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Rain Garden Basics
From Janice Hand’s March 17, 2011 presentation to the LGC about rain gardens…
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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
- 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
Width
9 inches x 100 = 5% slope
180 inches
Use the % slope information to determine how deep the rain garden must be (see table below).
Slope |
Rain Garden Depth |
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Less than 4% |
3-5 inches deep |
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5-7% |
6-7 inches deep |
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8-12% |
8 inches deep |
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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.
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3-5” depth |
6-7” depth |
8” depth |
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Sandy soil |
.19 |
.15 |
.08 |
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Silty soil |
.34 |
.25 |
.16 |
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Clay soil |
.43 |
.32 |
.20 |
- 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.
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All depths |
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Sandy soil |
.03 |
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Silty soil |
.06 |
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Clay soil |
.10 |
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
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Common Name |
Botanical Name |
Characteristics |
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Perennials |
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Arrowhead |
Sagittana latifolia |
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Aster - False |
Boltonia asteroids |
2-4’ tall; white flowers in the summer |
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Aster – Smooth, Marsh |
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Fall blooming |
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Aster - Tatarian (‘Jindai’) |
Aster tataricus |
4-6’ tall x 2’ wide; lavender flowers in late fall |
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Bee balm (‘Glenview Scarlet’) |
Monarda |
2-3’ tall x 1-2’ wide; bright red flowers in summer |
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Blazing Star |
Liatris spicata |
Summer-fall blooms |
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Blue Flag Iris |
Iris virginica shrevei |
2-3’ tall; purple flowers in May – July |
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Blue Phlox |
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Spring flowering |
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Blue/Great Lobelia |
Lobelia siphilitica |
3’ tall x 1-1.5’ wide; true blue flowers mid-late sumr |
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Bluebells |
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Spring ephemerals |
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Boneset |
Eupatorium perfoliatum |
Summer-fall blooming |
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Bullrush |
Scripus atrovivens |
Green, soft-stemmed |
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Canada Wild Rye |
Elmus Canadensis |
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Cardinal Flower |
Lobelia cardinalis |
2-5’ tall; red flowers in July – Sept. |
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Coneflower - Green Headed |
Rudbeckia laciniata |
5-8’ tall; yellow flowers in Aug. and Sept. |
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Coneflower – Purple |
Echinacea purpurea |
Summer bloomer |
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Coneflower - Sweet (‘Henry Eilers’) |
Rudbeckia subtomentosa |
4-5’ tall x 1.5-2’ wide; flowers bright yellow from summer into fall |
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Culver’s Root |
Veronicastrum virginiacum |
Summer-fall blooming |
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Cup Plant |
Silphium perfoliatum |
3-8’ tall; yellow flowers in June – Sept. |
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Fern - Sensitive |
Onoclea sensibilis |
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Ferns – Lady, Interrupted |
Athyrium filixfemina, Osmunda claytonia |
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Fowl Manna Grass |
Glyceria striata |
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Golden Alexander |
Zizia aurea |
Spring-summer blooming |
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Goldenrod - Ohio |
Solidago ohiensis |
3-4’ tall; yellow flowers in Aug. – Oct. |
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Goldenrod – Riddell’s |
Solidago riddellii |
2-5’ tall; yellow flowers in Aug. – Oct. |
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Great Angelica |
Angelica atropurpurea |
5-7’ tall; white flowers in May and June |
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Joe Pye Weed (Spotted JPW) |
Eupatorium maculatum |
4-6’ tall; pink flowers in summer |
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Little Bluestem |
Schizachyrium scoparium |
2-4’ tall x 1.5-2’ wide; blue-tinged grass that turns orange and red in the fall |
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Marsh Marigold |
Caltha palustris |
2’ tall; yellow flowers in April – June |
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Meadowrue - Tall |
Thalictrum dasycarpum |
3-6’ tall; white flowers in June and July |
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Michigan Lily |
Lilium michiganense |
3-7’ tall; orange flowers in summer |
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Monkey Flower |
Mimulus ringens |
Summer-fall blooming |
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Obedient Plant |
Physosetegia virginiana |
3-5’ tall; pink flowers in Aug. and Sept. |
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Penstemon |
Penstemon digitalis |
Spring-summer blooming |
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Queen of the Prairie |
Filipendula rubra |
4-5’ tall; pink flowers in summer; endangered in IL |
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Sedge - Burr or Hop Sedge |
Carex grayii/lupina |
Burr: 3’ tall, blooms late spring. Hop: 2-4’ tall, blooms Mar - May |
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Sedges – Bromelike, Oval-headed, Bottle-Brush, Gray, Broom, Tufted, Fox |
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Sneezeweed |
Helenium autumnale |
4-5’ tall; yellow flowers in Aug. – Oct. |
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Spiderwort |
Tradescantia virginiana |
1.5-2’ tall x 1.5’ wide; lingering lavender-blue blooms in summer |
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Swamp Milkweed |
Asclepias incarnate |
4-5’ tall; bright pink flowers (hummers & butterflies) |
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Torrey’s Rush |
Juncus torrey |
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Turtlehead |
Chelone glabra |
Fall blooming |
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Water Plantain |
Alisma subcordatum |
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Wild Bergamont |
Monarda fistulosa |
Summer-fall blooming |
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Wild Columbine |
Aquilegia Canadensis |
1-3’ tall; red and yellow flowers in spring; tends to mildew (hide by other plants after flowering) |
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Zig-zag Goldenrod |
Solidago flexicaulis |
3’ tall; yellow flowers in Aug. – Oct. |
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American Cranberry (‘Alfredo,' ‘Andrews,' 'Bailey Compact,' 'Compactum,' or 'Wentworth') |
Viburnum trilobum |
8-10’ tall x 6-8’ wide; full sun to part shade |
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Black Chokeberry |
Aronia melanocarpa |
6’ tall x 4’ wide; full sun to part shade |
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Elderberry |
Sambucus canadensis |
8-10’ tall x 6-8’ wide; full sun to part shade |
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Ninebark |
Physocarpus opulifolius |
8-10’ tall x 8-10’ wide; full sun to part shade |
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Spicebush |
Lindera benzoin |
6-12’ tall x 6-12’ wide; prefers shade |
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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)