Short answer: Bark washes off slopes because it is too fine, too round, or applied on top of smooth soil with nothing to hold it. On a slope, use coarse arborist chips or shredded bark that knits together, skip nuggets and fine bark, key the mulch into the soil, and add an edge or berm at the bottom of anything steeper than about a 1-in-5 grade.
If you live on one of the lake-facing slopes in Laurelhurst or Windermere, on the hill that drops from View Ridge toward Magnuson Park, or along the ravine edges in Ravenna, you already know the problem. You mulch in October and by February a good share of it is on the sidewalk, in the gutter, or piled against the fence at the bottom of the yard. Here is why, and how to stop it.
Why Northeast Seattle slopes are hard on mulch
Three things combine here. First, the terrain: these neighborhoods sit on glacial hills that drop steeply toward Lake Washington and Union Bay. Second, the rain: Seattle gets most of its 37 inches a year between October and April, often in long, soaking storms that saturate the soil and then keep coming. Third, the soil: much of it is glacial till, which is dense and sheds water once it is saturated rather than absorbing it.
Put light, loose mulch on a smooth, saturated, sloping surface and it floats. That is the whole story.
Mulch choices ranked for slopes
Arborist wood chips. Best option; irregular pieces lock together; leaf and twig fragments act like a net
Shredded bark ("gorilla hair" or shredded cedar). Very good solution; fibers mat together; the standard choice for steep ornamental beds
Medium bark. Fair option; fine on gentle slopes; migrates on anything steep
Compost. Poor choice; washes out in the first big storm; use only with a jute net
Fine bark. Also a poor choice; floats and crusts; avoid on any slope
Bark nuggets. Worst solution; round, light, and rolls downhill on its own
Crushed rock. Good, but with caveats; stays put, but only suits sun-loving, drought-tolerant plantings; heats up
Installation techniques that keep mulch in place
Rough up the soil first. Rake the slope horizontally to create shallow grooves across the grade. Mulch settles into those grooves instead of sliding over a smooth surface.
Key the top edge in. At the top of the slope, dig a shallow trench 2–3 inches deep and fill it with mulch. This anchors the layer and stops the whole sheet from sliding.
Build contour speed bumps. On long slopes, every 6–8 feet downhill, rake a low ridge of soil or set a line of rocks or a half-buried log across the grade. Each one catches mulch that starts to move.
Use a jute or coir net on steep or newly planted slopes. Pin the net to the soil, plant through it, then mulch over it. The net holds everything for a year or two until roots take over, then rots away.
Edge the bottom. A low berm, a row of stones, a steel edge or a timber at the base of the slope stops mulch from reaching the sidewalk or the lawn.
Go slightly thinner. 2–3 inches on a slope, not 4. A heavy layer is more likely to slide as a sheet.
Plant it up. The permanent fix is roots. Groundcovers like kinnikinnick, salal, creeping Oregon grape and sword fern hold slopes better than any mulch, and mulch is what gets them established.
Drainage and erosion red flags
Mulch loss is annoying. Soil loss is a different problem. Call someone, whether that is a landscaper, a drainage contractor or the city, if you see any of these:
Gullies or channels cutting into the slope after storms.
Water pooling at the top of the slope or against a foundation.
Soil, not just bark, on the sidewalk after rain.
A retaining wall that is bulging, leaning or weeping through the face.
Downspouts that discharge directly onto the slope.
The last one is the most common cause we see. Redirecting a downspout into a drain or a dispersion trench fixes more hillside erosion than any mulching technique.
Maintenance schedule for sloped beds
After each major fall and winter storm: Walk the slope; rake displaced mulch back uphill
Late October: Full mulch; refresh contour ridges and bottom edge
February: Repair bare patches at the top of the slope
April: Top-up; replace any failed jute netting
June: Check that plants are filling in; add groundcovers to bare spots
Frequently asked questions
Should I use rock instead of bark on a steep slope? Only if the plants there are sun-loving and drought-tolerant and the look fits the house. Rock stays put, but it heats the soil and offers nothing to rhododendrons, ferns or hydrangeas. For most Northeast Seattle slopes, shredded bark or arborist chips over a jute net is the better answer.
How thick should mulch be on a hill? 2–3 inches. Thicker layers are more likely to slide.
Does dyed mulch run in the rain? Yes. The dye leaches in heavy rain, staining sidewalks and leaving the mulch gray by spring. Avoid it everywhere, and especially on slopes.
Can I put mulch over landscape fabric on a slope? No. Mulch slides off fabric even faster than off bare soil. If you need something under the mulch, use jute netting, which has texture and rots away.
My slope is too steep to mulch at all. What then? Above roughly a 2-to-1 grade, mulch alone will not hold. That calls for terracing, a retaining wall or a planted jute-net system, and it is worth getting a professional opinion.
Serving View Ridge, Wedgwood, Laurelhurst, Windermere and Ravenna.