Hillside Landscaping: Erosion Control with Rock and Boulders

The PNW hillside problem is universal: bare slope erodes in the first heavy rain, lawn won't establish or hold, ivy might look like a solution at year one but becomes a different problem by year five. The combination that works is almost always the same: rock and boulders for the structural slope-breaking, deep-rooted plants for soil binding, and managed drainage for the concentrated water flow that breaks any plant-only solution.

This guide covers the design, materials, plant choices, and PNW-specific notes for turning an eroding or bare slope into a landscape feature that holds itself together. Specific to Kitsap County and the Puget Sound region: lots of clay subsoil, long wet winters, exposed slope properties on waterfront and bluff lots.

Diagnose the slope first

Before designing anything, look at three things:

• Slope angle. Under 15% (a 1-foot rise per 7 feet of run): easy. 15-30%: moderate, most residential erosion problems live here. Over 30%: difficult, often needs engineered retaining walls or professional help.
• Water source. Where is the water that's causing the erosion coming from? Downspout? Surface runoff from higher ground? Groundwater seeping out of the slope itself? Each needs a different solution. Many properties have multiple sources.
• Soil type. Most Kitsap properties have clay subsoil 12 to 36 inches down. Clay slopes shed water rather than absorb it, which is why surface drainage matters more here than on sandy or loamy slopes.

If you have an active erosion problem (visible rutting, exposed roots, soil migration onto the lower yard), address the water source first. A beautifully terraced slope still fails if it's getting hit by uncontrolled downspout discharge.

The four tools of PNW hillside landscaping

1. Boulders (slope breakers)

Mid to large boulders set partially buried across the slope. Each boulder breaks the slope into shorter water-flow segments and acts as a soil anchor. Use mixed sizes: 12 to 24 inches for the bulk of the placement, with three or four larger feature boulders (24 to 36+ inches) at slope transitions.

• Material: Natural Boulders ($91.99/ton, mixed sizes) for the main placement. Larger rockery rocks (Columbia Granite $83.99/ton, St. Helens Volcanic $107.99/ton) for feature stones.
• Placement: partially buried (one-third to one-half), long axis perpendicular to the slope, staggered (not aligned). See our landscape boulders guide for placement details.

2. Rock terracing

For steeper slopes (over 20% grade), break the slope into shorter terraces with low rockery walls. Each terrace becomes a manageable planting bed; the cumulative wall height handles a much larger grade than a single tall wall would.

• Material: Rockery rocks for the wall stones.
• Wall height: keep individual terrace walls under 4 feet to avoid permit requirements in most Kitsap jurisdictions. Multiple stepped terraces handle higher overall grade.
• Drainage: 3/4 clean drain rock behind each wall as backfill. See our rockery walls guide for the full technique including batter and drainage.

3. Deep-rooted plants

The biological tool. Plants with extensive root systems bind soil that rocks alone can't reach (between rocks, across the open soil surface, in the top 18 inches of soil where most erosion happens).

Best PNW native shrubs: Pacific ninebark, snowberry, red-twig dogwood, salal, low Oregon grape. All have strong root systems and tolerate slope conditions.

Best PNW native ground covers: sword fern, kinnikinnick (sun), Pacific bleeding heart (shade).

Best non-native tough options: creeping juniper (sun, drought-tolerant), spreading cotoneaster (sun, evergreen), ornamental grasses like blue fescue.

4. Managed drainage

For concentrated water flow (downspouts, surface flow from higher ground, gutter overflow): route it through a managed feature rather than letting it sheet across the slope. The best tool here is a dry creek bed running across the slope (not down it), which catches the flow and directs it to a discharge point.

• See: our dry creek bed design and installation guide for a full build.
• For underground/sustained flow: a buried French drain across the slope handles seeping groundwater that surface features can't reach.

Two design approaches

Approach A: Naturalistic boulder + plant slope

Best for slopes under 25% grade. 8 to 20 boulders distributed across the slope, partially buried, staggered. PNW-native shrubs and ground covers planted between and around them. Drainage handled by a dry creek bed if needed. Looks like a natural rocky hillside rather than an engineered structure.

Pros: casual look, lower cost, works with the existing grade. Cons: doesn't add usable flat space; harder to maintain plantings on steeper sections.

Approach B: Terraced retaining walls

Best for steeper slopes (over 25% grade) or where you want usable planted beds. Two or three low rockery walls stepping up the slope, each creating a flat planting bed. Plants chosen for each terrace based on its exposure (top terraces often drier, bottom terraces often wetter).

Pros: creates usable garden space, handles steeper grade, looks intentional. Cons: more expensive, more work, more visible engineering.

Install sequence (combined approach)

Step 1: Solve the water source first. Reroute downspouts. Add a French drain at the top of the slope if upslope water is the problem. Without this step, anything else you build will be undermined.

Step 2: Set foundation boulders. The largest stones first, at visual anchor points. Bury one-third deep. Long axis across the slope.

Step 3: Build terrace walls (if using). Low rockery walls along contour lines. See rockery walls guide for the stacking and drainage technique.

Step 4: Place mid-size boulders. Fill in the slope between foundation stones and terraces. Stagger sizes, irregular placement.

Step 5: Build any drainage features. Dry creek bed across the slope if needed. Buried French drain for groundwater.

Step 6: Plant. Deep-rooted natives in the soil between rocks. Tighter spacing than you'd use on flat ground (plants help stabilize while they establish).

Step 7: Surface mulch. 2 to 3 inches of bark or wood chip mulch over exposed soil. Holds moisture, suppresses weeds, and prevents the topmost layer of soil from washing during establishment.

