Rain Gardens

A rain garden is a natural or designed planted basin that collects rain runoff from roofs, driveways, walkways, and other surfaces.  These gardens are planted at a lower level than the surrounding ground so that water naturally flows into them.  They can be made of various soil, plant, and even tree types, all of which can be chosen based on location and climate.

Rain Garden (flickr: Montgomery County Planning Commission)

Rain Garden (flickr: Montgomery County Planning Commission)

Why are rain gardens important?  While they are beautiful additions to landscapes, they also serve an even more beneficial purpose, preventing two main issues:  flooding and water pollution.  Flooding results when water runs off since it cannot penetrate impervious ground surfaces such as roads, curbs, sidewalks, and driveways.  Meanwhile, as the human population continues to increase, the amount of pollution entering our water stream also increases.  Since all rain or tap water eventually reaches a river, lake, or ocean, these repositories are becoming more polluted over time.  By catching direct runoff from homes and buildings, rain gardens store and filter water, resulting in less flooding, water contamination, and strain on septic systems.

Rain Gardens:  Past, Present, Future

The earth’s natural ecosystems were the first rain gardens.  Prior to any land being settled, rain would filter through various soils, roots, and plants before returning to main waterways.  Over time, this process became less efficient as land was developed and water began to run directly into rivers and channels.

Due to this issue, rain gardens were engineered to mimic the original process for water movement.  They originated in 1990 in Prince George’s County, Maryland, where developer Dick Brinker had an idea to use them in place of traditional retention and drainage systems in a new subdivision.  Along with Larry Coffman at the county’s Department of Environmental Resources, Brinker implemented this bioretention strategy.  Each home in the subdivision had a 300- to 400-square-foot rain garden and the project showed immediate results.  Instead of the $400,000 that would have been needed for sidewalks, curbs, and gutters, the development’s rain gardens only cost $100,000.  Tests done in later years showed that these rain gardens reduced stormwater runoff by 75% to 80 %!

Since then, the use of rain gardens has greatly grown and they have become a very popular feature for both homes and businesses.  As of April 2021, 127 rain gardens were registered in the Greater Pittsburgh area, and each is helping reduce stormwater runoff and improve water quality.  Since July 2009, more than thirteen million gallons of rainfall were captured by these rain gardens! For more information, please refer to Three Rivers Rain Garden Alliance.

The Structure of a Rain Garden

The structure of a rain garden is quite simple.  A suitable area is filled with a preferred soil type and topped with a mulch or rock layer (rocks are used in certain circumstances to help better guide water), then various vegetation and shrubs are planted.  All of these elements are based on multiple factors, including location, site size, and climate, which work together to produce a rain garden with optimal efficiency for the specific space.

The overall structure of a rain garden varies depending on location.  Smaller rain gardens can be found in residential yards, while more complex gardens can be incorporated into commercial locations such as parking lots, sidewalks, parks, and green roofs.

How to Build a Rain Garden

In order to build a successful rain garden, three main factors must be determined: soil type, garden area, and depth.

Soil Type

While different soils can be used in rain gardens, keep in mind that their main goal is to absorb rain and that some soils accomplish this better than others.  Soils can mainly be broken down into three different component types: clay, silt/loam, and sand.  Clays are extremely fine and have a very slow infiltration rate.  Silts and loams have a higher infiltration rate, and sand has the highest infiltration rate of the three. Pennsylvania’s soil has a high clay content and, because of this, rain gardens will need to be larger or deeper to account for slower infiltration rates.

If the soil contains too much sand or clay, a soil mix can be a better option.  Below is the optimum rain garden soil mix:
50%-60% sand
30%-40% silt/loam top soil
10%-20% organic matter/compost

Adding organic matter or compost can really benefit a rain garden.  Compost (leafy is preferable) helps enrich the soil, increase the water infiltration rate, and break down pollutants.

The above percentages are not set in stone.  All yards, homes, and climates are very different, and therefore no two rain gardens are the same.  These percentages can be tweaked to form the optimal soil mix for one’s specific rain garden.

Garden Area

The overall size of your rain garden can be determined by its drainage area, which is the space from which water will flow into the rain garden.  If the garden is being placed beneath a slanted roof, for example, the roof area will be the drainage area; if it is being placed to receive runoff from a driveway, the driveway is the drainage area.

The drainage area can be determined by the simple area formula below:

Area=(length of area)*(width of area)

Once the drainage area is determined, the soil type is then used to factor the optimal size of the rain garden. If a rain garden is too small, it will not take in the amount of water that it ideally could; if it is too large, it will take up more space than is necessary.  Finding the optimal size takes full advantage of the land space and amount of water runoff.


A rain garden’s depth is one of its most important features, as it can be the deciding factor as to whether the garden succeeds or fails.  If too shallow, it may not be able to hold the water capacity of some rainfalls; if too deep, it may take water too long to filter through all of the soil.  The latter scenario can result in standing water, which can attract mosquitos and other pests.  To find the optimal rain garden depth, use the slope percent of the yard, found by dividing the change in yard height by the change in yard width:

Percent Slope=height/width*100

Once the percent slope has been calculated, the depth of the proposed rain garden can be determined.  The chart below displays typical depths in relation to percent slopes:

Percent Slope (%)

Typical Depth (in.)

≤ 4






≥ 12

Consider another location for your rain garden


The optimal depth of a rain garden is between four to eight inches.  If the percent slope is greater than or equal to 12%, a professional landscaper should be consulted or a different location considered.  Rain gardens should be level, as that will allow water to spread evenly across the whole garden as opposed to pooling and settling in one area.

All of these factors taken together can create an optimal rain garden, with values being altered as needed to make an individual’s ideal garden.  A few main points should always be taken into consideration:

  • Clays and silts are less permeable than sand and should therefore only be considered for larger or deeper rain gardens to account for slower water filtration
  • A rain garden should be flat to prevent pooling and settling
  • Compost and/or organic matter should be mixed into the soil to provide plant nutrients, break down water pollutants, and allow water to better permeate.

Three Rivers Rain Garden Alliance provides an online calculator to help determine appropriate garden specifications. 

Choosing Plants for Your Rain Garden

A variety of plants can be chosen for a rain garden, although native plants should certainly be considered since they require less maintenance to thrive in local climates.  (Additionally, they can provide a habitat for wildlife such as birds and butterflies.)  Rain gardens are in place year-round, so selected plants should be accustomed to both warmer and colder months. Plants can also be chosen in relation to the amount of sunlight or shade received by the garden.  For an extensive list of plants native to Western Pennsylvania, refer to the Three Rivers Rain Garden Alliance website.


The installation of a rain garden is just like that of a regular garden—it will take effort and thought, but there is no significant difference in the amount of work required.  The cost is no different either, as native plants recommended for rain gardens are comparable to other plants.

If the right vegetation is chosen, overall maintenance can be minimal.  Since native plants are adapted to seasonal changes in the area, they will not have to be replanted every year.  Mulching and weeding are helpful, especially when plants are first getting established.  Long-term maintenance includes removal of dead plants, addition of compost material, and upkeep around the garden’s edges.

Advantages of a Rain Garden

  • Simple, low-cost installation and maintenance
  • Filters stormwater runoff and pollution
  • Improves overall water quality
  • Enhances look and site aesthetics
  • Ideal for native plant species


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