Short answer: Seattle has two good mulching windows: late fall (October through November, after the leaves come down) and mid-spring (April through May, once the soil has warmed and dried a bit). If you only mulch once a year, do it in fall. Our wet, mild winters are when bare soil takes the most damage.
Most people think of mulching as a spring chore. In the Northwest it is really a fall chore with a spring touch-up, and getting the timing right makes the difference between mulch that works for two years and mulch that washes into the street by February.
Fall: October to November
This is the main event. Once the maples in Wedgwood and the oaks along Ravenna Boulevard have dropped, the beds are visible, the weeds are slowing down and the heavy rain has not fully arrived.
What fall mulch does:
Keeps winter rain from pounding bare soil into a compacted crust.
Suppresses the cool-season weeds (shotweed, chickweed, annual bluegrass) that germinate in our mild winters.
Insulates roots against the one or two hard freezes we usually get, including the cold snaps that come off the Fraser River in December and January.
Lets you use the free leaf mulch that is literally falling in your yard.
Do it after you have cut back perennials and cleared summer annuals, and before the ground is saturated. A dry week in late October is ideal.
Winter: December to February
Winter is a check-and-repair season, not a mulching season. The soil is too wet to work and new mulch tends to sit on top of a waterlogged bed.
Walk the beds after big storms. On sloped lots in Laurelhurst and Windermere, look for bark that has slid downhill, bare patches at the top of slopes and mulch piled against fences and walls. Rake it back into place. If a bed has gone bare, a thin top-up is fine, but save the full job for spring.
Spring: April to May
The second window. The main mistake people make is mulching too early. In March the soil in Seattle is cold and saturated, and a thick mulch layer traps that cold and wet, delaying growth for weeks. Wait until the soil has warmed and you can walk the beds without sinking in.
Spring is the time for:
Topping up beds mulched last fall to restore 2–3 inches.
Compost mulch on vegetable and perennial beds, applied after you have divided and replanted.
Mulching new plantings as soon as they go in.
Refreshing the look of front-yard beds before the summer growing season.
Time it so spring bulbs and emerging perennials have already shown themselves; it is much easier to mulch around visible shoots than to guess where they are.
Summer: June to September
Summer is for top-ups, not full re-mulching. Seattle gets very little rain from July through mid-September, and a good mulch layer applied in spring is what carries beds through that stretch. See our post on [how mulch cuts your summer water bill] for the details.
Two cautions:
Do not spread a thick layer of mulch over dry soil during a heat wave. It seals the dryness in. Water deeply first, then mulch.
Do not mulch right up against plants that are already stressed by heat; leave them room to breathe.
The one-glance calendar
January. Rake displaced mulch back; check slopes
February. Same; order mulch for spring if needed
March. Wait. Soil is too cold and wet
April. Full spring mulching once soil has warmed
May. Finish spring mulching; mulch new plantings
June. Deep water, then light top-up on thin spots
July. Top-ups only; never over dry soil
August. Leave beds alone
September. Light top-up ahead of fall; order fall mulch
October. Full fall mulching after leaf drop begins
November. Finish fall mulching before heavy rain sets in
December. Check mulch condition after storms
Special cases
New plantings. Mulch the day they go in, any month of the year, 2–3 inches, kept off the stem.
Newly seeded lawns. A very light layer of straw or peat, not bark. Bark will shade out the seed.
Vegetable beds. Compost in April, straw in June, pull everything into the soil in October.
Shady, damp lots. Beds under the big trees along the Ravenna Park ravine and in parts of Laurelhurst stay wet long into spring. Push the spring window later, into May, and use a slightly thinner layer so the soil can breathe.
Beds near Lake Washington. The lakefront lots in Windermere and Laurelhurst get more wind and reflected light. They dry out faster in summer, so make sure the spring layer is a full 3 inches.
Frequently asked questions
Can I mulch in the rain? Light rain is fine. Heavy rain means you are working saturated soil and compacting it, so wait for a break.
Is it too late to mulch in December? Not too late, but not ideal. A thin top-up on bare beds is fine. Save the full job for April.
How long does bark mulch last in Seattle's climate? Medium bark holds up for one to two years. Fine bark is usually gone in a year. Arborist chips last two to three.
Should I remove old mulch before adding new? No. Rake it to break up any crust, then add fresh on top until the total is 2–3 inches. Only remove it if it has gone moldy and matted.
Does mulch keep my plants from freezing? It moderates soil temperature and protects roots. It does not protect leaves and stems from a hard frost; use frost cloth for that.
Serving View Ridge, Wedgwood, Laurelhurst, Windermere and Ravenna