Rain Gardens

Urbs Pluvialis  The Rainy City

The Problem:

The average urban home is surrounded by impervious surfaces such as roofs, concrete sidewalks, asphalt and even grass lawns.  When the rain falls, it hits these surfaces and most of the water keeps moving, carrying toxins, pollution and an overflow of water toward the storm drains and sewer system.  The sewer system, in turn, becomes backed up and pollutes the local water ways.  The excess surface water also can cause erosion and flooding.  Luckily, there is an easy way to capture the rain and make better use of it.    

Rain Wise

The Solution:

Rain gardens absorb rainwater runoff and storm water overflow.  They create a basin that stores the water and slows percolation, allowing for total absorption into the soil.  The filtration process that occurs when water passes through the soil actually cleans the water and removes pollutants.

How it works: The key is to make a garden that is permeable and absorptive.  This is not your average veggie or perennial garden.  A rain garden must be placed in a specific spot in your yard where it will capture rain water running slightly down hill from your home and at least 10 feet away from any structural foundation.  It is best to use a flat area in partial to full sun that is not near a septic field, or right under a tree.

Rain Wise

How to build one: Before starting out, test the drainage capacity (watch the area in a heavy rainfall or dig a few holes to fill with water and observe the rate of absorption).  The placement of the rain garden can be either directly connected to your gutter (with the help of a length of pipe if necessary) or it can be placed in an area where it will catch runoff from other parts of your yard (e.g. at the bottom of a grassy lawn).   After calling to determine utility lines, dig out your rain garden (initially about 100 square feet and about 8 inches deep for the average home).  In order to maximize permeability, fill the bottom layer of the garden with crushed gravel, followed by compost-enriched soil and topped with mulch or shredded leaves and shredded wood.

What to plant: Natives.  You want your rain garden plants to do well in any season (in the northwest, the plants must be able to absorb heavy and consistent rain all winter followed by several dry months in the summer).  Look for plants with large and deep roots that will be most absorbant.  It's best to use established plants so that the root systems are already strong and ready to work in the rain garden.  The Brooklyn Botanical Garden has a good list of rain garden plants by region.

City Perk:

The city of Seattle is offering a 80-100% rebates for residents who install rain gardens in certain areas.  Watch this clip from King 5 News for more information:

What else you can do:

If a rain garden just won't work in your yard, there are a few other ways you can help prevent water runoff and pollution.

1. Buy a rain barrel or cistern to attach to your downspout.  This will store up filtered rain water for your personal use as needed.  Place small rocks around the barrel or cistern to catch any overflow.
2. Place small rock-filled trenches under downspouts and at the end of driveways to catch runoff.
3. Replace driveways and patios with porous pavers or at least replace of your impervious areas with green material to increase absorption capacity.
4. Install a green roof to absorb rain water before it even hits the ground.
5. Add compost and top dressings to your beds. This will increase the absorption rate of your plants and the compost will actually store many times its weight in water and keep your plants hydrated even when the rain stops.

Rain Garden Resources:

Rain Wise (Seattle Public Utilities)

Healthy Landscapes (University of Rhode Island)

Low Impact Development

Anne Lovejoy's Handbook of Northwest Gardening (Sasquatch 2003)