Frost dates Washington Tri-Cities

Frost dates in Tri-Cities, Washington

Last spring frost Apr 27 First fall frost Oct 11 NOAA 1991-2020

The average last spring frost in Tri-Cities is Apr 27 and the average first fall frost is Oct 11, measured at PASCO TRI CITIES AP, about 4 miles from the city centre. Below: how sure those dates are, and what to start, sow and set out each week.

Average last spring frostApr 2732°F, 50% probability
Average first fall frostOct 1132°F, 50% probability
Days between them167average frost-free season
How sure is that date? NOAA probability levels for a 32°F freeze
ProbabilityFreeze still comes after (spring)First freeze has come by (fall)
90%Apr 8Oct 27
50%Apr 27Oct 11
10%May 10Sep 28

Read it like this: in half of years the last 32°F freeze of spring comes after Apr 27, and in 1 year in 10 there is still a freeze after May 10. In 1 year in 10 the first fall freeze arrives before Sep 28.

Measured at: PASCO TRI CITIES AP, WA (NOAA station USW00024163), about 4 miles away at 407 ft, 21 years of record in 1991-2020.

Other stations nearby, for comparison:

  • KENNEWICK, WA (NOAA station USC00454154), about 5 miles away at 390 ft, 27 years of record in 1991-2020: last spring frost Mar 28, first fall frost Oct 30
  • RICHLAND, WA (NOAA station USC00457015), about 6 miles away at 373 ft, 30 years of record in 1991-2020: last spring frost Apr 6, first fall frost Oct 24

Seed-starting schedule for Tri-Cities, WA

Counted from a last frost date of Apr 27 (the average: a freeze comes later than this in half of years).

  1. Week of Jan 1914 weeks before last frost
    • Start seeds indoorsLeekJan 19 to Feb 2
    • Start seeds indoorsOnionJan 19 to Feb 2
  2. Week of Jan 2613 weeks before last frost
    • Start seeds indoorsParsleyJan 26 to Feb 2
  3. Week of Feb 212 weeks before last frost
    • Start seeds indoorsCeleryFeb 2 to Feb 16
    • Start seeds indoorsSpinachFeb 2 to Feb 16
  4. Week of Feb 1610 weeks before last frost
    • Start seeds indoorsBroccoliFeb 16 to Mar 2
    • Start seeds indoorsBrussels sproutFeb 16 to Mar 2
    • Start seeds indoorsCabbageFeb 16 to Mar 2
    • Start seeds indoorsKaleFeb 16 to Mar 2
    • Start seeds indoorsKohlrabiFeb 16 to Mar 2
  5. Week of Feb 239 weeks before last frost
    • Start seeds indoorsLettuceFeb 23 to Mar 2
  6. Week of Mar 28 weeks before last frost
    • Start seeds indoorsCauliflowerMar 2 to Mar 16
    • Start seeds indoorsEggplantMar 2 to Mar 16
    • Start seeds indoorsPepperMar 2 to Mar 16
    • Start seeds indoorsSwiss chardMar 2 to Mar 16
  7. Week of Mar 97 weeks before last frost
    • Start seeds indoorsTomatoMar 9 to Mar 23
  8. Week of Mar 166 weeks before last frost
    • Start seeds indoorsBasilMar 16 to Mar 30
    • Set transplants outSpinachMar 16 to Apr 6
  9. Week of Mar 304 weeks before last frost
    • Sow seed outdoorsRadishMar 30 to Apr 13
    • Sow seed outdoorsSwiss chardfrom Mar 30
    • Sow seed outdoorsTurnipMar 30 to Apr 13
    • Set transplants outBroccoliMar 30 to Apr 13
    • Set transplants outBrussels sproutMar 30 to Apr 13
    • Set transplants outCabbagefrom Mar 30
    • Set transplants outKalefrom Mar 30
    • Set transplants outKohlrabifrom Mar 30
    • Set transplants outLeekfrom Mar 30
    • Set transplants outLettuceMar 30 to Apr 6
    • Set transplants outOnionfrom Mar 30
  10. Week of Apr 63 weeks before last frost
    • Start seeds indoorsCucumberApr 6 to Apr 13
    • Set transplants outParsleyApr 6 to Apr 13
  11. Week of Apr 132 weeks before last frost
    • Start seeds indoorsMelonApr 13 to Apr 20
    • Start seeds indoorsPumpkinApr 13 to Apr 20
    • Start seeds indoorsSummer squashApr 13 to Apr 20
    • Start seeds indoorsWinter squashApr 13 to Apr 20
    • Start seeds indoorsZucchiniApr 13 to Apr 20
    • Plant seed potatoes outdoorsPotatoApr 13 to Apr 20
    • Set transplants outCauliflowerfrom Apr 13
    • Set transplants outSwiss chardfrom Apr 13
  12. Week of Apr 27Last frost week
    • Sow seed outdoorsBeanfrom Apr 27
    • Sow seed outdoorsParsnipfrom Apr 27
    • Set transplants outCeleryfrom Apr 27
  13. Week of May 41 week after last frost
    • Plant slips outdoorsSweet potatoMay 4 to May 11
    • Set transplants outCucumberMay 4 to May 11
    • Set transplants outTomatoMay 4 to May 11
  14. Week of May 112 weeks after last frost
    • Set transplants outBasilfrom May 11
    • Set transplants outEggplantMay 11 to May 18
    • Set transplants outMelonfrom May 11
    • Set transplants outPepperfrom May 11
    • Set transplants outPumpkinfrom May 11
    • Set transplants outSummer squashfrom May 11
    • Set transplants outWinter squashfrom May 11
    • Set transplants outZucchinifrom May 11

