Creating a Rain Garden

Interstate Commission on the Potomac River Basin

What is a Rain Garden?

A rain garden, also called a bioretention cell, is an attractive native plant garden with a special purpose, to reduce and filter the stormwater entering our local waters.  It is constructed as a place to direct stormwater from roofs, driveways and parking lots, allowing water to be held in the plants, mulch and soil.

Rain Gardens use the concept of bioretention, a water quality practice in which plants and soils remove pollutants from stormwater naturally.  Rain Gardens are created in low-lying area, with specific layers of soil, sand, and organic mulch.  These layers naturally filter the rain as it runs into the Rain Garden.  During the next few days after a storm, the soil absorbs and stores the rainwater and nourishes the Garden’s grasses, trees, and flowers.  The traditional system of curbs, gutters, and storm drains carries stormwater runoff directly to local streams and rivers without any bioretention filtering process.  Instead Rain Gardens filter and reuse the water, reducing stormwater pollution, while providing attractive landscaping.

Why Should I Create a Rain Garden?

The Environmental Protection Agency (EPA) has determined that up to 70% of the pollution in our surface waters comes from stormwater.  We tend to think that large industrial polluters cause most of this pollution, but this is not the case.  Many studies have found that nearly 50% of that pollution comes from small businesses, individuals and homeowners, due to lawn care, household chemicals, and automobile usage.

One way of reducing the non-point source pollution flowing into our streams and rivers is to install rain gardens on our property.  Rain Gardens aren’t just for houses! In fact, the use of rain gardens for stormwater management originated at commercial sites and public spaces where space is limited, and the installation and maintenance of conventional environmental practices, such as oil and water separators and stormwater ponds, is expensive.

In addition to their water quality benefits, rain gardens:

  • Promote environmental stewardship and community pride
  • Provide habitat for wildlife and native plants
  • Moderate air temperatures through evaporation
  • Increase real estate values by creating an aesthetically pleasing landscape

Developing Your Goals and Budget

Before charging ahead with a rain garden project it is crucial to determine the budget for your work and set your goals.  Let’s start with determining your budget, which will help determine your goals.

Cost

The cost associated with installing residential rain gardens average about three to four dollars per square foot, depending on soil conditions and the density and types of plants used in the installation.  Commercial, industrial and institutional site costs can range between ten to forty dollars per square foot.  In any rain garden design, the cost of plants varies substantially and can account for a significant portion of the garden’s price tag.

The costs associated with a rain garden installation are generally slightly higher than those of installing a typical landscaping treatment.  The increased costs are due to the increased number of plantings, the need for additional soil excavation and backfill material and the use of underdrains in some cases.

The cost of a rain garden is based on several factors including:

  • The area of the rain garden
  • The depth of the rain garden
  • Whether or not the soils found on site can be used in the garden (if they are less than ten percent clay)
  • If curb cuts are required to direct the flow from a roadway or parking lot into the garden
  • If the site requires an underdrain (a perforated pipe place under the rain garden in order to achieve a desired discharge rate)
  • If you are going to design and install the garden yourself or use contractors

We have included an automated spreadsheet that will help you determine the costs of installing your rain garden.  To use the spreadsheet, provide information about your project in the white boxes.  By changing the responses in these boxes, the estimated amount of materials needed changes and the total price of installation is updated.  Please remember that the prices are estimates only and will vary depending on the vendor used.

Financing

The options available for paying for a rain garden installation vary depending on where the rain garden will be installed.  If the rain garden will be installed on public land such as in a schoolyard or at a library or municipal building, grants are often available to fund construction.  Grantors include the Chesapeake Bay Trust, the Chesapeake Bay Foundation, Pennsylvania’s Growing Greener Program, the Chesapeake Bay Restoration Fund, and the National Fish and Wildlife Foundation’s Small Watershed Grants Program.  Your local government also may help pay for the garden’s installation.

If the rain garden will be on private lands, the options for financing are much more limited.  If the garden will be installed at a business location, ask the business owner, the property manager, and the property owner to help finance the project.  Let them know that rain gardens can reduce landscape maintenance costs, reduce stormwater utility fees, and increase property values.  If the rain garden will be on an individual homeowner’s property, the garden should be small enough that the costs will be reasonable for the homeowner to absorb.

Goals

You may be installing a rain garden in the yard of a member of your organization, or for a large apartment complex; you may be targeting a neighborhood with high storm water runoff, or targeting a specific audience such as a school in your watershed.  Whatever the case, you will want to write out your specific goals and let them guide your work plan.

