The rain garden on the ridge

Two downspouts discharging onto a path, a parking strip of dead grass, and an owner who wanted the roof water to stop being somebody else's problem.

The house sheds about eighteen thousand gallons a year off its roof, and all of it was going into a combined sewer. The clients wanted it kept on site, partly on principle and partly because the path flooded.

The basin is not large — about a hundred and twenty square feet, dished eighteen inches, with a designed overflow to the street for the storms it cannot take. What makes it work is the soil under it, which was excavated and replaced to two feet with a sand and compost mix, because a rain garden built into unamended Portland fill is a pond.

The parking strip is the part visitors ask about. It is planted almost entirely with things that will take being walked on, parked beside and never watered, and it has had no irrigation since the second summer.

Across 7 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 deep narrow trench cut through dark soil with severed pale roots visible in its walls, a shovel and a pick lying across the top. — Build, week oneRoots cut clean in a trench wall. Every one of them was a decision made before the trench was set out.

White trumpet-shaped flowers and a fern frond beaded with water droplets after rain, against soft out-of-focus green. — Year two, NovemberPhotographed wet, because for most of the year that is the condition a Portland garden is actually in.

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 four, FebruaryLayers, no bare ground, nothing standing on its own. The reference for most shade planting we draw.

Ferns growing out of a dark wet rock face, their fronds catching the only light in an otherwise deeply shaded frame of stone and leaf litter. — Year six, JanuaryWet basalt and fern — the combination the whole regional palette is built out of, and it needs no flowers.

Planting plan1" = 8'

Key — drawn at mature spread

CAO×3Carex obnupta — slough sedge
JUP×1Juncus patens — spreading rush
COS×1Cornus sericea — red-osier dogwood
SEA×3Sesleria autumnalis — autumn moor grass
ARU×3Arctostaphylos uva-ursi — kinnikinnick
ERU×2Eriogonum umbellatum — sulphur buckwheat
MAA×2Berberis aquifolium — tall Oregon grape
RIS×1Ribes sanguineum — red-flowering currant

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

Silt over sand and gravel — genuinely good drainage once you get past the fill. Two feet of engineered mix in the basin, and decompaction plus compost everywhere else.

Both roof downspouts, disconnected and piped to the basin, with an overflow to the kerb. The property qualified for the city's stormwater discount, which the owners applied for themselves.

What we would do differently

The overflow outlet is a rock-lined channel and it is the one part of the design that looks engineered rather than planted. It works perfectly. We should have put a shrub in front of it.

The backbone of the planting

CAOCarex obnuptaslough sedge3 × 3 ft at maturity
JUPJuncus patensspreading rush2.5 × 2 ft at maturity
COSCornus sericeared-osier dogwood9 × 9 ft at maturity
SEASesleria autumnalisautumn moor grass1.5 × 2 ft at maturity
ARUArctostaphylos uva-ursikinnikinnick0.8 × 6 ft at maturity
ERUEriogonum umbellatumsulphur buckwheat1.5 × 2.5 ft at maturity
MAABerberis aquifoliumtall Oregon grape7 × 5 ft at maturity
RISRibes sanguineumred-flowering currant8 × 7 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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