Rain Garden Basics
From Janice Hand’s March 17, 2011 presentation to the LGC about rain gardens…

Rain Gardens Must …

  • Be at least 10 feet from the house’s foundation
  • Not be over a septic system
  • Avoid areas of slow infiltration
  • Not be under trees (due to damage to tree roots when digging the garden)
  • Avoid underground utilities
  • Use flatter areas of the yard (on land with not over a 12% slope)

How do I determine my rain garden’s size?

As a general guideline, residential rain gardens are typically 100-300 square feet, but the final size will depend on (1) depth, (2) soil type, and (3) size of the area draining into the rain garden.

1.    Depth

  • Usually between 4 and 8 inches deep
    • - less than 4” and the garden may need to be excessively large so it can handle larger storms
    • - more than 8” and the garden might hold water too long, be unsightly, and a possible tripping hazard
  • The slope of the lawn determines the depth
Start with determining the slope of the lawn on which you will place the garden. To do that, set up two stakes – one at the top of the land and one at the bottom (about 15 feet apart).
  • Set a stake at the top of the (uphill end) of the rain garden site
  • Measure distance between stakes
  • Measure height of downhill stake
  • Divide height by width and multiply by 100

Height   x 100  = % slope


9 inches     x 100  =  5% slope

180 inches

 Use the % slope information to determine how deep the rain garden must be (see table below).


Rain Garden Depth

Less than 4%

3-5 inches deep


6-7 inches deep


8 inches deep

More than 12%

Select another site (or see a professional landscaper)

 2.  Soil type

  • Clay soils have slowest infiltration rate, while sandy soils have the fastest
  • A rain garden in clay soil will need to have more surface area than one in sandy soil


3. Size of the area draining into the rain garden

The size of the areas that will drain into the rain garden depends on location (less than or more than 30 feet from the house to the garden). To calculate the drainage area’s size, see below:

If the rain garden is close to a downspout (less than 30’)

  • Estimate what percent of the roof drains to that downspout,
  • Measure length and width of house and multiply together to get area, then
  • Multiply roof area by percent draining to downspout to get roof drainage area
  • Select the factor (from the table below), and multiply by drainage area to get garden surface size.

3-5” depth

6-7” depth

8” depth

Sandy soil




Silty soil




Clay soil





        If it is more than 30’ from a downspout:
  • Determine roof drainage area
  • Determine area of lawn that will drain into the garden (length x width = area)
  • Add #1 and #2 together to determine the drainage area, then multiply by the factor below to determine the rain garden’s surface area.


All depths

Sandy soil


Silty soil


Clay soil



Example (determining rain garden size)

Roof is 60 feet by 40 feet

Roof area: 60 ft x 40 ft = 2400 square feet

House has 4 downspouts, each handling about 25% of the runoff from roof (.25)

 Drainage area:

2400 sq. ft  x .25 = 600 sq. ft.

 The garden is closer than 30 feet from a downspout, will be in clay soil, and is to be 7 inches deep:

            600 sq. feet x .32 = 190 – 195 square feet in size


What is a Good Reference?

The most widely-cited source for information about rain gardens is the University of Wisconsin Extension and Wisconsin Department of Natural Resources Rain Gardens: A how-to manual for homeowners.

UWEX Publication:     GWQ037

DNR Publication:        PUB-WT-776 2003


Hardy Plants for a Partial-Shade Rain Garden

Common Name

Botanical Name




Sagittana latifolia


Aster - False

Boltonia asteroids

2-4’ tall; white flowers in the summer

Aster – Smooth, Marsh


Fall blooming

Aster - Tatarian  (‘Jindai’)

Aster tataricus

4-6’ tall x 2’ wide; lavender flowers in late fall

Bee balm (‘Glenview Scarlet’)


2-3’ tall x 1-2’ wide; bright red flowers in summer

Blazing Star

Liatris spicata

Summer-fall blooms

Blue Flag Iris

Iris virginica shrevei

2-3’ tall; purple flowers in May – July

Blue Phlox


Spring flowering

Blue/Great Lobelia

Lobelia siphilitica

3’ tall x 1-1.5’ wide; true blue flowers mid-late sumr



Spring ephemerals


Eupatorium perfoliatum

Summer-fall blooming


Scripus atrovivens

Green, soft-stemmed

Canada Wild Rye

Elmus Canadensis


Cardinal Flower

Lobelia cardinalis

2-5’ tall; red flowers in July – Sept.