Step 8: Water through year one. New plants need water through their first dry summer. After establishment (year two), PNW natives typically don't need supplemental water.

PNW hillside plant guide by exposure

Sunny slopes (south or west facing)

• Shrubs: snowberry, low Oregon grape, kinnikinnick (groundcover), evergreen huckleberry, native serviceberry, spreading cotoneaster (non-native).
• Grasses: blue fescue, native sedges (Carex), little bluestem.
• Ground covers: creeping juniper, beach strawberry, dwarf phlox.

Shady slopes (north or east facing)

• Shrubs: salal, low Oregon grape, sword fern, deer fern, Pacific ninebark, red-twig dogwood.
• Ground covers: Pacific bleeding heart, native ginger, foamflower, vanilla leaf.

Wet slopes (groundwater seepage)

• Shrubs: red-twig dogwood, Pacific ninebark, salmonberry, twinberry honeysuckle.
• Ground covers: native sedges, wet-tolerant ferns.

Avoid

Lawn grass (shallow roots, mowing dangerous on slopes), English ivy (invasive, undermines other plants over time), shallow-rooted annuals. Lawn specifically is the single most common PNW hillside mistake: it doesn't anchor soil deeply, it requires mowing on dangerous terrain, and it dies in the dry months exposing bare soil.

Cost: real numbers for a 25 ft × 15 ft slope

Add terrace wall stones if using Approach B (another $400 to $1,000 per terrace depending on wall length and stone choice).

Labor adds significantly for slope work. Most contractors charge a premium for slope projects because lifting and placing boulders on grade is physically demanding and slower than flat-ground work.

PNW and Kitsap-specific notes

Bluff and waterfront properties. Slopes along Hood Canal, Henderson Bay, and other shoreline areas often have specific erosion-control regulations administered through Kitsap County or the city. Critical-area review may apply. Don't undertake significant slope work near shoreline without checking permitting requirements.

Heavy clay layer. Most Kitsap and PNW slopes have a clay subsoil that does not absorb water. Surface erosion management matters more here than on permeable soils. Plan for water that runs across the surface; don't assume it will soak in.

Wet-season construction. Avoid major slope work November through March. The wet conditions undermine boulder placement (soil shifts as you work) and slow plant establishment. Plan slope projects for April through October.

Permitting. Retaining walls over 4 feet require permits in Kitsap County and most cities. Critical-area work (steep slopes, shoreline) may trigger additional review. For walls under 4 feet on a typical residential lot, no permit needed.

Slope stabilization vs landscaping. If you have an actively unstable slope (visible sliding, deep cracks, recent slope failure), this is a stability problem that needs an engineer or geotechnical assessment, not a landscaping project. Don't try to handle it with rocks and plants.

Common mistakes

• Trying to grow lawn on the slope. Doesn't anchor soil, dangerous to mow, dies in dry months.
• English ivy as a "solution." Invasive, suppresses other plants, undermines other plantings, almost impossible to fully remove once established.
• Boulders aligned in straight rows. Reads as artificial; defeats the natural look. Stagger.
• Surface-set boulders. Boulders sitting on the surface look placed (and can shift). Bury one-third.
• Ignoring the water source. A beautiful slope landscape fails when upstream water keeps hitting it. Solve the water source first.
• One tall wall instead of stepped terraces. Single tall walls are harder to build, require permits at lower wall heights, and look more imposing.
• Planting only ornamentals. Pretty plants without deep root systems don't stop erosion. Mix in deep-rooted natives.

Frequently asked questions

How do you stop erosion on a hillside?
Rock terracing or distributed boulders, deep-rooted plants, and managed surface drainage. Any one helps; the combination is what works.

What is the best material for hillside erosion control?
Mid-size boulders (12-24 inches), larger feature boulders at transitions, rip rap at concentrated flow points, combined with PNW-native deep-rooted plants.

How do you landscape a steep hill?
Break the slope into shorter segments with rock terraces or graduated boulder placements. Plant deep-rooted natives. Direct concentrated runoff through a dry creek bed across the slope.

How much does hillside landscaping cost?
For a 25 ft × 15 ft slope: $693 to $1,023 in bulk materials and plants (DIY). Labor adds significantly for contracted work.

What plants are best for a PNW hillside?
Deep-rooted native shrubs and ground covers: sword fern, salal, low Oregon grape, Pacific ninebark, snowberry, red-twig dogwood, kinnikinnick. Avoid lawn and English ivy.

Can you build a rock wall on a hillside?
Yes. Low rockery walls (under 4 feet, no permit in Kitsap) work well. Multiple stepped terraces handle steeper grade than one tall wall.

How do you stabilize a hillside without retaining walls?
Distributed boulders (8-20, partially buried, staggered) plus deep-rooted plant cover. Works on slopes up to about 30% grade.

What is the best ground cover for a PNW slope?
Sun: kinnikinnick, creeping juniper, beach strawberry. Shade: salal, low Oregon grape, sword fern. Avoid English ivy.

Get materials for your hillside project

We stock everything for a PNW hillside landscape at our Gig Harbor yard: natural boulders in mixed sizes, larger rockery rocks for feature stones and terrace walls, 3/4 clean drain rock for drainage features, rainbow drain rock for dry creek bed surfaces, and bark mulch for surface coverage. Pickup or delivery throughout Gig Harbor, Port Orchard, Bremerton, Silverdale, Poulsbo, and Kitsap County.

Call 253-857-5125 with your slope dimensions and we'll help with boulder mix, drainage rock, and delivery.

Related project guides: Landscape boulders · Rockery walls · Dry creek bed · French drain installation · Retaining wall rocks