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Last edited 20 Sep 2021

Rain garden

This article was written by, and republished with the permission of, The Low Impact Development Center, Inc.

A rain garden takes advantage of rainfall and stormwater runoff in its design and plant selection. It is often a small garden which is designed to withstand moisture extremities and the concentrations of nutrients that are found in stormwater runoff, in particular, nitrogen and phosphorus. It is usual for rain gardens to be sited close to the runoff source and serve to slow the stormwater as it travels downhill. This gives the stormwater more time to infiltrate and less opportunity to gain momentum and erosive power.

On the surface, a rain garden looks like an attractive garden. It may support habitat for birds and butterflies, be a formal landscape amenity, or be incorporated into a larger garden as a border or as an entry feature. What distinguishes it as a rain garden is the process by which it gets its water and what happens to that water once it arrives in the garden.

Below the surface of the garden, a number of processes are occurring which mimic the hydrologic action of a healthy forest. Soils are engineered and appropriate plants selected for the rain garden. The garden is a small bioretention cell in which stormwater is cleaned and reduced in volume once it enters the rain garden. Nitrogen and phosphorus levels and overall sediment loads in the stormwater are reduced by the action of the plants and growing media on the water. Multiple rain gardens over an area will have a positive cumulative effect on both the volume and quality of stormwater run off.

There are two basic types of rain gardens – under-drained and self-contained. Both types of rain gardens are used to improve stormwater quality, reduce runoff volumes and generally facilitate infiltration of cleaned water. Which type of garden is selected to be built is a balance of volumes of water to be treated, existing soil conditions, available space, and budget for the project.

In some cases where infiltration is not desired, the underdrain system can move excess water into a conventional storm sewer pipe system. Cases where infiltration would not be desirable would be if the bottom of the garden has less than 4 ft of clearance to the seasonal mean high water table or if the adjacent soils are contaminated and the cleaned water from the garden would become re-contaminated by coming in contact with the adjacent native solids.

Rain gardens are designed to be drained within four hours after a 1-inch rain event. Under-drained rain gardens typically are designed to drain within 2 hours of the design storm event. This is achieved through the use of highly porous planting media and underdrains which carry the cleaned rainwater away from the garden.

As a result, the plants selected for the bioretention cell need to be able to withstand both the extremes of flooding and drought. Plants on the upper edges of the garden are often xeric (very dry) in their cultural requirement descriptions with plants lower in the garden being more adapted to floodplain conditions. Many riparian edge species are particularly well suited to the extreme environments of rain gardens.

Rain gardens with no underdrain typically hold moisture longer, particularly in the lower areas of the garden. Plants selected for this garden should be able to tolerate inundation for a more extended period of time. However, as in the case of the underdrained rain garden, the surface is drained within four hours, although the soil may be saturated.

As in the bioretention cell, soils are amended with a very porous planting media, minimally to a depth of 8-inches and ideally to a depth of 2-3 ft. The lower the amount of soil amendment added when the garden is built, the more necessary it is to have plants adapted to prolonged periods of wetness. As with the underdrained rain garden, the plants on the upper edges of the garden will need to be more xeric in their cultural requirements than the plants in the lower areas.

In both types of gardens, the ground is excavated and the planting media is imported to the site. The imported planting media should be clean and weed seed free. A liner may or may not be used, depending on the local conditions.

In both the under-drained and self-contained rain gardens, the success of the garden is greater when you start with healthy and smaller, rather than larger, plants. Some plants listed are successful in rain gardens only when they are installed small and have a chance to adapt to the conditions as they grow.

Plants with deep fibrous roots tend to have a competitive advantage in a rain garden and provide the most cleaning and filtration benefits to the environment. Typical rain gardens are populated with natives or native cultivars because those are most well adapted to a locality, but other ornamental horticultural plants that are non-invasive but able to grow in the garden conditions can also be excellent choices.

Design principles:

Rain garden areas of about 10-20% of upstream impervious area are recommended. Optimum rain garden size is about 50 sq.m. draining 250 sq.m. of impervious area.
Smaller, distributed rain gardens are better than single large scale facilities.
Locate rain gardens a minimum 30.5 m from wells, 3 m downslope of building foundations, and only in areas where foundations have footing drains and are not above steep slopes.
Provide pre-treatment and erosion control, i.e. grass filter strip to avoid introducing sediment into the garden.
At point-source inlets, install non-erodable material, sediment cleanout basins, and weir flow spreaders.
Bottom width - 600 mm (min.) to 3,000 mm (desirable). Length-width ratio of 2:1.
Side slopes - 2:1 maximum, 4:1 preferred for maintenance. Maximum ponded level - 150 - 300 mm.
Draw-down time for maximum ponded volume - 72 hours.
Treatment soil depth - 450 mm (min.) to 1,200 mm (desirable); use soils with minimum infiltration rate of 13 mm/hr.
Surface planting should be primarily trees, shrubs, and groundcovers, with planting designs respecting the various soil moisture conditions in the garden. Plantings may include rushes, sedges and grasses as well as lawn areas for erosion control and multiple uses.
Apply a 50-75 mm layer of organic mulch for both erosion control and to maintain infiltration capacity.
Install a non-erodible outlet or spillway to discharge overflow.
Avoid utility or other crossings of the rain garden. Where utility trenches must be constructed below the garden, install trench dams to avoid infiltration water following the utility trench.
Drain rock reservoir and perforated drain pipe may be avoided where infiltration tests by a design professional show a subsoil infiltration rate that exceeds the inflow rate.