How to Build a Rain Garden

In this video, This Old House landscape contractor Roger Cook shows how to use rainwater to sustain a lush garden.

Steps:

1. Lay out the rough shape of the garden using a garden hose. For a natural-looking garden, eliminate straight lines and arrange the hose into gentle curves.

2. Mark the outline of the garden onto the ground with line-marking spray paint. Follow the shape of the hose, spraying just a few inches beyond the outline.

3. Move the garden hose out of the way, then dig out the garden to a depth of at least 12 inches using a mini-excavator.

4. Use pointed and square-blade shovels to shape the perimeter of the garden, creating a bowl.

5. Set four to six large boulders into place near the edges of the excavated garden.

6. Dig a shallow trench from the downspout to the garden.

7. Attach a 4-inch-diameter plastic 90 elbow to the bottom end of the downspout.

8. Run a length of 4-inch plastic pipe from the downspout elbow to the garden. Cover the drainpipe trench with topsoil.

9. Dig planting holes and set each plant into its hole.

10. Mix a handful of starter fertilizer into the topsoil and backfill around each plant.

11. Spread a thick layer of gravel over the bare ground between the planting beds.

12. Cover the soil around each plant with 2 inches of bark mulch.

13. Thoroughly water the plants each day until they’re established.

Pro Tip: Roger Cook, This Old House landscape contractor, notes that before you budget for a rain garden, check your local permitting requirements. On one project, he explained that homeowners “can’t get a building permit without doing this work,” meaning stormwater management costs were factored in “right from the get-go.” A perc test — the same type used for septic systems — will tell you how well your soil drains and help determine how much excavation and material you’ll actually need.

Keep in Mind: Material costs add up in layers. A proper rain garden typically requires stone at the base to allow water to drain, filter fabric to prevent weeds and keep sand from filling the stone voids, a layer of well-draining Class-5 sand, and about six inches of topsoil for planting. Then you’ll need native plants suited to varying moisture levels — wetland species for the bowl at the center and drought-tolerant varieties along the banks.

Pro Tip: David Hymel, who coordinated a rain garden project in Puyallup, Washington, for Stewardship Partners, a Seattle nonprofit, explains the purpose behind the design: “The whole idea is to have the water be absorbed back into the ground and filtered naturally, the way it used to be.” To maximize that filtering effect, be sure to include a rock-lined outlet for overflow so that excess water has somewhere to go during heavy storms.

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