Some example goals:

  • Reduce impacts of stormwater runoff for a sub-watershed of your local stream
  • Enhance community awareness of stormwater runoff and non-point source pollution
  • Foster environmental stewardship and an interest in science by students, teachers, administrators, and members of the community

Keep in mind that your goals and your audience may affect whether you use a contractor or install your own rain garden.  Installing your own rain garden helps develop a sense of pride and ownership, however using a contractor may be required for a large site because of the need for special equipment and expertise.  It may be a great teaching opportunity, for example, for young students to help install their own rain garden.  Even if your rain garden will require the use of contractors and engineers, you can invite the community to plant the rain garden.  This will reduce costs and instill a sense of ownership and pride.

Creating a Timeline

Your timeline will vary depending on how you have set up your project.  For example, if you are planning on financing your project through a grant, your first steps are to find an appropriate grant vehicle and apply for the grant.  Below is a sample timeline for a rain garden project on public property (a library) funded through a grant where individuals will aid in the installation.

  • Determine the property where you will install the rain garden
  • Examine the flow of rain water on the site to determine the size and location of the rain garden
  • Contact the utility companies to make sure that the site is free from obstructions
  • Calculate the cost of installing the rain garden
  • Using the information above, apply for a grant to fund the project
  • Assuming that the grant is funded, arrange for a location to store the supplies needed for the rain garden and purchase them
  • Arrange for supplies to be delivered or to pick up them up (NOTE: this may be an additional cost)
  • Solicit volunteers to aid in the installation of the garden and/or its planting (for example a watershed group, a friends of the library group, school group, boy or girl scout troop)
  • If possible, arrange for public officials to be present at the garden’s ground breaking, installation, planting, or dedication (could be all on the same day)
  • Purchase or rent the necessary tools and equipment for the installation
  • Issue press releases and follow up with media about the rain garden construction/planting day to ensure publicity
  • Create and print educational materials to provide to reporters, public officials, and those who aid in the construction
  • Construct the rain garden (can be done in advance of a planting or dedication event that involves the media and public officials)
  • Install the plants at the rain garden and hold a public ceremony
  • Follow up with the media to ensure the rain garden is publicized through additional press releases and phone calls
  • Follow up with event participants—sending public officials and volunteers thank you letters

Location and Size of your Rain Garden

Location

Finding a good location for a rain garden involves balancing a number of different factors:

The rain garden should be located in a place where it will receive runoff.  Check to make sure runoff flows to your site, or could flow with minor modifications, such as cutting a space out of a curb.

The rain garden must be located far enough from buildings to avoid damage to their foundations.  If the rain garden will be located near a building and it has a basement, the rain garden should be located at least 25 feet from the building.  If there is no basement, the rain garden should be at least 5 feet from the building.

Find out where underground utilities are buried.  Stay at least five feet horizontally and one foot vertically from any utilities.  Common utilities will include: water, sewer, electricity, natural gas, telephone, cable, and storm sewers.  Note: It is often difficult and time consuming to find out exactly where all of the utilities are buried on your site, so start early.  Write letters to each of the utilities, requesting plans showing the locations of underground utilities on your property.  Before construction, you should call Miss Utility, who will come to your site and mark the locations of utilities with spray paint, so you can be certain of avoiding them.

Check your soil.  If possible, locate the garden in uncompacted soils with low amounts of clay.  Since a lot of earth is moved during the construction of buildings, it is likely that your soils will be highly compacted, and will not drain well.  In order to compensate for this, you should use a special soil mix within the rain garden.  You should also install an underdrain at the bottom of the rain garden.

Size

When full, the ponded area of your rain garden should have a maximum depth of six inches.  Rain garden should have a minimum depth of 2.5 feet, but this depth may be constrained by the maximum depth of the underdrain at your site.  The underdrain must discharge down gradient from the rain garden so depending on the site’s topography, the bottom of the rain garden may need to be shallower than three feet below the ground surface.

If the rain garden is being designed for a new construction site, its size is matched to the site’s drainage area.  However, when a site is being retrofit, the size of the rain garden is often limited by the availability of suitable space.  It is important to estimate how effectively your proposed rain garden will capture, treat and infiltrate runoff from your site.
The included sizing tool will help you estimate how much runoff your garden can capture.  To use the spreadsheet, fill in only the shaded areas, the rest will calculate automatically.  First calculate the drainage area for the rain garden using site plans.  Then adjust the Device Storage Depth and Soil Depth to reflect your design.  Ideally, the rain garden should be large enough to infiltrate 90% of the runoff generated in the drainage area.  Adjust the rain garden area to get as close as you can to this goal while staying within the space constraints of your site.

