Rain gardens, also called bioretention facilities, are one type of method used to help soil absorb more rainwater runoff. They can also clean stormwater that has been polluted. Rain gardens are special garden areas designed to slow down the speed, reduce the amount, and lower the pollution in runoff from areas that do not let water soak in, such as rooftops, driveways, sidewalks, parking lots, and compacted grass areas. Rain gardens use plants and specially designed soil to hold stormwater, slow how quickly water soaks into the ground, and clean pollutants carried by runoff. These gardens also help reuse rainwater, reducing the need for extra watering. Planting rain gardens can lower the temperature of the air and water around them, which is especially helpful in cities with many surfaces that absorb heat, a problem called the heat-island effect.

Rain gardens often include plants found near wetlands, such as wildflowers, sedges, rushes, ferns, shrubs, and small trees. These plants take in water and nutrients from runoff and release water vapor into the air through a process called transpiration. Deep roots from these plants create paths for water to soak into the ground. These roots also help water move through soil more easily, keep soil from becoming too compacted, spread moisture, and support helpful microorganisms that clean water. These microorganisms break down organic materials and remove nitrogen from the water.

Rain gardens have many benefits. They improve water quality by cleaning runoff, help prevent flooding in nearby areas, create attractive landscapes, and offer opportunities to grow a variety of plants. They also support wildlife and increase biodiversity, connecting buildings with their surroundings in ways that help the environment. Rain gardens can improve the quality of water in nearby lakes, rivers, and groundwater supplies. They also reduce the amount of polluted water that flows into storm sewer systems, which can cause erosion, pollution, and flooding in surface water. Additionally, rain gardens help save energy by reducing the need for traditional stormwater systems to work as hard.

History

The first rain gardens were made to copy the natural areas that held water before cities were built. In 1990, residential rain gardens were created in Prince George's County, Maryland. A developer named Dick Brinker, who was building a new neighborhood, had the idea to replace traditional stormwater management ponds with a special area for water. He shared this idea with Larry Coffman, an environmental engineer and a county official. The result was the use of rain gardens in Somerset, a neighborhood where each home has a rain garden covering 300–400 square feet (28–37 square meters). This system was very cost-effective. Instead of using curbs, sidewalks, and gutters, which would have cost nearly $400,000, planted drainage swales cost only $100,000 to build. This was also less expensive than building traditional stormwater ponds that could handle large rain events. Later studies showed that rain gardens reduced stormwater runoff by 75–80% during regular rain events.

Some rain gardens were used before experts recognized them as important tools for Low Impact Development (LID). Any shallow garden that collects and filters rainwater to stop it from leaving the area is, by definition, a rain garden—especially if plants are used to help this process. Vegetated drainage areas, now called "bioswales," have been used as the main way to manage runoff in many places for a long time, even before concrete sewer systems became common in industrialized countries. What is new is the growing use of detailed scientific knowledge to understand how these methods can help create sustainable communities. This applies to both older neighborhoods that are adding rain gardens to existing systems and newer areas that want to build in a faster and more sustainable way.

Purpose

In natural areas, most rainwater soaks into the ground and is taken up by plants. This helps refill underground water supplies and reduces the amount of water that flows over the surface. Plants also slow water movement, which lowers the chances of flooding and soil erosion. In suburban areas, hard surfaces like asphalt or concrete cover about 5–10% of the land. In contrast, central urban areas are mostly covered by hard surfaces, which can cover 90–100% of the land. In these areas, most rainwater becomes surface runoff, which often overflows stormwater systems. Runoff in cities can be two to ten times greater than in natural areas. This increase causes flooding, erosion, and pollution in water systems. Rain gardens help manage stormwater by restoring natural water flow in built areas.

In urban areas, natural low areas where water would collect are often covered by hard surfaces like asphalt, pavement, or concrete for roads. Stormwater is directed into drains, which can cause pollution, erosion, or flooding in waterways. This water is often warmer than groundwater and can harm aquatic ecosystems by reducing oxygen levels. Stormwater also carries many pollutants, such as chemicals, pesticides, and metals, that wash off hard surfaces during rain.

Rain gardens are designed to capture the first rainwater and reduce pollution that flows directly into waterways. Natural processes help clean stormwater by filtering it through soil and plants. Pollutants are trapped in soil and plant membranes, where they begin to break down. This spreads contaminants across the area instead of concentrating them. Organizations like the National Science Foundation and the Environmental Protection Agency are studying ways to improve rain gardens by adding materials that reduce pollutants.

