Rain gardens

This article is about planted installations designed to capture surface runoff in an amended soil.
For more highly engineered systems, see Bioretention.

An image of a typical rain garden, with various vegetation, shrub species, and perennials to increase infiltration, filter out pollutants and reduce runoff/overland flow directed into the facility from rainwater collected from a building's roof. Rain gardens also provide food and shelter for pollinators, bird species, and other wildlife. More complex rain gardens with drainage systems and amended soils are often referred to as bioretention facilities. A note: The following is an "image map", feel free to explore the image with your cursor and click on highlighted labels that appear to take you to corresponding pages on the Wiki.

Disconnected downspout routing water into a rain garden.

Overview

Rain gardens capture roof, lawn and driveway runoff from low to medium density residential lots. These can be simple gardens constructed by the homeowner as a retrofit, or they can be professionally designed into a residential development.

Rain gardens are ideal for:

• Capturing the flow from disconnected downspouts
• Community design/build projects for raising awareness about stormwater capture and functional landscapes

The fundamental components of a rain garden are:

• An amended soil mix
• Suitable planting

Additional components may include:

• A splash pad of stone to prevent the downspout flow from eroding the soil
• A simple overflow swale to another area of the garden

Planning Considerations

Raingardens look like typical gardens. However, their infiltration and bioretention functions are much greater than normal gardens. By landscaping areas to include a surface depression of approximately 150 mm, rain gardens allow runoff from residential properties to collect and to easily absorb into the ground. Within the rain garden, special types of soil, 'filter media' or soil amendments increase the amount of water absorbed by the garden and infiltrated into native soils.

Location is very important to a functioning rain garden. When looking at potential locations, look for low areas where runoff can be easily be directed into the garden. Rain gardens are a great option for residential LID retrofits because homeowners can customize the garden to suit their desired styles. They can be planted with a variety of vegetation including shrubs, grasses and flowers.

A well-designed rain garden can be maintained with minimal care, which is a great marketing point for persuading homeowners. In the first two years, the plants will need watering to ensure they become established. After this establishment period, the garden should only need simple maintenance, e.g. weeding.

Design

Plant Selection

The design of a rain garden allows for several different planting zones.

• A variety of plant material can be selected based on the desired aesthetic, including turf grass, ornamental grasses and perennial flowers, shrubs, and trees.
• The aesthetic can range from very naturalistic to quite formal, depending on the planting plan.
• Vegetation must also be capable of tolerating prolonged wet and dry periods unless an irrigation plan is in place
• If the facility is intended to be relatively inconspicuous, then the tallest plant material should be placed into the deepest part of the rain garden. Otherwise, choose taller plants for the back of the garden tapering the height down to the front of the garden. .

Since rain gardens are typically smaller scale homeowner projects smaller stock plants may be considered. Smaller containers may be easier to purchase, transport, and plant, but it may take longer to have a fully vegetated garden. Plant material per Canadian Standards for Nursery Stock, Eighth Edition. Seed planting is not recommended for rain gardens. Plants should be container grown, balled and burlapped or wire basket.

Below are recommended plant sizes for planting rain garden features:

Deciduous Shrubs - 60 - 80 cm height

Coniferous Shrubs / Broadleaf Evergreens - 40 cm spread (roughly #3 or 3 gal. container)

Perennials/Grasses - 15 cm to #1 or 1 gal. container stock

Ground Cover/ Vines - 10 cm to #1 or 1 gal. container stock

Hydric Zone Criteria

Low Zone	Often referred to as the extended detention or shoreline fringe area. This area is frequently inundated during storm events, and is well-drained between rainfall events.
Mid Zone	Often referred to as the floodfringe area. This zone is inundated less frequently (2 – 100 year storm events) and has periodically high levels of moisture in the soil. The ecology of this zone is a transition from the Mineral Meadow Marsh/Beach-type community to an upland community.
High Zone	Often referred to as upland area. The ecology of this zone is terrestrial due to its elevation in relation to the filter bed. The zone most closely resembles a Cultural Meadow or a Cultural Thicket community, depending on the mix of grasses, herbaceous material, shrubs and trees utilized.

Low Zone

• Mineral Meadow Marsh plant community
• Grasses, Sedges, rushes, wildflowers, ferns and shrubs that have an ‘Obligate’ to ‘Facultative’ designation
• Wetland ‘Obligate’ species that are flood tolerant as they will persist in average years and flourish in wetter years.
• Plants that are likely to occur in wetlands or adjacent to wetlands.
• Plants with dense root structure and /or vegetative cover are favoured for their ability to act as pollution filters and tendency to slow water velocity
• Be advised these practices are not constructed wetlands and are designed to fully drain within 48 hours.

Mid Zone

• Plants able to survive in soils that are seasonally saturated, yet can also tolerate periodic drought.
• Species include grasses and groundcovers, as well as low shrub species.

High Zone

• Plants should have deep roots for structure, be drought-tolerant and capable of withstanding occasional soil saturation.
• Trees and large shrubs planted in this zone will aid in the infiltration and absorption of stormwater.
• This area can be considered a transition area into other landscape or site areas.
• A variety (min. five) species should be used to prevent a monoculture.

Landscaping Design Consideration

• In natural areas, a diversity of plant types not only adds beauty but also create a thick underground root matrix that keeps the entire plant community in balance. In fact, 80% of the plant mass in native prairie communities is underground. Once the rain garden has matured and your sedges, rushes and grasses have established a deep, thick root system, there will be less change in species location from year to year, and weeds will naturally decline. .

• Smooth out any sharp corners to create flow in your garden.
• Neat edges can be established using pavers, walls or bands of turf create an attractive amenity within the residential landscape.

See Also

• Bioretention
• Downspout disconnection

External Links

• Raingarden tour
• RainScapeTO

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The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

A lot level bioretention cell designed to receive and detain, infiltrate and filter runoff, typically used for discharge from downspouts.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

Open space floodway channels, road reserves, pavement expanses and other flow paths that convey flows typically in excess of the capacity of the Minor Drainage System.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

Human application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47

A stormwater design features that provides for the gradual release of a volume of water in order to increase settling of pollutants and protect downstream channels from frequent storm events.

A vegetated area such as a bog, fen, marsh, or swamp, where the soil or root zone is saturated for part of the year.

Surface runoff from at-grade surfaces, resulting from rain or snowmelt events.

Low impact development is a stormwater management and land development strategy applied at the parcel and subdivision scale that emphasizes conservation and use of on-site natural features integrated with engineered, small scale hydrologic controls to more closely mimic pre-development hydrologic functions.

A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.