Advertising your Rain Garden Project

There are lots of ways to advertise for an event and, unfortunately, no one way works consistently.  Here are several options for you to choose from.  The best outreach campaigns make use of several techniques and only you can know what is best for your project.  Advertising options include:

  • Creating and distributing door hangers to target a specific neighborhood where you would like to work
  • Creating and distributing a press release to your local paper, radio stations, television stations, and outdoor/environmental groups (see the Resources section for information on how to write and distribute a press release)
  • Creating flyers and placing them in public libraries, recreation centers, and walking trails
  • Add the event to social media
  • Running an advertisement in a local paper, or on radio stations and/or television stations
  • Discussing the event at a targeted community association, church group, youth group, and/or service organization meeting
  • Advertising your event on free on-line environmental calendars such as the Chesapeake Network
  • Sending out an email to people you think would be interested and asking them to tell their friends as well
  • Sending out a mass mailing to your targeted audience

Organizing a Rain Garden Construction/Planting

Materials

Please keep in mind that this is a general list of materials.  The amounts you need will vary and depend on the size of the rain garden you are creating.

  • Topsoil
  • Mulch
  • Plants
  • Screwdrivers
  • Hammers
  • Shovels
  • Rakes
  • Garden Hose
  • String and Stakes
  • Wheelbarrows
  • Gardening Gloves

Selecting Plants

The success of your rain garden will depend on this important information analyzing the sites characteristics carefully.  Take notes and track what is happening at the site in the rain and on sunny days.  Write down how much sun it gets – how many hours of afternoon sun? morning sun?  Is there a reflection off of an adjacent building that provides more light or seems to make the heat more severe?  Figure out if there are any unusual ‘microclimates’ at the site and then begin researching the plants that like these conditions.

Plant your rain garden with plants that thrive in that environment.  Whether in the sun or shade, rain gardens can be planted with shrubs and flowers that are beautiful and low maintenance – as long as you select the plants that love those conditions.  Native plants – or plants that grow naturally in this climate or region — can thrive without a lot of care, extra water, or extra fertilizer.

This is an opportunity to learn more about the types of plants that love the sun, love the shade, love to have their ‘feet wet’ for a day or two, or don’t mind being dry for days on end.  It is like a puzzle.  See if you can pick out plants that are different heights, have different leaf color, and bloom different colors throughout the spring, summer, and fall seasons.  Remember that even in the winter, plants without leaves can provide berries for birds and have an unusual structure that can be accentuated in the snow.

Once you know your site, make a scale drawing of your future rain garden and design where your plants will go.  This step is part art and part science.  Use circle templates of different sizes to represent the different possible plant and draw the plants in your rain garden.  Select several 4-6 foot shrubs, a lot of perennials (flowers that come back every year), and depending on the size of the rain garden a medium-sized tree or two 15-20 feet at full growth.  For the science of planting – remember to think about the plant’s tolerance to moisture, sunlight, and temperature in placing the plants.  In terms of the art of planting consider the colors of the plants at different times of the year and the pattern of the plants in the garden (irregular patterns tend to look more natural).

Also consider these things when selecting plants:

  • Choose native plants when possible
  • Avoid planting non-native invasive plants
  • Choose fragrant or edible plants when possible
  • Avoid toxic/poisonous plants
  • Avoid plants that produce excessive pollen

Construction

Construction techniques are extremely critical in ensuring the success of a rain garden. Various construction guidelines and inspection points are given below, as well as a general construction schedule adapted from the Prince George’s County Bioretention Manual.  Local or regional rain garden/bioretention cell guides may also have useful information.

Remember that each rain garden is unique.  The sequence of construction described below is for a large-scale rain garden.  A small rain garden may not require some of the steps below and some of the steps may take less time and therefore be combined.

Install sediment control devices (1/2 day) – These are for larger projects.  Check with your local building permit office.  It’s a good idea to surround the down gradient part of the site with straw bales or silt fence.  It’s readily available and inexpensive.

Grade site to elevations shown on plan (1 day) – If applicable, construct curb openings and/or remove and replace existing concrete.  Curb openings should be blocked or other measures taken to prohibit drainage from entering construction area.  Equipment such as a backhoe may be rented for this step.  Make sure any Miss Utility is notified before any digging.  Safety fence should be used around any construction area or excavation.

