Rain Gardens GWQ037 User Manual
A how-to manual
for homeowners
Homeowners in many parts of the country are catching on to rain gardens – land-
scaped areas planted to wild flowers and other native vegetation that soak up rain water,
mainly from the roof of a house or other building. The rain garden fills with a few inches
of water after a storm and the water slowly filters into the ground rather than running off
to a storm drain. Compared to a conventional patch of lawn, a rain garden allows about
30% more water to soak into the ground.
Why are rain gardens important? As cities and suburbs grow and replace forests and
agricultural land, increased stormwater runoff from impervious surfaces becomes a
problem. Stormwater runoff from developed areas increases flooding; carries pollutants
from streets, parking lots and even lawns into local streams and lakes; and leads to costly
municipal improvements in stormwater treatment structures.
By reducing stormwater runoff, rain gardens can be a valuable part of changing these
trends. While an individual rain garden may seem like a small thing, collectively they
produce substantial neighborhood and community environmental benefits. Rain gardens
work for us in several ways:
g Increasing the amount of water that filters into the ground, which
recharges local and regional aquifers;
g Helping protect communities from flooding and drainage problems;
g Helping protect streams and lakes from pollutants carried by
urban stormwater – lawn fertilizers and pesticides, oil and
other fluids that leak from cars, and numerous
harmful substances that wash off roofs and
paved areas;
g Enhancing the beauty of yards and neighborhoods;
g Providing valuable habitat for birds, butterflies
and many beneficial insects.
Your personal contribution to cleaner water
2
Frequently asked questions
Does a rain garden form a pond?
No. The rain water will soak in so the rain garden is dry
between rainfalls. (Note: some rain gardens can be
designed to include a permanent pond, but that type of
rain garden is not addressed in this publication).
Are they a breeding ground for mosquitoes?
No. Mosquitoes need 7 to 12 days to lay and hatch eggs,
and standing water in the rain garden will last for a few
hours after most storms. Mosquitoes are more likely to lay
eggs in bird baths, storm sewers, and lawns than in a
sunny rain garden. Also rain gardens attract dragonflies,
which eat mosquitoes!
Do they require a lot of maintenance?
Rain gardens can be maintained with little effort after the
plants are established. Some weeding and watering will be
needed in the first two years, and perhaps some thinning
in later years as the plants mature.
Is a rain garden expensive?
It doesn’t have to be. A family and a
few friends can provide the labor. The
main cost will be purchasing the plants,
and even this cost can be minimized by
using some native plants that might
already exist in the yard or in a
neighbor’s yard.
Who should use this
manual?
This manual provides
homeowners and landscape
professionals with the
information needed to
design and build rain
gardens on residential lots.
Guidelines presented in this
manual can also be used to
treat roof runoff at commercial and institutional
sites. However, the manual
should not be used to
design rain gardens for
parking lots, busy streets
and other heavily used
paved areas where
stormwater would require
pretreatment before
entering a rain garden.
Rain Gardens – A how-to manual for homeowners
3
Sizing and Siting the Rain Garden
This section of the manual covers rain gar-
den basics – where to put the rain garden,
how big to make it, how deep to dig it, and
what kind of soils and slope are best. Following
the instructions in this section is the best way
to ensure a successful rain garden project.
If you already know the size you want your rain
garden to be, then skip ahead to the section
about building the rain garden. However, take time read the pointers about location, and
do find the slope of the lawn. If the location has a slope more than about 12%, it’s best to
pick a different location because of the effort it will take to create a level rain garden.
Where should the rain garden go?
Home rain gardens can be in one of two places – near the house to catch only roof runoff or farther out on
the lawn to collect water from the lawn and roof. (Figure 1 shows the possible locations on a residential lot.)
To help decide where to put a rain garden, consider these points:
• The rain garden should be at least 10 feet from the house so infiltrating water doesn’t
seep into the foundation.
• Do not place the rain garden directly over a septic system.
• It may be tempting to put the rain garden in a part of the yard where water already
ponds. Don’t! The goal of a rain garden is to encourage infiltration, and your yard’s wet
patches show where infiltration is slow.
