GreenlyWhat to plant in August in the Pacific Northwest
Worked out for 11 cities from their own weather and frost dates, by zone.
What to sow and plant in August in the Pacific Northwest, by zone: frost-free, highs above 82 °F (Portland, Boise and 6 more): zucchini, green beans, basil, bok choy and borage; frost-free, highs of 72–82 °F (Seattle, Olympia and 1 more): zucchini, green beans, sweet corn, basil, bok choy and 2 more.
In the Pacific Northwest, August is summer.
Frost-free, highs above 82 °F: Portland, Boise, Spokane, Salem and 4 more
In August, on average, these cities have highs of 85 to 93 °F and lows of 55 to 65 °F, 0.2 to 0.6 in of rain on 1 to 3 days; the day lasts 13 h 56 min to 14 h 19 min (on the 15th). A dry month: whatever is sown needs watering.
No frost in August in 10 years of data: frost-tender crops can go in too, if it is warm enough for them.
| How | What | Where |
|---|---|---|
| sow direct | Zucchini, green beans, basil, bok choy, borage | all |
| sow direct | Watermelon, pumpkin, sweet corn, chayote, corn, Florence fennel, kiwano, luffa, moringa, Napa cabbage, okra, peanut, soybean | Portland and Salem |
Compared with July: no longer sown — cucumber, melon and achocha.
Not starting in August: tomato (March, started indoors), bell pepper (March, started indoors), lettuce (September, started indoors), peas (September, sown direct) and spinach (September, started indoors) — dates for Salem.
Ready to harvest in August in Salem, if sown in the right month: cabbage (April), zucchini (June), green beans (June), sweet corn (May), basil (June), bok choy (June), borage (June) and corn (May).
Frost-free, highs of 72–82 °F: Seattle, Olympia and Tacoma
In August, on average, these cities have highs of 79 to 81 °F and lows of 56 to 58 °F, 1.0 to 1.3 in of rain on 4 to 6 days; the day lasts 14 h 16 min to 14 h 19 min (on the 15th).
No frost in August in 10 years of data: frost-tender crops can go in too, if it is warm enough for them.
| How | What | Where |
|---|---|---|
| sow direct | Broad bean, peas, cabbage, beet, parsley, cilantro, arugula, chamomile, chickpea, dill, mustard greens | Seattle |
| plant sets | Onion, shallot | Seattle |
| start indoors, plant out in September | Lettuce, cauliflower, Swiss chard, kale, kohlrabi | Seattle |
| start indoors, plant out in October | Broccoli | Seattle |
| sow direct | Zucchini, green beans, sweet corn, basil, bok choy, borage, corn | all |
| sow direct | Watermelon, pumpkin, chayote, Florence fennel, kiwano, moringa, Napa cabbage, peanut, soybean | Tacoma |
Compared with July: new — lettuce, Swiss chard, kale and kohlrabi; no longer sown — cucumber, melon, Brussels sprouts, luffa, okra and achocha.
Not starting in August: tomato (March, started indoors), bell pepper (March, started indoors) and spinach (September, started indoors) — dates for Olympia.
Ready to harvest in August in Olympia, if sown in the right month: broad bean (May), peas (June), cabbage (April), beet (June), green beans (June), sweet corn (May), parsley (June) and cilantro (June).
How we work it out
For each city we use its average weather in August and the months after (highs, lows and rain for 2016–2025, ERA5 reanalysis via Open-Meteo) and its average frost dates. A crop starts in August if the month's mean temperature reaches the minimum soil temperature for germination on its card (we use the mean air temperature as a stand-in for the soil's) and if, over its first months (up to three), the average highs stay within its group's range, with no frost for crops that cannot take it. Crops started indoors count from the month they are planted out.
Temperature groups from Colorado State University Extension: cool-season crops prefer 60–80 °F and lose quality in heat; hardy ones grow in days as cool as 40 °F, semi-hardy ones from 40–50 °F (we use 50 °F); warm-season crops need days above 55 °F, and the most tender (tomato, pepper, eggplant, melon, watermelon, squash) above 60 °F, up to 95 °F. Crops the guide does not name go into the stricter group by their frost hardiness. Fava (broad) beans, which the guide does not name, count as hardy: hardy varieties are sown in fall and overwinter (RHS). Garlic and bulbs: a month before the first frost, and later only while the ground is not frozen (as on each city's page).
This is a general rule from averages: your garden can be a week or two earlier or later, and a greenhouse or a cold frame brings the season forward. Greenly works out the dates for your location.
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