Short answer: A 3-inch layer of bark or wood-chip mulch keeps bed soil cooler and sharply reduces evaporation from the surface, which means you can water less often during the July-to-September dry stretch. For a typical Northeast Seattle lot, that is a real reduction in outdoor water use during exactly the months when Seattle Public Utilities charges its highest summer rates.
The Seattle summer paradox
Seattle has a reputation as a wet city, and it earns it from October through May. Then the rain stops. July and August are reliably among the driest months anywhere in the lower 48, often with less than an inch of rain each, and the dry stretch has been running longer and hotter. The June 2021 heat dome, when the city hit 108°F, is the extreme example, but 90-degree days in August are now ordinary.
That is the window when your garden needs the most water and the city charges the most for it. Seattle Public Utilities uses a tiered summer rate from mid-May through mid-September, so the water you use for irrigation in August costs more per unit than the water you use in January. Check your own bill or the Seattle Public Utilities water rates page for the current tiers.
How mulch actually saves water
Mulch does four things to the soil under it, and all four reduce how much water you need to add.
It blocks evaporation. Bare soil loses water straight to the air, especially on a hot, breezy afternoon. A mulch layer breaks that path. Soil under 3 inches of bark stays noticeably damper a week after watering than bare soil next to it.
It keeps soil cooler. Mulched soil on a 90-degree day can be 10 or more degrees cooler at root depth than bare soil. Cooler soil loses less water and stresses roots less.
It stops crusting and runoff. Bare soil in Seattle's clay-heavy lots crusts over in summer, so water from a sprinkler beads up and runs off into the street. Mulch keeps the surface open so water soaks in.
It holds water in the root zone. Organic mulch acts like a sponge, absorbing water and releasing it slowly into the soil below instead of letting it drain past the roots.
The combined effect is that plants in mulched beds can go longer between waterings and show less wilt when you miss one.
Rough math for a typical lot
Take a 6,000-square-foot lot in View Ridge or Wedgwood with about 1,500 square feet of planted beds. Watering those beds with a sprinkler or hose during the dry season, three times a week, is a common schedule.
Mulching the beds and watering deeply instead of often usually lets homeowners cut that to once or twice a week without plant stress. The savings depend on the plants, the sun exposure and how you water, so we will not promise a specific dollar figure. Over a 14-week dry season, though, cutting bed irrigation by a third to a half is a reasonable expectation, and it comes out of the most expensive tier on the bill.
The mulch itself, for 1,500 square feet at 3 inches, is about 14 cubic yards. That is a one-time cost that lasts two summers.
Mulch plus three other cheap wins
Mulch works best as part of a simple system:
Drip irrigation under the mulch. A $40 drip line puts water at the roots, under the mulch, where none of it evaporates. This is the single biggest upgrade after mulch itself.
Water early in the morning. Watering at 6 a.m. instead of 6 p.m. means less evaporation and less fungal disease.
Water deeply and less often. One long soak a week grows deeper roots than a short sprinkle every day. Mulch makes this possible because it holds the water in the root zone between soaks.
Rethink the lawn. Lawns are the thirstiest thing in most Seattle yards. Converting a strip of lawn to a mulched bed of drought-tolerant plants saves more water than any amount of mulch on the existing beds.
Which mulch holds water best?
For water retention, coarse organic mulch beats everything else. Arborist chips and medium bark are the best choices; they absorb water, shade the soil and stay loose. Fine bark crusts and sheds water. Rock and gravel get hot and reflect heat onto plants, so while they stop evaporation they can increase plant stress. See [What kind of mulch should I use?] for the full comparison.
Frequently asked questions
Does mulch mean I never have to water? No. It means you water less often and your plants survive the gaps better. Established drought-tolerant shrubs under good mulch may need no summer water at all. Hydrangeas, annuals and vegetables still need regular water.
Can mulch hold too much water in winter? In very wet, poorly drained spots, a thick mulch layer can keep soil soggy. Use a thinner layer, 2 inches, in those beds and never pile it against stems.
Does mulch help my lawn? Not directly. Mulch is for beds. For lawns, mow higher in summer, leave the clippings, and water deeply once a week.
Will mulching help with Seattle's summer watering restrictions? Seattle rarely imposes mandatory restrictions, but SPU does issue voluntary conservation requests in dry years. A mulched garden is already ahead of those requests.
Is it too late to mulch for this summer if I wait until June? No. Water the beds deeply first, then mulch. Just do not mulch dry soil during a heat wave.
Serving View Ridge, Wedgwood, Laurelhurst, Windermere and Ravenna.