The clay slope

A slope that shed water into the house, a lawn nobody could mow safely, and a seep that four previous drainage attempts had not touched.

The wet patch was thirty feet upslope of where anybody had ever dug. It was groundwater moving laterally above the clay, surfacing where the ground cut across it — a textbook Bull Mountain seep, and the reason the three french drains already in the garden had achieved nothing.

We cut an interceptor drain along the contour above the seep line, down to the clay, with a real outfall to the street. That single trench was nineteen thousand dollars and it is the reason everything else on this project worked. It is also completely invisible, which is why it is the hardest part of a budget to sell and the part we will not reduce.

Above it, three dry-laid basalt terraces, cut into the slope rather than built out from it. Below, planting that tolerates a clay soil that is saturated in February and set hard in August — which rules out most of what a garden centre will sell you for a slope.

Across 5 years

Every frame is labelled. The build photographs are in here too, because the fortnight where a garden is a trench is part of what you are buying.

A perforated corrugated drainage pipe laid along the bottom of a trench in reddish clay soil and surrounded by clean pale drainage gravel, running away from the camera. — Build, week twoPerforated pipe in clean drain rock. The rock is the drain; the pipe only carries away what the rock collects.

A man in work gloves and a grey shirt building a low stone retaining structure behind timber shuttering in a garden, an old fruit tree and a wheelbarrow behind him, looking down at the stone he is placing. — Build, week sixA retaining wall is a drainage detail that happens to hold soil up.

A forest floor entirely covered in deep undulating moss with small ferns pushing through it, in flat diffuse light that shows no direct sun. — Year two, OctoberLayers, no bare ground, nothing standing on its own. The reference for most shade planting we draw.

A large rhododendron covered in pink flower trusses, standing among lower evergreen shrubs with the dark silhouettes of conifers and a heavy grey cloud bank behind it. — Year four, MayThe two weeks it does this are not the reason to plant one. The other fifty are.

Planting plan1" = 10'

Key — drawn at mature spread

MAA×3Berberis aquifolium — tall Oregon grape
RIS×2Ribes sanguineum — red-flowering currant
SEA×3Sesleria autumnalis — autumn moor grass
AMH×2Amsonia hubrichtii — blue star
ARU×3Arctostaphylos uva-ursi — kinnikinnick
CAO×2Carex obnupta — slough sedge
JUP×1Juncus patens — spreading rush
PHC×1Physocarpus capitatus — Pacific ninebark

Solid outlines are evergreen. Dashed outlines lose their leaves — which is what a plan should tell you before you build it, not after your first winter.

The ground, and the water

Clay over basalt, with a perched water table above the clay through the winter. We did not attempt to amend it wholesale — the terraces are backfilled with a free-draining mix and the planting below is chosen for the clay as found.

An interceptor drain above the seep, a dispersal trench at the bottom, and a rain garden taking the two downspouts on the uphill side of the house.

What we would do differently

We used a five-foot terrace rise where four would have done. It saved a course of stone and it makes the middle terrace feel like a corridor rather than a room. The client has never mentioned it. We notice it every time we visit.

The backbone of the planting

MAABerberis aquifoliumtall Oregon grape7 × 5 ft at maturity
RISRibes sanguineumred-flowering currant8 × 7 ft at maturity
SEASesleria autumnalisautumn moor grass1.5 × 2 ft at maturity
AMHAmsonia hubrichtiiblue star3 × 3 ft at maturity
ARUArctostaphylos uva-ursikinnikinnick0.8 × 6 ft at maturity
CAOCarex obnuptaslough sedge3 × 3 ft at maturity
JUPJuncus patensspreading rush2.5 × 2 ft at maturity
PHCPhysocarpus capitatusPacific ninebark12 × 10 ft at maturity

Yours would be drawn the same way.

A measured base plan, a planting plan at mature spread, and a written note on what the garden will look like in year one so that nothing about the first winter is a surprise.

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