |
|---|---|---|---|
| Rosalia, WA (used) | 2,420 ft | May 7 | Sep 28 |
| St John, WA | 1,952 ft | Apr 29 | Oct 4 |
| Spokane 17 Ssw, WA | 2,267 ft | May 30 | Sep 4 |
| Plummer 3 Wsw, ID | 2,905 ft | May 11 | Oct 5 |
Frost dates near Thornton
- Oakesdale, WA 99158 · last May 7, first Sep 28
- Rosalia, WA 99170 · last May 7, first Sep 28
- Malden, WA 99149 · last May 7, first Sep 28
- Steptoe, WA 99174 · last May 7, first Sep 28
- Saint John, WA 99171 · last Apr 29, first Oct 4
- Belmont, WA 99104 · last May 7, first Sep 28
- Garfield, WA 99130 · last May 6, first Sep 29
- Farmington, WA 99128 · last May 15, first Sep 21
Frequently asked questions
- When is the last frost in Thornton, WA?
- For ZIP code 99176, the average last spring frost (32°F) is May 7, based on NOAA 1991-2020 climate normals for the Rosalia, WA weather station. There is a 1-in-10 chance of a frost after May 25.
- When is the first frost in Thornton, WA?
- The average first fall frost (32°F) is September 28. There is a 1-in-10 chance of frost by September 11, so tender plants are at risk from that point on.
- How long is the growing season in 99176?
- In half of years, the frost-free season (between the last and first 32°F frost) is at least 144 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.