Rain Garden Commonly Asked Questions
What is a rain garden?
A vegetated, bowl-shaped landscape feature that absorbs runoff from roofs and driveways to help reduce pollution and flooding in local streams.
Why should I build a rain garden?
Rain gardens are attractive, easy to maintain, and serve an important environmental function.
In Gresham, rain from roofs and driveways typically goes untreated into city pipes under the street and on to local streams. Stormwater carries harmful water pollutants such as gas, grease, oil, pet waste, weed killers and fertilizers, which are unhealthy for fish and other wildlife and can be a danger to people, too.
Large volumes of stormwater also cause erosion and flooding. By absorbing stormwater into your landscaping with a rain garden, you’ll help reduce pollution, recharge groundwater and improve wildlife habitat.
Is every yard suitable for a rain garden?
No. Some soils, slopes and spaces are not ideal for rain gardening. Read the City's Rain Garden Guidelines to be sure your site is safe. The City provides a free safety inspection and technical assistance to help residents get started, so please contact us before you begin your project.
Do rain gardens breed mosquitoes?
No. Mosquitoes require at least 4 ½ days to mature from larva to adult. A properly functioning rain garden dries out in 24 hours or less, making it impossible for mosquito larvae to survive.
Is a rain garden or disconnected downspout a threat to my basement, crawlspace or foundation?
Safety guidelines have been developed by engineers and reviewed by building and plumbing inspectors in order to prevent property damage from residential rain gardens. Safety inspections by City staff ensure that setbacks and other requirements as described in “Rain Garden Guidelines” are followed.
How is a rain garden different from a pond?
Rain gardens should drain completely within 24 hours and are not likely to have permanent aquatic wildlife inhabiting them. They are not suitable for fish.
Do I need to water my rain garden in the summer and fall?
By using plants native to Oregon or other drought-tolerant species, you will only need to water during the first two summers in order to establish the plants. After that, the plants should not require additional watering.
- Be sure to water deeply but infrequently (3 to 4 inches every three to four weeks) the first two summers to encourage plant roots to grow deep where they will find moisture even in the driest months.
- Watch your plants for signs of heat stress and water more frequently if temperatures are above 90 degrees.
What if my rain garden overflows?
Rain garden sizing recommendations have been designed to hold and infiltrate typical Gresham rainfall. Despite this, rain gardens must be designed with a safe overflow location to accommodate large, heavy rain events. Review the City’s Rain Garden Guidelines for more detailed instructions on constructing a safe overflow.
How big should my rain garden be?
A general rule of thumb for rain gardens in Gresham is about 10% the size of the area draining to them. This will accommodate most rainfall events. Information on sizing your rain garden is in the Rain Garden Guidelines.
Do I need a permit to install a rain garden in my yard?
You do not need a permit to install a rain garden at a single family residence in Gresham. However, you are required to pass a free safety inspection by city staff before beginning construction. Call 503-618-2793 to request an appointment. Property owners are responsible for the safety and function of drainage systems on their property.
Drainage modifications to commercial or multi-family buildings in Gresham must be performed by a state-certified plumber, and design drawings of the new drainage system must be completed by a licensed civil engineer. Inspections by City staff are also required. For more information on commercial stormwater management, contact Rob Stahle at 503-618-2621 or Robert.Stahle@GreshamOregon.gov.
What kinds of plants should I put in my rain garden?
Use native plants. Native Oregon plants are adapted to local weather, pests and disease and don’t require fertilizers, pesticides or water once established.
- Fertilizers and pesticides should never be used in your rain garden because they are a source of water pollution.
- Plants chosen for the bottom, or basin, of the rain garden should be tolerant of both drought and standing water. Sedges (Carex species) and rushes (Juncus species) work well for this portion.
- Plants chosen for the upper edge, or rim, of the garden should be drought tolerant.
- The Rain Gardens homepage has links to plant lists, sun and shade plant palette design plans, and native plant suppliers.
Are there examples of rain gardens I can visit in Gresham?
Yes. We have a map of Gresham rain gardens on our Rain Garden page.