Stabilize grading within Limit of Disturbance  (1/2 day) – except for the bioretention area, which will be planted.  Surrounding the garden with a biolog, straw bales, or compost berm to keep sediment out of the bioretention cell.

Excavate bioretention area (1/2 day) – to the proposed depth and scarify the existing soil surfaces, taking care not to compact the in-situ materials.  A contractor may be hired to dig the hole.  Make sure that there are no open areas or pits open at the end of the day and no excavation over local jurisdiction or OSHA limits (Generally bioretention cells are less than 3 feet deep and therefore within OSHA limits).

Install underdrain system and observation wells (1/2 day) -, if specified.  Use perforated four inch HDPE pipe and cover the pipe with about two inches of pea gravel.

Backfill bioretention area with planting soil (1/4 day) – Fill the garden with eight inch layers of soil.  Saturate each layer and let it drain and then place the next layer.  Be careful not to compact the soil layers with equipment.

Plant vegetation (1/4 day) – Use a map of the rain garden that you have created in advance.  When planting, try to avoid compacting the soil as much as is possible.  After planting, rake the soil that was compacted to restore its original condition.

Mulch and install erosion protection at entrance points (1/4 day) – Place the planters over the installed plants so that the mulch lands around the plants and not on them.  Remove the planters and spread the mulch evenly.  Remove sediment control practices or entrance blocks with inspector authorization if this project requires a permit.  It is sometimes recommended to leave perimeter biodegradable controls to reduce sediment loads to rain garden.
Total Estimated Construction Time:  5.5 Days

Planting Event

If you are going to have a press event to publicize your group and the rain garden installation, we recommend holding it when the rain garden is to be planted.  There are several reasons for choosing this time.

  • The other stages of rain garden construction can be technical and may take longer than expected
  • There is a great sense of accomplishment from having a site go from being bare to being covered with shrubs, trees and flowers. It can therefore instill a sense of ownership in the participants
  • This planted garden creates a great visual backdrop for photos and television footage
  • A planting event can use many volunteer and no great skill or strength is required

It is not enough to have the plants ready for your event—some final preparations need to be made to make your construction/planting a success.

In advance of your event make sure that you have:

  • Enough tools to share among participants
  • Refreshments and snacks if possible or at least provide water
  • Created and printed a sign-in sheet
  • Prepared a task list so that you use your volunteer’s time and skills wisely
  • Done as much prep work as is possible—for example, placed the plants where they are to be planted
  • Called key volunteers and asked them to arrive early and be Team Leaders
  • Put together an emergency medical kit and set up an emergency plan in case of an accident
  • Made sure that bathroom facilities are available for volunteers
  • Made sure to have access to water hose if possible
  • Contacted local newspapers,
  • Designated a volunteer to take photos of the event

Maintaining Your Rain Garden

A rain garden requires additional maintenance after installation, as do any new plantings.  The garden should be watered thoroughly once a day, unless there is a significant rainfall, for fourteen day or until the plants are established.  Once established, a routine maintenance schedule should be followed for the rain garden.  The example presented here is adapted from the Prince George’s County Bioretention Manual.

Maintenance Schedule

Soil

  • Visually inspect and repair erosion monthly. Use small stones to stabilize erosion along drainage paths
  • Check the pH once or twice a year; apply an alkaline product, such as limestone, if needed

Mulch

  • Re-mulch any void areas by hand as needed
  • Every 6 months, in the spring and fall, add a fresh mulch layer
  • Once every 2 to 3 years, in the spring, remove old mulch layer and apply a new one

Plants

  • Immediately after the completion of cell construction, water plant material for fourteen consecutive days unless there is sufficient natural rainfall
  • When trees have taken root, or at least by 6 months, remove stakes and wires
  • Once a month (more frequently in the summer), visually inspect vegetation for disease or pest problems
  • If pest treatment is warranted, use the least toxic approach
  • Twice a year, between March 15th to April 30th and between October 1st to November 30th, remove and replace all dead and diseased vegetation considered beyond treatment
  • During times of extended drought, look for physical features of stress (un-revived wilting, yellow, spotted or brown leaves, loss of leaves, etc.) and water in the early morning as needed
  • Weed regularly
  • Prune excess growth annually or more often, if desired

General

After rainstorms, inspect the garden and make sure that drainage paths are clear and that ponding water dissipates over four to six hours (the water may pond for longer periods during the winter and early spring).

Source:  Low Impact Development Center

Resources