Sustainability 62: Rain Gardens

Return water to the earth
Return water to the earth

One way in which each of us can make a relatively simple, yet potentially profound, contribution to a more sustainable world is through the creation of rain gardens.

A rain garden is essentially a vegetated trap for rainwater. Typically consisting of a portion of low-lying land — whether dished or swaled — to which rainwater runoff is diverted, rain gardens are planted with a variety of plant types that absorb and filter that water, allowing it to percolate into the ground to a natural water table.

Rain gardens do their best work for the planet when they take in runoffs from roofs, driveways, patios, sidewalks, and any densely compacted lawn or soil areas. In keeping such impervious surface runoff out of municipal sewer systems, rain gardens lower the overall burden on those systems of water management, filtration and treatment. They also thereby keep surface pollutants, such as fertilizers, fuel residue, and road grime, out of the municipal waste stream, employing an appropriate mix of plant types to do the necessary pollutant remediation.

For optimal efficacy, a rain garden must be sized to take in the likely total of all expected rainwater runoff. Unlike a mere drainage swale, which simply channels water to a final destination, a rain garden is intended as THE final destination for rainwater. In some cases involving large drainage areas, a series of successive intermediate rain gardens may be chained together, to allow for some holding and remediation of rainwater at each stage, before any remainder reaches the final rain garden.

A rain garden must first consist of suitably loose and aerated permeable soil of an appropriate mix (ideally 1 part topsoil, 1 part compost and 3 parts sand), to readily allow water to percolate and plant roots to anchor. The rain garden’s plant mix should consist largely of naturally indigenous species common to local wetlands: sedges, ferns, rushes, wildflowers, and certain shrubs or small trees. Plant types should require no fertilizer, and should be tolerant of local climatic conditions.

The configuration of the rain garden — the shape of its depression, its depth and slopes, its rainwater runoff entry points, etc. — should be such as to allow the rain garden to typically dry itself completely within a day or two. This is an indication that it is properly doing its job of absorbing and percolating rainwater, and will also insure there are no areas of long-term standing water to allow mosquitoes to breed.

Of course, rain gardens offer more than just water collection and absorption. Their use mitigates local erosion, flooding and waterway pollution, while restoring essential groundwater. Their wetlands ecology also supports a variety of wildlife, and can be developed in conjunction with vegetable and flower gardens. The design of rain gardens can also enhance the overall aesthetic of one’s yard and home.

Rain garden at full flood
Rain garden at full flood | Source

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