• It is better to build the rain garden in full or partial sun, not directly under a big tree.
• Putting the rain garden in a flatter part of the yard will make digging much easier.
For example, a rain garden 10 feet wide on a 10% slope must be 12 inches deep to be
level, unless you import topsoil or use cut and fill.
4
• • • • •
Step
An extension of PVC pipe helps direct downspout water
to this rain garden.
not within 10'
of foundation
> 30'
from
down spout
street
roof and lawn
drainage
area to back
rain garden
roof drainage
area to front
rain garden
rain garden
length
rain
garden
width
rain
garden
width
close to
down spout
rain garden
length
When considering placement of your rain
garden, design with the end in mind.
Carefully consider how the rain garden can
be integrated into existing and future
landscaping. Also, pay attention to views
from inside the house as well as those
Rain Gardens – A how-to manual for homeowners
5
throughout the landscape. Determine how
far or how close you want your rain garden
to outdoor gathering spaces or other play
areas. Why not locate it near a patio where
you can take advantage of the colors and
fragrances for hours on end!
Figure 1 A rain garden can
be built in the front or back
yard. Pick a pleasing shape
for the rain garden. Crescent,
kidney, and teardrop shapes
seem to work well.
Consider your overall landscape
6
How big should the rain garden be?
The surface area of the rain garden can be almost any size, but time and cost will always be important considerations in sizing decisions. Any reasonably sized rain garden will provide some stormwater runoff control.
A typical residential rain garden ranges from 100 to 300 square feet. Rain gardens can be smaller than 100
square feet, but very small gardens have little plant variety. If a rain garden is larger than 300 square feet it
takes a lot more time to dig, is more difficult to make level, and could be hard on your budget.
The size of the rain garden will depend on
•how deep the garden will be,
• what type of soils the garden will be planted in, and
•how much roof and/or lawn will drain to the garden.
This information, along with the sizing factor from the tables on
page 9, will determine the surface area of the rain garden.
> 10'
from
foundation
< 12%
slope
berm
berm
street
> 30'
from
down spout
close
to
down
spout
Guidelines are not rules…
The sizing guidelines
described in this manual are
based on a goal of controlling
100% of the runoff for the
average rainfall year while
keeping the size of the rain
garden reasonable. Establishing a 100% runoff goal helps
compensate for some of the
errors that creep into the
design and construction of
any rain garden.
If you follow the guidelines in
the manual and decide the
calculated surface area is just
too large for your goals, it is
perfectly acceptable to make
the rain garden smaller. The
rain garden can be up to 30%
smaller and still control almost
90% of the annual runoff. On
the other hand, it is fine to
make the rain garden bigger
than the guidelines indicate.
Figure 2
Rain gardens should
be located at least 10 feet from
the house, on a gentle slope
that catches downspout water.
Digging with a rented backhoe.
Rain Gardens – A how-to manual for homeowners
7
How Deep Should the Rain Garden Be?
A typical rain garden is between four and eight inches deep. A rain garden more than eight inches deep
might pond water too long, look like a hole in the ground, and present a tripping hazard for somebody
stepping into it. A rain garden much less than four inches deep will need an excessive amount of surface
area to provide enough water storage to infiltrate the larger storms.
No matter what the depth of
the rain garden, the goal is to
keep the garden level. Digging
a very shallow rain garden on a
steep lawn will require bringing
in extra topsoil to bring the
downslope part of the garden
up to the same height as the
up-slope part of the garden. As
the slope gets steeper, it is easier to dig the rain garden a little
deeper to make it level.
The slope of the lawn should determine the depth of the rain garden. Find the slope of your lawn by
following these steps. (Figure 3 shows how the stakes and string should look.)
1. Pound one stake in at the uphill end of your rain garden site and pound the other stake
in at the downhill end. The stakes should be about 15 feet apart.
2. Tie a string to the bottom of the uphill stake and run the string to the downhill stake.
3. Using a string level or the carpenter’s level, make the string horizontal and tie the string
to the downhill stake at that height.