Coneflower - Green Headed

Rudbeckia laciniata

5-8’ tall; yellow flowers in Aug. and Sept.

Coneflower – Purple

Echinacea purpurea

Summer bloomer

Coneflower - Sweet (‘Henry Eilers’)

Rudbeckia subtomentosa

4-5’ tall x 1.5-2’ wide; flowers bright yellow from summer into fall

Culver’s Root

Veronicastrum virginiacum

Summer-fall blooming

Cup Plant

Silphium perfoliatum

3-8’ tall; yellow flowers in June – Sept.

Fern - Sensitive

Onoclea sensibilis


Ferns – Lady, Interrupted

Athyrium filixfemina, Osmunda claytonia


Fowl Manna Grass

Glyceria striata


Golden Alexander

Zizia aurea

Spring-summer blooming

Goldenrod - Ohio

Solidago ohiensis

3-4’ tall; yellow flowers in Aug. – Oct.

Goldenrod – Riddell’s

Solidago riddellii

2-5’ tall; yellow flowers in Aug. – Oct.

Great Angelica

Angelica atropurpurea

5-7’ tall; white flowers in May and June

Joe Pye Weed (Spotted JPW)

Eupatorium maculatum

4-6’ tall; pink flowers in summer

Little Bluestem

Schizachyrium scoparium

2-4’ tall x 1.5-2’ wide; blue-tinged grass that turns orange and red in the fall

Marsh Marigold

Caltha palustris

2’ tall; yellow flowers in April – June

Meadowrue - Tall

Thalictrum dasycarpum

3-6’ tall; white flowers in June and July

Michigan Lily

Lilium michiganense

3-7’ tall; orange flowers in summer

Monkey Flower

Mimulus ringens

Summer-fall blooming

Obedient Plant

Physosetegia virginiana

3-5’ tall; pink flowers in Aug. and Sept.


Penstemon digitalis

Spring-summer blooming

Queen of the Prairie

Filipendula rubra

4-5’ tall; pink flowers in summer; endangered in IL

Sedge - Burr or Hop Sedge

Carex grayii/lupina

Burr: 3’ tall, blooms late spring. Hop: 2-4’ tall, blooms Mar - May

Sedges – Bromelike, Oval-headed, Bottle-Brush, Gray, Broom, Tufted, Fox



Helenium autumnale

4-5’ tall; yellow flowers in Aug. – Oct.


Tradescantia virginiana

1.5-2’ tall x 1.5’ wide; lingering lavender-blue blooms in summer

Swamp Milkweed

Asclepias incarnate

4-5’ tall; bright pink flowers (hummers & butterflies)

Torrey’s Rush

Juncus torrey



Chelone glabra

Fall blooming

Water Plantain

Alisma subcordatum


Wild Bergamont

Monarda fistulosa

Summer-fall blooming

Wild Columbine

Aquilegia Canadensis

1-3’ tall; red and yellow flowers in spring; tends to mildew (hide by other plants after flowering)

Zig-zag Goldenrod

Solidago flexicaulis

3’ tall; yellow flowers in Aug. – Oct.

Native Shrubs

American Cranberry (‘Alfredo,' ‘Andrews,' 'Bailey Compact,' 'Compactum,' or 'Wentworth')

Viburnum trilobum

8-10’ tall x 6-8’ wide; full sun to part shade

Black Chokeberry

Aronia melanocarpa

6’ tall x 4’ wide; full sun to part shade


Sambucus canadensis

8-10’ tall x 6-8’ wide; full sun to part shade


Physocarpus opulifolius

8-10’ tall x 8-10’ wide; full sun to part shade


Lindera benzoin

6-12’ tall x 6-12’ wide; prefers shade

Winterberry Holly (‘Red Sprite,’ ‘Winter Gold,’ ‘Berry Nice®’)

Ilex verticillata

3-5’ tall and wide; deciduous; need female and male individual plants

(Sources: multiple sources, all cross-checked for accuracy)