Rain Gardens

Small Urban Space Rain Gardens

January 29, 2010

flower-after-rain

Rain gardens aren’t just for homeowners with large tracts of land.  A rain garden planted in a small urban area can make a big difference in the water quality and environment of its surrounding area.

When it rains in densely populated urban areas, impervious surfaces such as roads, sidewalks, and roofs not planted with gardens, trees, or turf, produce runoff that goes straight into storm sewers.  Some storm drains carry water to treatment plants, while water from other storm drains washes directly into lakes, rivers, and oceans.  Any time a large influx of water pours into an aquatic ecosystem, the balance of oxygen and nutrients is disturbed, causing death to aquatic life, and other disruptions of the ecosystem.

Rain-Garden1
Photo, City of Kingston, Melbourne, Australia

In addition to impervious surfaces made of concrete or asphalt, many urban areas have vacant, muddy lots.  Apartment buildings might have courtyards of hard-packed dirt that will turn to mud when it rains.

Planting a rain garden, even a small one, can help divert water and keep it within the aquifer and out of streams and lakes.  Rain gardens can be beautiful and beneficial additions to urban properties.  Here’s how to plant a rain garden in a small space:

Urban_Rain_Garden_at_Chelsea_Show
Small urban rain garden at 2008 Chelsea Flower Show

Rain Garden Basics
Whether the garden is in a small space of land between a building and a sidewalk, or part of a larger yard, there are a fundamentals to rain garden construction that apply to all garden design.

• Rain gardens need to be constructed to drain within four hours of a 1” rainfall.  This is to protect the health of the plants while absorbing the water.
• Plants in rain gardens need to be able to experience wide variations of wet and dry conditions. For this reason, native plants work best for rain gardens, because they are well-adapted to local conditions.  Favorable plants for rain gardens will vary from region to region.
• The soil in which the garden is planted must be lightweight and porous, so it will likely need to be amended prior to planting.  Leaf compost, sand, vermiculite, and loamy soil additions are good mixtures for rain gardens.
• For the garden to be most beneficial, it needs to be located in an area where water naturally collects, or drains from a downspout or gutter. To select the best place (if there is a choice), observe the area after a heavy rain.

Eupatorium_purpureum
Eupatorium is suitable for rain gardens.

Plants for Small Rain Gardens
Many plants suitable for rain gardens are large and rangy.  Small, urban gardens require more compact species and dwarf cultivars of larger native plants.  Here are some “best bets” for rain gardens in small spaces.

• Campanula (bellflower)
• Carex (sedge)
• Lobelia (cardinal flower)
• Eupatorium (DWARF Joe Pye weed and boneset)
• Ruellia ‘Katie’
• Garden phlox
• Baptisia  (false indigo)
• Liatris (blazing star)
• Solidago (goldenrod)
• Aster
• Tradescantia (spiderwort)
• Helenium (sneezeweed)

There are numerous trees and shrubs that are also at home in the rain garden, but most grow too large to be part of a smaller garden. With smaller plants, and careful attention to soil amendments, a small rain garden in an urban space can be a beautiful and ecologically beneficial addition to the landscape.

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