Designing rain gardens is challenging because it requires predicting the types and amounts of pollutants they can handle during heavy rains. Pollutants include organic materials like animal waste and oil, as well as inorganic materials like heavy metals and fertilizer nutrients. These can harm water systems by causing excessive plant and algae growth. Predicting pollution levels is especially difficult after long dry periods, as the first rain often carries accumulated pollutants. Rain garden designers have focused on using strong native plants and improving soil filtration. Recently, they have also added materials that chemically reduce pollutants. Some plants store nutrients and release them when they die, while others absorb heavy metals. Removing these plants after their growth cycle helps clean polluted soil and water, a process called phytoremediation.

Rain gardens use soil layers to trap pollutants from stormwater. Plants also help by absorbing pollutants and encouraging microbes in the soil near their roots. The type of soil and plants used greatly affects how well the garden works. Soils with more sand improve water absorption but may reduce nutrient retention. Plants with deep or fibrous roots help remove nitrogen and process pollutants.

Studies show that rain gardens with soil rich in organic matter and deeper layers can remove over 60% of nitrogen under ideal conditions. Data from the International Stormwater BMP Database shows rain gardens can remove about 80% of solid particles, 50–60% of heavy metals, and reduce nutrients like nitrogen and phosphorus. These results support rain gardens as effective tools for managing urban stormwater.

Stormwater management works across entire watersheds to protect water quality. A watershed includes areas where water collects, stores, and flows underground. Natural watersheds are damaged when covered by hard surfaces, which send polluted runoff into streams. Urban areas have more pollution due to human activities. Rain on hard surfaces creates runoff with oil, bacteria, and sediment that eventually reaches streams and groundwater. Stormwater control methods like rain gardens treat this runoff and return clean water to the soil, helping restore watersheds. The success of these systems is measured by how much rainfall becomes runoff and how quickly runoff is released. Even small rain gardens can help reduce urban runoff over time. Adding more permeable surfaces, like rain gardens, reduces pollution in natural water systems and recharges groundwater faster. Rain gardens also help reduce the strain on public stormwater systems.

The bioretention method, including rain gardens, uses natural processes to move, store, and filter stormwater before it becomes runoff. It also reduces hard surfaces that allow polluted water to flow. Rain gardens work best when connected to other stormwater control systems. This integrated approach is called the "stormwater chain," which includes methods to stop runoff, store it for infiltration or evaporation, slow its release, and direct rainwater to storage areas. Rain gardens affect the larger water system by filtering water through soil and plants before it reaches groundwater or drains. Any remaining runoff from a rain garden is cooler than runoff from hard surfaces, reducing harm to water systems. Increasing permeable surfaces with rain gardens reduces pollution in natural water systems and recharges groundwater more quickly.

Bioretention

Low-impact design (LID) for stormwater management uses a method called bioretention. This approach combines landscape and water design to use the natural abilities of soil, plants, and microorganisms to manage water flow and improve water quality on a site. Bioretention systems are mainly used to control water, treating runoff from cities, stormwater, groundwater, and in some cases, wastewater. Special constructed wetlands are needed to handle sewage or grey water, which can affect human health more than other types of water. Benefits of bioretention include more wildlife, better habitats, and less energy use and pollution. Using natural bioretention systems helps avoid covering land with hard, impermeable surfaces.

Bioretention manages stormwater quantity by capturing rainwater, allowing it to soak into the ground, and releasing it through evaporation and plant processes. Rain is first collected by plant leaves and stems and stored in tiny spaces in the soil. Water then moves downward through the soil, where it is held until the soil can no longer absorb more. When the soil is full, water pools on the surface. This pooled water and water from plants and soil evaporates into the air. Designers aim to keep pooled water shallow to increase evaporation. Water also evaporates from plant leaves, a process called evapotranspiration.

Bioretention improves water quality by allowing particles to settle, filtering out debris, absorbing nutrients, and breaking down pollutants. When water pools on bioretention systems, heavy particles sink to the bottom. As water moves through soil and plant roots, smaller particles and dust are filtered out. Plants take in nutrients for growth or store them. Chemicals in the water attach to soil and root surfaces, making them less harmful. Microorganisms in the soil break down remaining chemicals and organic matter, turning pollutants into harmless soil material.