Not in the city centre? Look up your own ZIP code

More in Washington

Where the numbers come from

Sources and method

Frost dates. U.S. Climate Normals 2020: U.S. Annual/Seasonal Climate Normals (1991-2020), NOAA National Centers for Environmental Information (Palecki, Durre, Applequist, Arguez and Lawrimore, 2021). These are 30-year station statistics for 1991 to 2020. "Frost" here means an air temperature of 32°F or lower at the station. We use 6,539 stations that have at least 10 years of record and a published date; 176 more are stations where a freeze is too rare for NOAA to date, and 180 are stations cold enough to have frost risk all year.

Probability level. The headline dates are NOAA's 50% level: in half of years the last spring freeze comes later than that date, and in half of years the first fall freeze comes earlier. The 10% and 90% levels are shown next to them. For tender plants, the 10% spring date is the safer one to plan from: a freeze follows it in only 1 year in 10.

Which station. Your ZIP code is matched to the nearest of those stations by straight-line distance from the ZIP code's centre point (GeoNames, geonames.org, CC BY 4.0; 40,979 ZIP codes in the 50 states and DC). Nearest is not always most alike: a station on a hill, in a valley or by the water can differ from your garden by a week or more. That is why the station's name, distance and elevation are shown with every result, along with two more stations nearby. NOAA makes the same point: the normals are limited between stations as distance grows horizontally and vertically.

Seed-starting offsets. Growing Vegetables: When to Plant Your Vegetable Garden, a fact sheet from University of New Hampshire Extension. It gives each crop's weeks from seed to transplant and when it is safe to set out or sow relative to the last frost. The method is theirs too: take the planting date and count back the seed-to-transplant weeks. Where the fact sheet gives a range of planting dates, the start-indoors dates count back from the earliest one. The figures used, as the fact sheet states them:

  • Basil: Seed to transplant 6-8 weeks; set out 2 weeks after last frost.
  • Bean: Plant seed in garden after last frost.
  • Broccoli: Seed to transplant 4-6 weeks; set out 2-4 weeks before last frost.
  • Brussels sprout: Seed to transplant 4-6 weeks; set out 2-4 weeks before last frost.
  • Cabbage: Seed to transplant 4-6 weeks; set out 4 weeks before last frost.
  • Cauliflower: Seed to transplant 4-6 weeks; set out 2 weeks before last frost.
  • Celery: Seed to transplant 10-12 weeks; set out after last frost.
  • Cucumber: Seed to transplant 3-4 weeks; set out 1-2 weeks after last frost.
  • Eggplant: Seed to transplant 8-10 weeks; set out 2-3 weeks after last frost.
  • Kale: Seed to transplant 4-6 weeks; set out 4 weeks before last frost.
  • Kohlrabi: Seed to transplant 4-6 weeks; set out 4 weeks before last frost.
  • Leek: Seed to transplant 8-10 weeks; set out 4 weeks before last frost.
  • Lettuce: Seed to transplant 4-5 weeks; set out 3-4 weeks before last frost.
  • Melon: Seed to transplant 3-4 weeks; set out 2 weeks after last frost.
  • Onion: Seed to transplant 8-10 weeks; set out 4 weeks before last frost.
  • Parsley: Seed to transplant 9-10 weeks; set out 2-3 weeks before last frost.
  • Parsnip: Plant seed in garden after last frost.
  • Pepper: Seed to transplant 8-10 weeks; set out 2 weeks after last frost.
  • Potato: Plant seed potatoes in garden 1-2 weeks before last frost.
  • Pumpkin: Seed to transplant 3-4 weeks; set out 2 weeks after last frost.
  • Radish: Plant seed in garden 2-4 weeks before last frost.
  • Spinach: Seed to transplant 4-6 weeks; set out 3-6 weeks before last frost.
  • Summer squash: Seed to transplant 3-4 weeks; set out 2 weeks after last frost.
  • Sweet potato: Plant slips in garden 1-2 weeks after last frost.
  • Swiss chard: Seed to transplant 4-6 weeks; set out 2 weeks before last frost; or plant seed in garden 4 weeks before last frost.
  • Tomato: Seed to transplant 6-8 weeks; set out 1-2 weeks after last frost.
  • Turnip: Plant seed in garden 2-4 weeks before last frost.
  • Winter squash: Seed to transplant 3-4 weeks; set out 2 weeks after last frost.
  • Zucchini: Seed to transplant 3-4 weeks; set out 2 weeks after last frost.

Not on the calendar. The same fact sheet times these by soil temperature, not by frost date, so they get no date here:

  • Pea: as soon as the soil thaws.
  • Beet: when the soil warms to 45°F.
  • Carrot: when the soil warms to 60°F.
  • Corn: when the soil warms to 60°F.
  • Cucumber, pumpkin, summer squash, winter squash, zucchini: direct-sow when the soil warms to 70°F.

Rutabaga is left out: the fact sheet gives it New Hampshire calendar dates, not a frost offset.

The fact sheet was written for New Hampshire. The offsets are relative to the last frost, so they carry to other places with a spring frost, but where winters are mild many cool-season crops are grown in fall and winter instead. Your state's extension service has the local calendar. Dates are 30-year statistics, not a forecast: check the forecast before setting tender plants out.

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