4. Measure the width (in inches) between the two stakes.
5. Now measure the height (in inches) on the downhill stake between the ground and string.
6. Divide the height by the width and multiply the result by 100 to find the lawn’s percent
slope. If the slope is more than 12%, it’s best to find another site or talk to a professional
landscaper.
Using the slope of the lawn, select the depth of the rain garden from the following options:
• If the slope is less than 4%, it is easiest to build a 3 to 5-inch deep rain garden.
• If the slope is between 5 and 7%, it is easiest to build one 6 to 7 inches deep.
• If the slope is between 8 and 12%, it is easiest to build one about 8 inches deep.
EXAMPLE
Todd measures the length of the string between the stakes; it is 180 inches long. The height
is 9 inches. He divides the height by the width to find his lawn’s percent slope.
With a 5% slope, Todd should build a 6 inch deep rain garden.
height
x 100 =% slope
width
9 inches
x 100 =5% slope
180 inches
Figure 3
The string should be tied to
the base of the uphill stake, then tied to
the downhill stake at the same level.
✓
downhill
stake
height
the string must be level
width
uphill
stake
Rain gardens more than 30 feet from the downspout
1. If there is a significant area of lawn uphill that will also drain to the rain garden, add
this lawn area to the roof drainage area. First find the roof drainage area using the steps
above for a rain garden less than 30’ from the downspout.
2. Next find the area of the lawn that will drain to the rain garden. Stand where your rain
garden will be and look up toward the house. Identify the part of the lawn sloping into
the rain garden.
3. Measure the length and width of the uphill lawn, and multiply them to find the lawn area.
4. Add the lawn area to the roof drainage area to find the total drainage area.
8
How big is the area draining to the rain garden?
The next step in choosing your rain garden size is to find the area that will drain to the rain garden. As the
size of the drainage area increases so should the size of the rain garden. There is some guesswork in determining the size of a drainage area, especially if a large part of the lawn is up-slope from the proposed garden
site. Use the suggestions below to estimate the drainage area without spending a lot of time.
Rain gardens less than 30 feet from the downspout
1. In this case, where the rain garden is close to the house, almost all water will come from
the roof downspout. Walk around the house and estimate what percent of the roof feeds
to that downspout. Many houses have four downspouts, each taking about 25% of the
roof’s runoff.
2. Next find your home’s footprint, the area of the first floor. If you don’t already know it,
use a tape measure to find your house’s length and width. Multiply the two together to
find the approximate area of your roof.
3. Finally, multiply the roof area by the percent of the roof that feeds to the rain garden
downspout. This is the roof drainage area.
What type of soils are on the rain garden site?
After choosing a rain garden depth, identify the lawn’s soil type as sandy, silty, or clayey. Sandy soils have
the fastest infiltration; clayey soils have the slowest. Since clayey soils take longer to absorb water, rain
gardens in clayey soil must be bigger than rain gardens in sandy or silty soil. If the soil feels very gritty and
coarse, you probably have sandy soil. If your soil is smooth but not sticky, you have silty soil. If it is very
sticky and clumpy, you probably have clayey soil.
EXAMPLE
Todd’s house is 60 feet by 40 feet, so the roof area is 2400 square feet. He estimates that
the downspout collects water from 25% of the roof, so he multiplies 2400 by 0.25 to get a
downspout drainage area of 600 square feet.
Roof Area: 60 ft by 40 ft = 2400 square ft.
Drainage Area: 2400 square ft. x 0.25 = 600 square ft.
. If the rain garden
is far from the
house, and you
don’t want a swale
or downspout
cutting across the
lawn, run a PVC
pipe underground
from the downspout to the rain
garden. In this
case do calculations
as for a rain
garden less than
30 feet from the
house.
✓
Rain Gardens – A how-to manual for homeowners
9
Using the Rain Garden Size Factors
Having estimated the drainage area, soil type, and depth for your rain garden, use Table 1 or Table 2 to
determine the rain garden’s surface area. Use Table 1 if the rain garden is less than 30 feet from the downspout, and use Table 2 if it is more than 30 feet from the downspout.
Table 1 Rain gardens less than 30 feet
from downspout.
