Rain Garden FAQ

in the picture, one of two small rain gardens frame the front step entrance to residences

A rain garden is a sunken garden bed that captures stormwater runoff from hard surfaces like rooftops, sidewalks and driveways, and allows it to soak back into the ground naturally. They are planted with hardy, native perennials that filter pollutants commonly found in stormwater runoff. This helps reduce the overall amount of runoff and pollution that gets into our streams, and maintains the natural hydrology so streams don’t go dry during the hot summer months.

Check out the diagram below for a visual explanation of how rain gardens take in stormwater runoff, and learn some basics in the sections below!
illustration of a rain garden

Click or tap on any of the questions below to explore that topic!

Why is EMSWCD promoting rain gardens?

According to the US Environmental Protection Agency (EPA), 70% of all water pollution comes from stormwater runoff. Urban residents contribute to water pollution by leaving pet waste uncollected, using chemicals on landscapes, washing cars where the wash water drain to sewers, and flushing oils, grease, paint, etc. down the drain or storm sewer. Rain gardens are an important tool landowners can use to reduce stormwater pollution and improve local water quality.

What are the benefits of rain gardens?
  • Rain gardens reduce the load on city infrastructure, reduce the risk of local flooding, and may get you a discount on your water bill.
  • Rain gardens provide habitat for pollinators, songbirds, butterflies, and other wildlife.
  • Rain gardens are a proven way to reduce water pollution in local streams and rivers.
  • Rain gardens are an affordable, do-it-yourself option for homeowners. They require minimal time and skill to maintain, and actually beautify your landscape!
What makes a rain garden different from any other perennial garden?

Rain gardens are bowl-shaped depressions which receive water from your roof via a downspout extension. The water then soaks into the ground and is naturally cleaned, rather than going into the storm sewer. Just like any other perennial garden, however, you can plant many beautiful plants in your rain garden. They are both attractive and functional – the best of both worlds!

Why is stormwater (rain water runoff) a problem?

Yard care products and chemical pollution from household activities are a leading cause of water pollution in our communities. When it rains, the water flowing across our rooftops, driveways, and lawns is called stormwater runoff. As this runoff flows across the landscape, it picks up pollutants like excess fertilizer, gas and motor oil, tire and brake dust, pet waste, etc. which is then carried into local streams and rivers. Water pollution hurts or kills aquatic life, and makes water unsuitable for human use or recreation.

What is an impervious surface?

An impervious surface is any surface that prevents rain water from soaking into the soil, including roofs, roads, driveways, sidewalks, and even lawns and gravel driveways when they get too compacted. The less water that can soak in, the more stormwater runoff we have, which not only means more pollution, but the sheer volume and speed of the runoff also erodes streambanks and destroys aquatic habitat.

How can I reduce impervious surface on my property?
  • Remove or reduce the size of concrete patios, sidewalks, and driveways.
  • Consider using permeable pavers, or pavers with gaps between them, that allow runoff to soak into the soil.
  • Replace poor draining, shallow-rooted lawns with deep-rooted native plantings.
  • Install a rain garden to capture runoff from existing impervious surfaces.
How do rain gardens prevent flooding?

When stormwater flows into the street, it is just one clogged drain away from a localized flood. Storm sewers can handle a lot of rain, but not an infinite amount. Rain gardens infiltrate water into the ground, which reduces the demand on our storm sewers and makes flooding less likely. With a rain garden, many properties can handle the majority of their stormwater on site. That can add up to a lot of water! In our climate, a rain garden receiving runoff from just 500 square feet of roof absorbs around 13,000 gallons of rain a year!

To figure out how much runoff your roof (or other impermeable surface) receives, you can use this online rainfall volume calculator.
Will my rain garden have standing water?

Rain gardens are designed to filter and absorb water into the soil, not to hold water. (Think of them as “drain gardens”). They are designed to drain within 24 hours. If it rains heavily for several days in a row, it is possible that your rain garden may have standing water until the rain stops and the water has time to soak into the soil.

Will rain gardens attract or breed mosquitoes?

Not if they are built correctly. Mosquitos need at least a week for eggs to mature into a flying adult. Rain gardens drain while it rains, and are supposed to infiltrate all water within about 24 hours after the rain stops. It’s extremely unlikely a well-designed rain garden would have standing water long enough for mosquitoes to successfully reproduce.

Will a rain garden attract pollinators or other wildlife?

Rain gardens can double as pollinator paradises! Plant them with a variety of native trees, bushes, and perennial and annual flowers. Install a pollinator bath in the center with a drip to keep the water fresh, and rockpiles and/or brush piles in various places to provide shelter. Then watch your rain garden come alive!

What types of plants are used in rain gardens?

Native plants are ideal for planting in rain gardens, for many reasons!

