The Rain Garden: A Simply Elegant Solution to Stormwater Pollution

Dec 9, 2023

Last winter's epic rainstorms have caused many of us to think about what we can do to mitigate the impact of heavy downpours on our communities. As it happens, the impervious surfaces that keep us dry and mud-free (our homes, patios, driveways, etc.) also keep water from seeping naturally into the ground. Instead, rainfall is typically managed by gutters and downspouts which collect surface water and funnel it into drains and culverts. Unfortunately, what comes along for the ride are a lot of pollutants: pesticide residue, heavy metals, bacteria, etc., all of which end up contaminating our waterways and potable water supplies. According to the US Environmental Protection Agency, rainwater runoff accounts for an estimated 70% of all water pollution. Now for the good news: there's a simple solution to keeping that water from going down the drain that will not only beautify your landscape but do great things for the environment, too. It's called a “rain garden.”

What is a Rain Garden?

Known more technically as “bio-retention cells,” rain gardens are landscaped areas designed to capture and retain runoff water where it can be slowly absorbed into the surrounding soil. In the process, the interaction of plants and soil microbes makes significant improvements in the quality of the water, reduces flooding and helps to recharge groundwater. A well-constructed and maintained rain garden can remove up to 90% of the pollutants present in rainwater runoff and can absorb significantly more water than conventional landscape areas such as lawns. Unlike natural areas that are slow to drain, the water in a properly designed and constructed rain garden will allow water to infiltrate in 48 hours or less, which means mosquitoes will not have enough to time to breed.

How Rain Gardens Work

The name may conjure up watery images of ponds and lily pads but “rain gardens” are dry most of the time and blend seamlessly into the existing landscape. During a rain event, runoff is directed to a location in the ground that has been excavated to be slightly shallower than the surrounding area, forming a basin to temporarily contain a quantity of water. In the “basin” are plant varieties that are strategically selected and placed to survive the extremes of periodic flooding and drought conditions. When water flows into the rain garden, it irrigates the plants while soil microbes go about the job of breaking down pollutants and providing nutrients for the plant roots to absorb.  As the plants grow, they provide an attractive habitat for birds, butterflies, and other animals. 

Size Matters

How big should a rain garden be? Ideally, it should be large enough to capture and contain the water that comes off the roof of your home, patio or other paved area.  During a storm that dumps an inch of rain, about 200 gallons of water will be shed from a 400 square foot roof. This would require a rain garden approximately 20% of the size of the roof or about 50 square feet. Size calculations will vary according to soil types and other factors but even a small rain garden can make a big difference.

Site Selection

The location of your rain garden should be at least 10 feet away from structures and in an area where water from downspouts or drains can be directed. This can be in a natural depression, flat area or gentle slope no greater than 12%.  Ideally, the location should receive full sun to partial shade for plants to do their best and soil must be able to percolate water at the rate of approximately ½ inch per hour. Depending on the terrain, the long side of a shape should be arranged perpendicular to a slope to catch as much water as possible. Areas with heavy clay soils, trees and tree roots, utilities, septic tanks, drain fields and well heads are to be avoided.

Design & Construction

Rain gardens can be any shape, size or style. They are simple to construct and usually require little in the way of supplies and equipment. If you have a shovel and the willingness to do some digging, you can probably do it yourself.  You will need plenty of organic compost to amend the soil, which is essential for increasing infiltration and promoting plant growth. Depending on the type of soil, other amendments might be needed to facilitate drainage. After plants are in place, you will need to top everything off with a layer of organic mulch to help retain moisture and suppress weeds. Maintenance is also simple: a bit of weeding, pruning and adding more mulch is usually what's required.

Plants and Planting

Think of a rain garden as having 3 different zones: “bottom,” mid slope and perimeter or berm. Plants in the bottom zone need to tolerate periods of flooding and will receive the most water for the longest time. Plants on the next level need to be a bit smaller and must also withstand moisture but for shorter duration. At the top or outermost zone are plants that are more typical of California and Mediterranean style gardens. Since our climate is dry for much of the year, all of the plants you select will need to do well with minimal water.

To slow the velocity of water entering the “basin,” rain gardens should be more densely planted than other types of landscaped areas. Grasses, sedges and other plants that form “clumps” are key to holding soil, supporting other plants and preventing erosion. Many California natives are well suited to these conditions but non-native plants with similar characteristics are also good options.

Plant Recommendations

  • “Bottom” plant varieties include: douglas Iris (Iris douglasiana), creeping wildrye (Elymus triticoides), yerba mansa (Anemopsis californica) and torrent sedge (Carex nudata).
  • Mid slope varieties: salt grass (Distichlis spicata), common or spreading rush (Juncus patens), wood rose (Rosa gymnocarpa).
  • Top level or berm varieties: common yarrow (Achillea millefolium), ceanothus (Ceanothus spp.) California fuchsia (Epilobium canum), pitcher sage (Salvia spathecea).

More Solutions

If a rain garden isn't possible, there are other ways to retain direct and utilize excess rainwater. Rain barrels attached to gutter downspouts collect and store large amounts of water very quickly.  Swales, rock-lined areas that mimic streambeds, can divert water, promote infiltration and add interesting landscape features. Swales can also be used in conjunction with rain gardens.

Rebates for rain gardens and rain barrels are available to San Mateo County residents. For more information, see San Mateo Countywide Water Pollution Prevention Program under “Resources” below.

To see a great video on residential rain gardens, go to:


-UCANR Coastal California Rain Gardens:

-San Mateo Countywide Water Pollution Prevention Program: 

-US Environmental Protection Agency:

 -The Xerxes Society:



 Maggie Mah is a UC Master Gardener who is in awe of nature's power and wisdom hopes lots of people will be encouraged to build rain gardens.

UC Master Gardeners of San Mateo-San Francisco County are volunteers who are trained under the auspices of the University of California to provide science-based information on plants, horticulture, soil and pest management at no charge to the public. For more information and to find out about classes and events in your area, visit our website where you can also sign up for our newsletter and contact our Helpline:

By Maggie Mah

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