A rain garden is a shallow little bowl-shaped dip in the landscape that collects rainwater. It is often used remediate water issues in residential areas.
It's also an ideal place to grow many native foods like gooseberries, salmon berries, camas, oxalis, and other edible greens,
By directing rain runoff from roofs or paved areas and overflow from other rainwater collection systems, rainwater gardens manage naturally wet areas in your yard and draw water away from the base of your home.
They also add beauty to your landscape, filter the water, and provide habitat for birds, bees, and other pollinators. They're a win, win, win.
The rainwater garden in these photos has red huckleberries, evergreen huckleberries, salmonberries, nootka rose, wild cranberry, thimbleberry, kinnikinnick, wild strawberry, oxalis, miner's lettuce, pacific waterleaf, and california blackberry, all super healthy, no maintenance native foods.
How Rain Gardens Work
Rain gardens are typically filled with water from roofs or paved areas and directed to the garden with downspout extensions or lined streambeds, where it is collected and stored until it can safely soak into the ground.
(The photo above shows that adapter we used to redirect the water runoff from the roof and out into our garden area.)
Before the dirty runoff reenters the waterways biofiltration plants, grasses and bushes, along with the soil itself, filter out chemicals, dirt, and other pollutants picked up by the rainwater as it washes over hard surfaces, protecting our rivers and streams from pollution.
They also help replenish groundwater, and reduce the risk of sewer backups and overflows into the Willamette River.
Summary of Rain Garden Design Requirements
Ultimately, rainwater gardens are "not that deep" and can be created without professional help. Here are some essential safety measures and City of Portland regulations to consider before you start your project.
Placement:
The edge of the garden must be:
At least 6 feet away from a building with a basement
2 feet from a building without a basement
At least 5 feet away from neighboring properties
At least 5 feet from the base and 10 feet from the top of retaining walls higher than 3 feet
The deepest part of the rain garden must be:
at least 10 feet from any neighboring structures
When building a rain garden, it’s also important to keep in mind:
Infiltration rates of the soils (how well water soaks into the ground). An infiltration test is recommended prior to rain garden construction. Find information on how to do an infiltration test in the How to Build a Residential Rain Garden guide.
Depth & Size. The minimum suggested ponding depth of a finished rain garden should be 6 to 12 inches and be large enough to completely drain within 24 hours of a rain event and be able to handle all of the runoff directed to it.
Sizing will depend on tested infiltration rates and by determining how much other runoff is directed to the catchment area.
Be cautious of underground utilities. Do not build over water, gas lines or oil tanks. Call before you dig, 1-800-332-2344, 8-1-1, or schedule an appointment online to locate all underground utilities.
Slope. Rain gardens are not suitable for steep locations — property with more than a 10 percent slope.
Avoid compacting any native soils.
Water Overflow. Every garden should have a safe escape route for water overflow. Plan where the rain will go when the garden is full and direct it away from structures and neighboring properties.
When choosing plants for your rain garden:
Install plants from the Stormwater Management Manual plant list or choose plants appropriate for the native plant community type as described in the Portland Plant List.
Environmental Services prohibits plants on the Portland Nuisance Plant List and the Required Eradication List. Both categories can be found in the Portland Plant List.
When to Call a Professional
A professional native plant landscape designer is not technically required to design and build most home rain gardens but hiring Urban Rewilding will yield several advantages, here are just a few.
Ecological Suitability: At Urban Rewilding we understand the local ecosystem, including specific soil types, climates, micro climates, and biodiversity of the region.
We're able to choose native plants that have evolved to thrive in this climate and the multiple micro climates we find in Portland.
We use the plants best adapted to the conditions found in your yard to create a mini ecosystem supporting native plants and animals that ultimately won't require maintenance.
Biodiversity Support: Native plants play a crucial role in supporting local wildlife, including insects, birds, and other species.
We can design a rainwater garden that not only manages stormwater but also contributes to the conservation of native flora and fauna.
Water Conservation: Native plants are generally adapted to local rainfall patterns and soil conditions, requiring less water once established.
We can design a rainwater garden that promotes water conservation, allowing the long term health of the plants without human intervention.
Low Maintenance: Native plants are often well-suited to the local environment, requiring less maintenance than non-native species.
We will select plants that are resilient and thrive with minimal intervention, making the rainwater garden more sustainable in the long run.
Aesthetic Appeal: We have an eye for designing aesthetically pleasing landscapes that integrate seamlessly with the surrounding environment.
We can create visually appealing rainwater gardens that enhance the beauty of your property.
