The new rain garden at the Forbes House (11829-97th Ave NE), located at Juanita Beach Park is much more than a pleasing collection of beautiful plants and shrubs. It also serves as a demonstration project and helps improve water quality. A rain garden is a landscaped depression designed to help to slow and soak up storm runoff.  How the Garden Works Rain Garden Anatomy Project Benefits Planting Plan Design Features Recruitment Flyer Plant Choice & Installation Project Tasks Cost Recognition Resources

Before Construction Finished Rain Garden

How the Garden Works:

The rain garden collects stormwater runoff from the house’s roof and allows it to naturally soak into the ground. The bowl shape of the garden aids in the collection of water and amended soils improve its filtration properties. This garden design allows for absorption of stormwater runoff and filtration of pollutants.

Project Benefits:

• The rain garden design provides for stormwater infiltration to recharge the groundwater and incorporates deep-rooted plants to naturally filter the runoff, which enhances water quality.
• Over 200 plants provide a low-maintenance, drought-tolerant landscape that will require little to no watering and no fertilizers or pesticides, which will further protect water quality.
• The project enhances the beauty of the landscape and attracts beneficial wildlife.

Design Features:

At approximately 330 square feet and with a maximum ponding depth of about 12 inches, the flat-bottomed rain garden is designed to collect runoff from about 1300 square feet of roof area. This size of rain garden, which is equal to about 25% of the area of the roof that drains to it, in well-draining soils, is likely to capture almost all of the runoff in a given year. Runoff from the roof is directed to the garden through a yard drain and any overflow exits the garden through an outlet located in the southwest corner of the garden. For this project, it was easier to re-slope the roof downspouts, rather than using several pipes to get the water to the yard drain. The bottom of the garden contains a variety of sedges and rushes while the drier slopes and top edges are planted with drought-tolerant plants (see planting plan for details).

In planning and locating the garden, we took several factors into consideration including:

• Proximity to building foundation - at least 10 feet from foundation,
• Proximity to septic system - at least 50 feet from septic system
• Proximity to underground utilities - utilities located & marked before digging
• Location of water source - water drains to garden from house downspouts
• Soil type and slope
• Overflow - water can be directed safely away from house
• Potential for viewing by pedestrians

Plant Choice & Installation:

The rain garden contains a collection of perennial shrubs and herbaceous plants that are tolerant to both temporary pooling and drought. We selected low-maintenance plants based on suitability to the site and aesthetic appeal.

The rain garden currently contains the following plants:

The house beds currently contain the following plants:

Compost was spread over the entire garden before planting. Mulch was applied on the sloping sides of the rain garden to help conserve water, suppress weed growth, and prevent erosion.

The plants will occasionally need to be watered, especially during our dry summers. They are otherwise low-maintenance plants and should not have fertilizer or pesticide added. Other periodic maintenance includes weeding, mulching, and keeping the inlet and overflow clear.

Project Tasks:

Plan for garden depth, area, & shape

Location

• Forbes House identified as location for demonstration rain garden
• Location of garden in relation to house determined (at least 10 feet from foundation)
• Soil drainage test conducted

Planning & Design

• Rain garden depth, area, and shape determined
• Amount of amended soil to order calculated
• AutoCAD drawings of rain garden completed
• Planting plan/plant list for rain garden and existing house beds developed
• Plants, soil mix (60% sand, 40% compost), and mulch ordered
• Planting plan routed to Parks for review and approval
• Temporary informational sign fabricated
• Volunteers recruited for planting phase of project

Construction

Excavation of native soils

• Utilities located before excavation
• Outline of rain garden marked
• Soil mix delivered
• Downspouts rerouted and yard drain installed as entry for water into rain garden
• Native soils excavated
• Bottom of rain garden leveled
• Rain garden soil mix installed (12 inch depth), filled to a level that left 12 inches below edge of rain garden for ponding
• Rock-lined overflow provided
• Temporary sign describing project and recruiting volunteers posted

Planting

Volunteers installing garden

• Plants delivered
• Plants placed in desired locations
• Set-up and preparations for volunteers made
• Volunteer event (17 volunteers) – plants installed, exposed soil covered with mulch & compost, plants watered

Site maintenance

• Mulching as needed to prevent erosion and weeds
• Watering as needed
• Weeding as needed
• Keeping inlet and outlet clear of debris

Cost:

Materials for the rain garden have an approximate value of $2360. This garden is large and was installed by City staff and volunteers, but there are ways to keep down the cost of home installation including:

• Recruit friends to excavate garden by hand and to install plants.
• Purchase smaller plants. Emergents can be bought in tubes for about $1 each.

Cost of materials for rain garden (not including house beds):

Recognition:

The Forbes House rain garden was installed in October 2008 as a demonstration project. It is the result of a collaborative effort of the Kirkland Public Works and Parks departments. 17 community volunteers also donated their time to install plants and spread mulch on the site.

Resources:

Hinman, Curtis. Rain Garden Handbook for Western Washington Homeowners. Washington State University Pierce County Extension. http://www.pierce.wsu.edu/Water_Quality/LID/Raingarden_handbook.pdf