Residential Rain Gardens

University of Minnesota Master Gardener Program

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A rain garden is a

water-quality tool

that you can use

in your own yard.

What We’ll Cover

• What is a “rain garden”?

• Functions and benefits

• How to make one

• Plant selection

• Maintenance

• Cost

• Helpful resources

What is a “rain garden”?• A shallow sunken garden that recycles the rain

– less than 8” deep• Rainwater runoff is directed toward it• Runoff soaks in

Photo: Mary Nolte, Fulton neighborhood, Minneapolis

Plants help recycle the rain

• Plant roots absorb water

• Water goes up the stem

• Leaf surfaces release moisture

• Water returns to the atmosphere as a vapor

A rain garden is also known as ...

• Mini wetland

• Water quality garden

• Stormwater marsh

• Planted swale

• Bio-retention pond

• Strategically placed puddle

Functions of a Rain Garden

• Diverts runoff from paved surfaces– Driveways, roofs, streets, patios, walks

• Water moves “sideways” – runs off pavement

• Keeps runoff on site– Instead of flowing untreated into streams

and storm sewers

• Soil acts like a living sponge– Water moves “down” - into the ground

Benefits of a Rain Garden

• Soaks up 30% more runoff than lawns

• Filters polluted runoff• sediments, fertilizers, pesticides

• Recharges groundwater

• Helps prevent flooding

• Provides habitat/food for butterflies, birds

• Beautifies a low spot in the yard

Polluted runoff harms water quality

• Flows into waterways untreated

• Harms fish and wildlife

• Kills vegetation• Fouls drinking

water supplies• Makes recreation

areas unsafe

Every curb is a shoreline

Grass clippings and leaves are the main source of phosphorus in lakes and streams

Rain gardens act as filters and remove:– 94% of sediment– 43% of phosphorus– 70% of nitrogen

Design Features are Flexible

Variables include:

• Location

• Soil

• Size and shape

• Plants

www.montgomerycountymd.gov/mc/services/dep/greenman/rain.htm

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Location of the rain garden

Go with the flow• Observe the drainage pattern in your yard

• Locate the garden:– in a natural low spot– near sidewalks, driveways, or

other paved surfaces– down-slope from roofs, gutters, downspouts,

sump pump outlet

• Direct water into rain garden– channel or buried plastic pipe

Project and photo by Kestrel Design Group

Locate the rain garden at least 10 feet from the house, on a gentle slope

Bird’s-eye view of rain garden locations

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Traditional path of roof runoff

Graphic: Applied Ecological Services, Inc.

Average runoff from a roof is 24,000 gallons per year

That would fill 600 bathtubs!

Roof runoff directed to rain garden

Rain Gardens in Home Landscapes

In a back yard catching runoff from the garage

In a front yard catching runoff from a downspout

The soil must drain!

• A rain garden is NOT a pond.• Percolation test:

– fill a 6-inch-deep hole with water– should drain within 24 hours– if not, don’t put a rain garden there– or amend soil

• “Rain garden soil mix”– 50-60% sand, 20-30% topsoil, 20-30a% compost

How big?

• No standard size

• Rule of thumb: 1/3 of drainage area– e.g., 170 sq. ft. (10’ x 17’) garden for

500 sq. ft. of drainage area

• Factors include slope, soil type, distance from runoff point

• Even a small rain garden is beneficial

Calculating Drainage Area

• Length of house 100 feet

• Width of house 20 feet• L X W = 2000 sq ft• 2000 sq ft ÷ 4 =

500 sq ft draining to the rain garden

Area of roof going to down spout

Width

Length

Graphic: UW Extension Service

Designing the rain garden

• Call Gopher State One: 800-252-1166

• Outline it with rope or hose• curvy shape

• Remove sod and dig to desired depth

Designing the rain garden – cont’d

Features:

– gently sloping sides– flat in the deepest spot– berm at low end– grass filter strip on top edge– mulch – shredded bark

A soil berm acts like a bumper – keeps water from flowing over edge

Photo: UW Extension Publication GWQ037

Berm

Rain Garden Collects Parking Lot Runoff

Photos & design: Kestrel Design Group

Spray paint outlines the garden shape

Parking lot runoff flows into the rain garden instead of into the street

Runoff pools in the garden, then seeps into the ground

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Graphic: UW Extension Publication GWQ037

Bird’s-eye view of a rain garden

Layout for a 140-sq.ft.-garden

Graphic: UW Extension Publication GWQ037

Photos: Mary Nolte

Before

After

Buried pipe connects to downspout

Berm

Plants for Rain Gardens

Select perennial plants that

• Tolerate both wet and dry spells– plants that like wet feet in deepest part

• e.g.: Blue flag iris, marsh milkweed, big bluestem, sedges, red-twigged dogwood, buttonbush

• Tolerate de-icing salts (if near roads)

• Match up with soil and light conditions

Native plants have advantages

• Adapted to the climate and native pests

• Deep rooted– long roots make

channels in the soil for water to follow

• Havens for butterflies, birds, beneficials

Prairie plants have deep roots

Graphic: Conservation Design Forum Inc.

Rain gardens in Maplewood, MNinstead of curbs and gutters

Photo: City of Maplewood

Sun gardens

Photos: City of Maplewood

Garden for light shade

Photo: City of Maplewood

An easy shrub gardenShrubs:• Annabelle

Hydrangea• American

Highbush Cranberry

• Anthony Waterer Spirea

Perennials:• Marsh Milkweed• Stello d’Oro Daylily

Photo: City of Maplewood

Maintenance

• Pull weeds (esp. the 1st year)

• Water ~3 x a week until established– also during a dry spells

• Mulch – shredded hardwood won’t float away

• Cut back or mow down

What about mosquitoes?

• A rain garden is not a pond

• No standing water between rainfalls

• Mosquitoes need at least 7 days in standing water to hatch– will not survive if wetland dries out in less than

a week.

Estimated cost and plants needed

• Do-it-yourselfers: – about $3 to $5/sq. ft.

• Professionals: – about $10 - $12/sq.ft

• Plants needed for 300 sq. ft. garden– 100 for wet zones – 200 for upland zones

Summary• Shallow saucer

shape• Strategic location• Plants match soil

and moisture levels• Soil drains• Size appropriate for

yard and drainage area Photo: Mary Nolte