  • Native plants are well-adapted to our local climate – they are hardy and require very little maintenance. (Remember, our rain gardens are intended to reduce water pollution, so they need to be planted with species which need no chemical use or artificial fertilizer.)
  • Unlike ornamentals, native plants need little to no summer watering – especially considering all the water they will get in a rain garden!
  • Native plants grow well in our native soils, maintaining the rain garden’s shape and preventing erosion as rainwater soaks in.
  • Native plants provide excellent habitat for birds, butterflies, and beneficial insects that will eat any pests in your yard!

Learn more about the benefits of native plants on our Native Plants page.

Can I build a rain garden myself?

Yes! Rain gardens are a perfect do-it-yourself landscape project. To help you get started, EMSWCD offers both live and recorded Rain Garden 101 workshops that will help you better understand the planning, design, and installation process for a residential scale rain garden. Register for a workshop today!

Where should I place the rain garden?

A rain garden can go in a variety of locations as long as it is big enough (see below). They need to be located away from retaining walls, underground utilities, basements, septic systems, and tree roots, and set back from sidewalks, driveways, and property boundaries. The overflow needs to be able to safely flow into a lawn or adjacent garden bed – but not into your neighbor’s yard! Check your local jurisdiction and the Oregon Rain Garden Guide for more details.

How big should I make my rain garden?

It depends on how much impervious surface will drain to it, and how fast the soil in your yard drains. A good rule of thumb is to make the rain garden 10% of the size of the area that drains to it. For example, if you are capturing water from 120 square feet of roof, the garden only needs to be 12 square feet in area. This percent can very depending on how fast your soil drains (slow-draining sites may need to be 15-20% rather than 10%; it never hurts to make your garden bigger than you think you will need). Check your soil drainage rate with a percolation test; it needs to drain at least 0.5 inches per hour to be suitable for a rain garden.

How do I measure the impervious surface draining to my rain garden?

You can measure the length and width of your driveway, sidewalk, etc. with a simple tape measure. To measure your roof, go to your property in Google Maps and set it to Satellite view. Right click on a corner of your roof that will drain to your rain garden, and at you will see a drop-down menu. Click on “Measure Distance” at the bottom. Then left click on the corners or around the outside edge of the section(s) of roof that will drain to the rain garden. As you click, you will see a box at the bottom of the page showing the length of the line you are making. Once you have gone all the way around, close the shape by clicking on the first point again. The box at the bottom should now show the area of the shape as well as the length of the line. Whatever the number is, make your rain garden 10% of it (with caveats; see above).

I have heavy clay soils. Can I plant a rain garden in my yard?

Maybe. Rain gardens must be able to absorb water within 48 hours. In clay soils, you may need to design a larger, shallower rain garden to ensure timely drainage of the water. In some locations, amending clay soil with compost can help improve drainage.

Where can I buy rain garden plants?

Most native plant nurseries have a wide selection of plants that are appropriate for rain gardens. Check out our Local Sources of Native Plants page. Watch for native plant sales in the fall and winter for inexpensive native plant options. The Oregon Rain Garden Guide has a complete list of suggested rain garden plants.

Where can I find someone to build a rain garden for me?

If you would like assistance building a rain garden, visit the EMSWCD Conservation Directory for a list of landscape designers and contractors who have experience with rain garden projects. The Conservation Directory is not an endorsement for any of the listed vendors, but rather is intended to be a starting point for your research.

What is the cost of a rain garden?

Costs vary depending on a number of factors, including the size and complexity of the garden and whether you’re hiring a professional. For a do-it-yourself rain garden, expect to pay between $3 and $5 per square foot. If you’re hiring a contractor, the costs can exceed $10 to $15 per square foot. You can save money by purchasing smaller plants, and by recruiting volunteer help from friends and neighbors.

EMSWCD also offers CLIP funding to residents of Gresham’s Downspout Disconnection Zone to build a rain garden or for other types of stormwater management.

What kind of maintenance is involved?

Rain gardens can be designed to require very little maintenance by choosing the right plants, spacing plants closely together, applying mulch annually, and performing some occasional weeding, they may need as little as 15 minutes of care per month during the summer. Remember, native plants don’t need watering after their first year or two!

Will I need to water my rain garden during dry periods?

Plan on watering your rain garden during the first two years to help young plants get established. Water deeply and infrequently to make them grow deep roots and build drought resilience: every one to two weeks for the first year, and every two to four weeks for the second year (and also only if it looks like the plants actually need it). After the first two years, rain gardens should require little to no watering during the summer – especially if you have an abundance of native plants!

Do I need to fertilize my rain garden?

No, rain gardens do not need to be fertilized, and in fact should not be fertilized. Remember, one of the benefits of rain gardens is their ability to clean and filter excess nutrients and chemicals from stormwater runoff. Adding artificial chemicals or nutrients would reduce the rain garden’s effectiveness.

How can I get a yard sign for my rain garden?

For new and prospective rain garden owners, hearing from friends and neighbors about their rain garden experience is incredibly valuable and helpful information. Therefore, if you register your rain garden and share your own rain garden “lessons learned,” we’ll send you an attractive 7”x9” aluminum yard sign for your rain garden! Learn more on the Rain Garden Registration page.