3-5 in. 6-7 in. 8 in.
deep deep deep
Sandy soil 0.19 0.15 0.08
Silty soil 0.34 0.25 0.16
Clayey soil 0.43 0.32 0.20
EXAMPLE
Todd’s rain garden is less than 30 feet from the downspout, and his lawn has a 5% slope, so
he will have a 6-inch deep rain garden. His lawn is silty, so Table 1 recommends a size factor
of 0.25. He multiplies the downspout drainage area, 600 square feet, by 0.25 to find the
recommended rain garden area, 150 square feet.
600 square ft. by 0.25 = 150 square ft.
1. Find the size factor for the soil type and rain garden depth.
2. Multiply the size factor by the drainage area. This number is the recommended rain
garden area.
3. If the recommended rain garden area is much more than 300 square feet, divide it into
smaller rain gardens.
Table 2 Rain gardens more than 30 feet
from downspout.
Size Factor, for all depths
Sandy soil 0.03
Silty soil 0.06
Clayey soil 0.10
✓
Simple soil tests
Two small tests can ensure your soil can handle a rain garden:
• Dig a hole about 6 inches deep where the rain
garden is to go and fill the hole with water. If
the water takes more than 24 hours to soak in,
the soil is not suitable for a rain garden.
•Take a handful of soil and dampen it with a
few drops of water. After kneading the soil in your fingers,
squeeze the soil into a ball. If it remains in a ball, then work
the soil between your forefinger and thumb, squeezing it
upward into a ribbon of uniform thickness. Allow the ribbon
to emerge and extend over the forefinger until it breaks
from its own weight. If the soil forms a ribbon more than
an inch long before it breaks, and it also feels more smooth
than gritty, the soil is not suitable for a rain garden.
The map is a starting point for assessing what type of soils you might find in your yard. However, the soil on a
small plot of a yard can be very different from the soils indicated on the map. Use the simple soil test described
here for a more accurate representation of the soils in the possible rain garden location. More information about
sampling and testing lawn and garden soils can be obtained at county UW-Extension offices.
How long and how wide should the
rain garden be?
Before building the rain garden, think about how it will catch
water. Runoff will flow out of a downspout and should spread
evenly across the entire length of the rain garden. The rain
garden must be as level as possible so water doesn’t pool at
one end and spill over before it has a chance to infiltrate.
The longer side of the rain garden should face upslope; that is, the length of the rain garden should be
perpendicular to the slope and the downspout. This way the garden catches as much water as possible.
However, the rain garden should still be wide enough for the water to spread evenly over the whole bottom
and to provide the space to plant a variety of plants. A good rule of thumb is that the rain garden should be
about twice as long (perpendicular to the slope) as it is wide.
When choosing the width of the garden, think about the slope of the lawn. Wide rain gardens and rain
gardens on steep slopes will need to be dug very deep at one end in order to be level. If the rain garden is
too wide, it may be necessary to bring in additional soil to fill up the downhill half. Experience shows that
making a rain garden about 10 feet wide is a good compromise between the effect of slope and how deep
the rain garden should be. A rain garden should have a maximum width of about 15 feet, especially for
lawns with more than about an 8 percent slope.
To determine the length of the rain garden:
1. Pick the best rain garden width for your lawn and landscaping.
2. Divide the size of your rain garden by the width to find your rain garden’s length.
10
Choose a size that is best
for your yard
Remember that these are only
guidelines. The size of the
rain garden also depends on
how much money you want to
spend, how much room you
have in your yard, and how
much runoff you want to control. Again, you can reduce
the size of your rain garden
by as much as 30% and still
control almost 90% of the
runoff. If the sizing table suggests that the rain garden be
200 square feet, but there is
only enough room for a
140-square-foot rain garden,
that’s fine. A smaller rain garden will usually work to control most stormwater runoff,
although some bigger storms
might over-top the berm.
EXAMPLE
Todd wants a 10-foot wide rain garden, so he divides 150 by 10 to find the rain garden
length, 15 feet.
✓
rain garden area
= length
width
150 ft
2
= 15 ft
10 ft
Runoff flows into a new rain garden (shown before plants are fully grown).
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