Although natural water purification relies on planted areas, the success of bioremediation depends on soil quality and microorganism activity. Plants support these processes by creating space in the soil, reducing soil compaction, providing homes for microorganisms on their roots, and bringing oxygen into the soil.

Design

Stormwater garden design uses bioretention principles to manage rainwater. These systems are arranged in a sequence that follows how rainwater moves from buildings and permeable surfaces to gardens and eventually to water bodies. A rain garden needs a space where water can collect and soak into the ground. Plants in the garden help maintain the rate at which water soaks into the soil, support microorganism growth, and store water. Since infiltration systems reduce stormwater runoff and peak flow rates, rain garden design must start with a site analysis to understand rainfall loads on the proposed system. This analysis helps determine the best plant choices and soil types for the garden. Rain gardens should be designed to handle the peak runoff rate during the most severe storm expected. The amount of water the system will handle determines the best flow rate for the garden.

Existing gardens can be changed to function like rain gardens by redirecting downspouts and paved surfaces to planting areas. Even though these gardens may have loose soil and established plants, they might need more space or additional plants to increase infiltration capacity. Some plants cannot handle long periods of wet roots and may not survive increased water flow. Rain garden plants should match the site conditions after determining the location and storage needs of the bioretention area. Rain gardens also help reduce urban runoff and can support habitats for native butterflies, birds, and insects.

Rain gardens are sometimes confused with bioswales. Bioswales slope toward a destination, while rain gardens are flat. However, a bioswale may end with a rain garden as part of a larger stormwater system. Drainage ditches can be managed like bioswales and may include rain gardens in series to save on maintenance costs. A garden that always holds water, such as a pond or wetland, is not a rain garden. Rain gardens also differ from retention basins, where water soaks into the ground more slowly, over a day or two.

Water in rain gardens filters through layers of soil or engineered growing soil, called substrate. When the soil becomes saturated, extra water pools on the surface and then soaks into the natural soil below. The bioretention soil mix should usually include 60% sand, 20% compost, and 20% topsoil. Soils with more compost improve groundwater and rainwater filtration. Non-permeable soil must be replaced regularly to ensure the system works well. Sandy soil (bioretention mix) should not be mixed with surrounding soil that has less sand, as clay particles can settle between sand particles and form a solid, non-porous substance, as found in a 1983 study. Compact lawn soil does not hold groundwater as well as sandy soil because its tiny pores cannot retain large amounts of runoff.

If an area’s soil is not permeable enough to allow water to drain and filter properly, the soil should be replaced, and an underdrain installed. A drywell with gravel layers near the lowest point in the rain garden can help water soak into the ground and prevent clogging. However, a drywell placed at the lowest point may become clogged with silt, turning the garden into an infiltration basin and defeating its purpose. The more polluted the runoff, the longer it must stay in the soil to be purified. Longer purification times can be achieved by using several smaller rain garden basins with soil deeper than the seasonal high water table. In some cases, lined bioretention cells with subsurface drainage are used to retain small amounts of water and filter larger amounts without letting water soak too quickly. A five-year study by the U.S. Geological Survey shows that rain gardens in urban clay soils can work without underdrains or replacing native soil with bioretention mix, as long as pre-installation infiltration rates are at least 0.25 inches per hour. Type D soils require an underdrain paired with sandy soil mix to drain properly.

Rain gardens are often placed near a building’s roof drainpipe (with or without rainwater tanks). Most rain gardens are designed to be the endpoint of a building or urban site’s drainage system, allowing all incoming water to soak through layers of soil or gravel beneath the plants. A French drain may direct some rainwater to an overflow location during heavy rains. If the bioretention site receives extra runoff from building downspouts or if the existing soil filters water faster than 5 inches per hour, the rain garden’s substrate should include a gravel or sand layer beneath the topsoil to handle the increased load. If a site was not originally designed with a rain garden, downpipes can be disconnected and redirected to a rain garden for retrofit stormwater management. This reduces the water load on traditional drainage systems and directs water for infiltration and treatment through bioretention features. By reducing peak stormwater discharge, rain gardens mimic natural water cycles, allow groundwater recharge, and reduce stormwater volumes. However, they may not improve pollution unless remediation materials are included in the filtration layers.