Erosion Control: Native plants are often effective in preventing soil erosion, a critical consideration when designing rainwater gardens. The deep root systems of many native plants help stabilize soil, reducing the risk of erosion caused by stormwater runoff.
Knowledge of Local Regulations: We are familiar with local regulations and guidelines related to landscaping and stormwater management.
We can ensure that your rainwater garden design complies with city codes and environmental regulations.
Educational Value: We will educate curious homeowners about the ecological benefits of native plants, fostering a greater appreciation for the local environment. This knowledge can empower homeowners to make informed decisions about further landscaping projects and conservation efforts.
Customization to Site Conditions: Every property has unique characteristics, not only can we can tailor the rainwater garden design to specific site conditions, taking into account factors such as soil composition, sunlight exposure, and existing vegetation, but your budget as well.
Long-Term Success: With our expertise, we can contribute to the long-term success of the rainwater garden. Proper plant selection and design considerations can result in a resilient and thriving ecosystem that continues to provide benefits over time.
Hire Urban Rewilding for rainwater garden design, builds or maintenance and we will ensure that your project is ecologically sensitive, aesthetically pleasing, and compliant with local regulations, promoting the well-being of the local environment.
Costs and Permits
Rain gardens are not approved for any new construction or redevelopment project that activates the Stormwater Management Manual requirements. See the manual for more information.
Cost
The cost of installing a rain garden depends on many factors such as size, plant selection and density, and other possible work such as removing paved surfaces or rerouting downspouts.
A project can range from $1500.00 - ???
An area with an established layout may not need designs or large machinery to remove sod, dig the bowl, and grade the area. We may be able to sculpt existing features by adding rocks and plants and redirecting and digging out waterways, in a smaller area.
This is much less expensive than turning existing landscaping. or bare ground into a rainwater garden requiring designs, heavy machinery for excavation in large areas, and re routing downs spots.
Landscaping generally costs between $4 and $12 per sq ft. Other factors may contribute to the overall cost of the project including permits.
Permits
You will need a City permit if your project involves any of the following activities or if the following conditions apply to your property.
You excavate or remove more than 10 cubic yards of dirt (enough to fill one standard-size dump truck).
Your property has a 10 percent slope or more.
Your property is within 50 feet of a wetland or waterbody.
Your property is in a floodplain.
If any of these conditions apply to your property, you may need to take extra steps to install a rain garden safely. Contact the Private Property Drainage Inquiries team to discuss safe options for your property.
Maintenance Requirements
Like any garden, a rain garden requires some regular maintenance. Once the garden becomes established, which happens in about two years, maintenance should be minimal.
Other ongoing maintenance tasks include:
Watering the plants deeply once a week during dry months (May to October) to encourage root growth and keep plants strong, especially while plants are getting established during the first two summers.
Pull weeds by hand before they become established (avoid chemical weed killers).
Watch for erosion, and replace plants as needed.
Regularly check gutters, downspouts and inlet pipes to ensure they are free and clear of debris and that rainwater can enter the garden.
At Urban Rewilding, we are experts at native plant garden maintenance. Our bi weekly services include all of the traditional mow and blow tasks along with an emphasis on soil health, biodiversity, native plant, wildlife and pollinator habitat, and water management.
We never use any chemicals or fertilizer or gas powered machinery. Hire a native plant maintenance professional to take such good care of your yard, you stop needing them!
Stormwater Management Manual
Consult the City’s Stormwater Management Manual for the complete set of requirements on how to safely site, build, and maintain a rain garden or other stormwater management solution on your property.
Clean River Rewards Eligibility
Residential and commercial property owners who install qualified stormwater management solutions may be eligible for a discount on the stormwater charges of their sewer, stormwater, and water bills through the Clean River Rewards program.
Rain gardens that meet the safety and space requirements outlined above and are at least 10 percent of the roof's area draining to it are eligible for Clean River Rewards.
Find More Resources
How to Build a Residential Rain Garden. This printable how-to guide from Environmental Services will walk you through the steps to plan, design, and build your rain garden.
Download PDF file How to Build a Residential Rain Garden 2.95 MB
Low-Impact Development Fact Sheet on Rain Gardens by Oregon State University Extension Service provides a detailed overview of all aspects of rain gardens from site conditions to maintenance. Get the Low Impact Development Fact Sheet
Free Classes. East Multnomah Soil and Water Conservation District offers free classes on building rain gardens. Learn more or sign up for Rain Gardens 101.
Stormwater Management Manual's Plant List provides a list of plants recommended for rain gardens.
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