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Saturday, March 30, 2013

Harvesting Rain: Rain Gardens and Vegetated Swales

Rain garden in Mother Nature's Backyard

Your home roof is likely the largest impervious surface in your garden.   Last month (February 2013) we talked about gutters, downspouts and rain chains which can be used to collect and direct roof water.  This month and next we consider methods for infiltrating the water from impervious surfaces into the ground.

Why infiltrate rainwater?    Rainwater is clean, free water. As our planet heats up, methods to capture and use rainwater become increasingly important.  Some of the benefits of keeping rainwater in your own garden include:

  1. Saving money – rain water is free water
  2. Conserving a valuable and increasingly scarce resource – water
  3. Helping replenish the ground water
  4. Providing habitat for insects and birds
  5. Providing a place to plant interesting stream and pond-side plants
  6. Teaching children the values of conservation and water-wise gardening
  7. Deep watering your garden’s trees and shrubs (in winter/spring in our area)


One simple but effective infiltration tool is the rain garden or vegetated swale.  Rain gardens/swales capture water that would otherwise run into the storm drains - and allow it to percolate into the ground.   They make sense for many home gardens and are becoming increasingly popular throughout the world.

What is a rain garden?  A rain garden is a small garden where runoff from impervious surfaces (roof, driveway, patio, walkway) is directed. Rain gardens are like leaky ponds – they hold rainwater just long enough for it to soak into the soil.     Because they only have standing water right after a rainstorm, you don’t have to worry about mosquitoes and other problems associated with standing water.  Rain gardens are often planted with wetland plants that help hold the soil and make the garden beautiful and life-friendly.

What do rain gardens look like?  
Almost all rain gardens have these common features:
  • An intake area (often a downspout, pipe, channel, etc.)
  • A shallow depression (like a shallow pond) to hold water briefly
  • A layer of gravel or rocks to help the water to soak in
  • Wetland plants that take occasional flooding & hold the soil
  • May have: an infiltrator device - to help water soak in faster
Rain gardens can be any shape – from round like a pond to linear like a dry stream bed (these are called vegetated swales).   The size of the rain garden is based on the amount of water that needs to soak in.  The shape depends on your needs and tastes – and the available area.
Rain garden designs often have a regional ‘look’.  For example, rain gardens in places with summer rain (for example, Pennsylvania) are often kidney-shaped flower gardens located in low areas of the lawn.  But there’s no reason that all rain gardens need follow this design.  In summer-dry regions like Southern California, rain gardens can be completely dry from late April through October.  In these regions, a rain garden can serve a dual-purpose - functioning as a water infiltrator in winter and something else during the dry season. 
A little creativity can suggest ways to make your home rain garden meet your needs and tastes.  Here are just a few ideas of what a Southern California rain garden might look like:
  • An unmowed ‘lawn’ in a slight depression
  • A concrete planter featuring plants that can survive winter flooding
  • A summer pathway with gravel and pavers
  • A summer seating area, often featuring pavers or stone slabs set in gravel
  • A ‘dry creek bed’ planted local native streamside plants
  • A Japanese (Zen Garden) or Mediterranean style gravel garden (maintained as a formal gravel garden during the dry season)
As rain gardens become more common, landscape architects and homeowners alike are developing more creative and functional designs.   We suggest you search the internet for designs that inspire and work well with your lifestyle, climate and garden.
Does my rain garden need to be planted?  No, in fact some very successful rain gardens contain no plants at all.  However, there are several good reasons to select a planted rain garden.  First, the plants serve as habitat for birds, butterflies and other insects.  They may even provide food for your family.  So, a planted rain garden is usually more life-friendly than one with no plants.
A second reason to plant your rain garden is that it will be cooler.  In our climate, planted areas play an important role in keeping gardens cooler during the hot, dry summer.  If you live in a mediterranean climate, this can be an important advantage.
Another reason to include plants in a rain garden is that their roots help rainwater infiltrate more quickly.  Roots help keep the soil texture open, allowing water to infiltrate better.  This is obviously an advantage in a rain garden.
Finally, a rain garden provides the ideal place to grow plants that like a little extra  water.  These include some wonderful native pond-side and stream bank plants.  Planting a rain garden with these plants – and using rainwater to water them during the rainy season – allows you to enjoy these beautiful plants.
Building a Rain Garden
Check your soil drainage to determine if your soil is suitable.   Dig a 1x1 foot hole and fill it with water.  Let the water drain out completely.  Fill the hole again and check after 1 hour.  If the water drains at least 1-2 inches per hour your soil is suitable for a rain garden.
Choosing a location is key to designing a successful rain garden.  The location should either be near the impervious surface(s) it drains or in a place where water can be easily diverted to it.   Ideally, a rain garden that infiltrates water from a large surface (such as a roof) should be at least 8-10 ft. rom any structures or utility lines and 3 ft. rom sidewalks and walkways.  Rain gardens that drain patios or walkways are usually located fairly close to the surface they drain – usually 1-2 feet away.   In Mother Nature’s Backyard, the water from half of our small roof runs down our rain chain, into an infiltration pot and then directly into our rain garden. Ideally, the rain garden would be located a bit further from the structure.  However, the small amount of drainage area, coupled with the depth of our rain garden, made our location feasible.
Rainwater from the north half of the roof flows down the
 rain chain into a  large pot and then into the rain garden
Determining the size and depth of the rain garden is fairly easy. First you need to determine how much rain falls per hour during a good rainstorm.  Then you convert this amount to cubic feet.  In our area of Southern California, we rarely get sustained rainfall that exceeds ¾ inch per hour.  Therefore a rain garden in our area should be able to contain at least 3/4 inch (.0625 cubic ft.) of rain that falls on the impervious drainage surface. This is probably the least amount of rain fall you should plan for – even in the desert, a rainstorm can drop an inch or more of rain in an hour.   
If you live in an area with heavier rainfall, you’ll need to increase your estimate accordingly: 1 inch per hour = 0.083 cubic feet; 1.25 inches = 0.104 cubic feet; 1.5 inches = 0.125 cubic feet; 1.75 inches = 0.146 cubic feet.    To convert any rainfall per hour to cubic feet simply multiply the rainfall amount by 144 and divide this number by 1728. 
Next determine the area of the impervious drainage surface by multiplying the length of the surface times its width.  For example, a 20 by 30 ft. roof has an area of 600 square feet.  Multiply this by the cubic feet of rainfall (0.0625 in our area) to get the total cubic feet of water from the drainage area (600 x 0.0625=37.5 cubic feet).    This is the amount of water your rain garden will need to contain before it infiltrates into the ground.
Most home rain gardens are 1-3 feet deep (with most of the depth filled with gravel or dirt – see below).  Decide on a depth and divide the cubic feet of water (37.5 cubic feet) by the depth (2 feet) to get the surface area of your rain garden (37.5 / 2 ft. = 18.75 square feet).   In this example, an oblong rain garden that is 3 ft. wide by 6 ½ feet long and 2 feet deep will do the trick.   It’s prudent to make your rain garden a little larger than the calculated size and to also plan an overflow area in case of heavy rainfall.  Mother Nature is full of surprises these days!
The shape of the rain garden may be dictated by the site or designed by you.  Gardeners often find that rounded shapes are most pleasing; but the design depends on your needs and the design of your home and garden.  You can easily lay out the basic shape for your rain garden using a garden hose.  As long as the size is adequate, the shape of the rain garden is up to you.   Our rain garden is oblong with rounded edges and a dry-stone wall on the walkway side to provide stability and a decorative appearance.