Rain gardens have inlets and outlets, which are where stormwater enters and leaves the garden. Inlets are important because they spread runoff evenly, reduce erosion, and lower sediment entering the system. Since rain gardens handle peak runoff during severe storms, inlet areas should be reinforced with erosion-resistant materials like gravel or rock to protect the soil and plants.

Outlets are designed to move excess water from a rain garden during storms that exceed the system’s capacity. Outlets are critical because too much water can damage plants, erode soil, or harm infrastructure. Outlet structures often include overflow weirs, standpipes, curb openings, or stabilized spillways built into perimeter berms. Overflow levels are set to keep ponding depth low and allow excess water to drain once storage limits are reached. Many guidelines suggest rain gardens should drain within 24–48 hours after storms. Overflow structures help ensure this happens during heavy or long rain events. These outlets are often protected with stone or concrete to prevent erosion and damage downstream.

Typical rain garden plants include herbaceous perennials and grasses, chosen for their porous roots and fast growth. Trees and shrubs can also be planted to cover larger areas on bioretention sites. Specific plants are selected based on the site’s conditions.

Projects

• The Healthy Waterways Raingardens Program teaches people how to build rain gardens at home. These gardens help manage stormwater and protect waterways. The program’s goal was to have 10,000 rain gardens built in Melbourne by 2013.
• Melbourne Water keeps a list of projects that use Water Sensitive Urban Design, including 57 examples of rain gardens and bioretention systems. Melbourne Water is a government group in Victoria that manages water sources for Melbourne.
• Water By Design is a program that helps people learn about Water Sensitive Urban Design, including rain gardens, in South East Queensland. It was created in 2005 by the South East Queensland Healthy Waterways Partnership as part of a plan to improve water quality.

• The London Wetland Centre includes a rain garden.
• In London, Islington Borough Council worked with Robert Bray Associates to build a rain garden in the Ashby Grove development in 2011. The garden uses water from a 30m² roof and includes tools to measure water, soil, and quality. It can hold 2.17m³ of water, enough for a 1 in 100 storm plus extra for climate change.
• The Day Brook Rain Garden Project added rain gardens to a residential street in Sherwood, Nottingham.

• The 12,000 Rain Garden campaign in Puget Sound aims to build 12,000 rain gardens by 2016. It provides resources for the public, professionals, and officials. The campaign hopes to reduce 200 million gallons of polluted runoff yearly, improving water quality.
• Maplewood, Minnesota, encourages residents to install rain gardens. A partnership between the city, the University of Minnesota, and a watershed district included a focus group to guide other communities.
• Some governments offer grants for rain gardens. In Dakota County, Minnesota, residents can get $250 and help through the Landscaping for Clean Water program.
• In Seattle, a project called SEA Street redesigned a street with rain gardens, reducing stormwater by 99%.
• The 10,000 Rain Gardens initiative in Kansas City, Missouri, encourages property owners to build gardens, aiming for 10,000 total.
• In Grand Rapids, Michigan, the West Michigan Environmental Action Council runs a rain garden program. In Oakland County, a pamphlet encourages rain gardens to improve water quality in the River Rouge watershed. In Washtenaw County, homeowners can get free professional garden designs and build gardens themselves.
• Portland, Oregon, offers discounts and workshops through its Clean River Rewards program to help residents install rain gardens.
• In Delaware, rain gardens were created by the University of Delaware and groups like the Appoquinimink River Association.
• In New Jersey, Rutgers Cooperative Extension built over 125 rain gardens to help reduce flooding and improve water quality. A rain garden manual was created with the Native Plant Society of New Jersey.
• According to Massachusetts’ Department of Environmental Protection, rain gardens can remove 90% of suspended solids, 50% of nitrogen, and 90% of phosphorus.
• Dr. Allen P. Davis, a professor at the University of Maryland, studied rain gardens for 20 years. His research showed rain gardens capture pollutants and improve water clarity. A rain garden at the University of Maryland’s Center for Young Children helps teach sustainability.
• At the University of Technology in Xi’an, China, a rain garden was studied over four years. It retained water from 23 of 28 large storms, overflowing only five times.
• In Xi’an, rain gardens are part of Low Impact Developments (LID).
• China plans to implement rain gardens as part of its environmental efforts.