Seattle's summer dry season, and why plant choice has to satisfy both extremes

Seattle's 1991–2020 normals run to roughly 36–39 inches of precipitation a year depending on the station, concentrated in the cool half of the year. July is the driest month at around half an inch; November the wettest at close to six inches.

That seasonal split, not the annual total, is what gardens have to survive. The University of Washington's Climate Impacts Group reported in September 2025 that summer 2025 was particularly dry statewide, with below-normal June and July precipitation and above-normal June–August temperatures, and that projections show Washington summers are likely to become drier. Washington's Department of Ecology lists drier summers among its headline climate indicators.

The heat ceiling has already moved. During the June 2021 heat dome, Sea-Tac reached 108°F on 28 June, an all-time record, after 104°F the day before. In the previous 126 years of records, Seattle had reached 100°F three times in total; it did so on three consecutive days that week.

Successful Seattle gardens therefore need species that tolerate winter saturation and summer drought — a narrower set than either extreme demands on its own. Anything that rots in a wet winter or collapses in a rainless August fails here regardless of its hardiness rating.

Timing beats volume. Mulch applied in spring, while the soil still holds winter moisture, conserves far more than the same mulch applied in August after the profile has dried. Watering deeply and infrequently through July and August encourages the deep rooting that carries plants through the next dry season.

We have not found an official NOAA figure quantifying how many days longer Seattle's dry season has become; widely quoted numbers of that kind come from third-party analyses rather than the record itself.