Digging the rain garden can usually be done with a shovel and a little effort.  We had a little mechanical help, but did much of the digging with picks and shovels – even in our heavy, debris-filled soil. The excavated area should have gently sloping edges.   Pile the excavated soil around the edges to create a small berm around the rain garden.    You can see our fully excavated rain garden above.  Because our area in Mother Nature’s Backyard is limited, our rain garden is fairly deep - about 3 ½ feet at its deepest point.  Because the rain garden is filled with gravel and a layer soil, only the only the top part is visible (see photo below).

Rain garden is filled with gravel and smooth.  Next a 6 inch layer
of soil was placed over the gravel layer.
Fill the bottom of the rain garden with at least 6-12 inches of gravel. This will help the water to infiltrate better.   If your rain garden is deeper than 2 feet you can use more gravel.   The surface of the gravel layer should be at least 8-10 inches below the level of the surrounding soil.  Rake the gravel to make it level.

Cover the gravel with 4-6 inches of garden soil (use the excavated soil).  This will be sufficient for the bottom plants, many of which will grow through the gravel layer and into the soil beneath the rain garden.

Smooth/compact the berm to create gently sloping edges.

Plant the rain garden with appropriate plants (see below).

Mulch around the plants with an appropriate mulch.  Depending on the plants, this may be an organic or inorganic mulch (see our July 2012 posting on ‘Garden Mulches’).   In Mother Nature’s Backyard we used chunks of sandstone left over from the stone pathways to create a color-coordinated inorganic mulch at the bottom of the rain garden.  The plants on the berm are lightly mulched with wood chips until they grow to cover the area.

Choosing Plants for Rain Gardens

 If you live in Southern California,  we suggest choosing California native plants for their beauty, life-friendliness and suitability. Otherwise, consider using plants native to your own local area.   You will need to choose two types of plants:

Plants for the bottom of the rain garden.  These plants will be briefly flooded after a rainstorm and the ground will be seasonally moist.   You need to choose plants that normally grow along streams or around ponds and lakes (riparian plants).  These plants are often short (1-3 feet) and grass-like.  Some typical examples are rushes, spike rushes, and some annuals and perennials like annual Monkeyflowers (Mimulus) and even Miner’s Lettuce (see February 2013 Plant  of the Month).

Plants for the upper sides and berm.  These are plants that like damp soil in winter and spring. This area will likely be too damp for the most water-wise local plants (like local native Salvias), particularly if your soil drains slowly.  However, there are many lovely plants to choose from.  These range in size from grasses and sedges to large shrubs and even trees. 

A list of California native plants appropriate for the bottom and sides of rain gardens is available at: http://www.slideshare.net/cvadheim/south-bay-native-plants-rain-gardens-swales-213
A talk on rain gardens & vegetated swales (complete with discussion of some appropriate plants: http://www.slideshare.net/cvadheim/infiltration-gardens-2015


Additional Resources on Rain Gardens


LA Rain Gardens - http://www.laraingardens.org/  


You can also e-mail your Water Zone questions to: mothernaturesbackyard10@gmail.com 


  1. This is one of the best posts I have seen on rain gardens. Very well done!

  2. On the surface, a rain garden looks like an attractive garden. It may support habitat for birds and butterflies, it may be a formal landscape amenity or it may be incorporated into a larger garden as a border or as an entry feature.

  3. Just found your blog - really great information ... Thank you!