RAIN GARDENS: Practical and Beautiful

By Sandy Metzger, Sonoma County Master Gardener

Visualize this: The first rain of the season is pouring down, running off your roof, down your driveway and sidewalks, into the streets, storm drains, creeks, ultimately to the ocean. Some of it is actually absorbed by your gardens and lawn, but most of it is run-off, carrying with it debris, excess fertilizers, pet waste, herbicides and pesticides. What if instead, you could capture much of that run-off and use it for a garden, with the excess percolating downward to recharge the aquifer below, purifying itself as it goes through the soil?
You can, by adding a rain garden to your property. Besides being a lovely landscaping feature, a rain garden mimics nature: it intercepts run-off, decreases erosion, reduces pollutant transport, irrigates the plants in it, and acts as a mini-filtration system as it sends the rainwater back down into the aquifer. A rain garden is not a pond, pool, bog, or swamp. It’s a depression in your yard that you have specifically built to capture run-off; and it should drain within 24 to 48 hours, thereby eliminating mosquito egg development, which takes four to 10 days.
Okay, but what exactly IS a rain garden? It’s a concave area, very often oval or kidney-shaped, that has been dug out, the soil improved with sand and/or organic matter, planted with mostly natives from your area (or regionally adapted plants), and that collects rainwater directed from your roof, driveway or sidewalks. Native plants develop deep roots because they have evolved here for millennia and, especially here in Sonoma County, are accustomed to heavy sporadic rain for half the year and drought the other half. They thrive on drenching and drought, and this means little or no irrigation requirements during dry summer months.

You, as a homeowner, can probably construct a rain garden, depending upon the amount of rain to be captured; that figure, together with type of soil, determines the size, depth, and volume of gravel, sand, or compost to be added to the depression. Particularly large rain gardens likely need to be engineered professionally. As an example, if your total roof area is 2400 sq. ft., one downspout on one end of one side will carry one quarter of the rain from a 600 sq. ft. area to your rain garden. One inch of rain from this one quarter of your roof generates approximately 374 gallons of water or 187 toilet flushes. If you have sandy soil or improved clay soil, the size of your garden ought to be about 20-30% of the drainage area, i.e. 20% of 600 sq. ft., which is 120 sq. ft. or 10’ x 12’.
First, walk around your property to determine where to put your beautiful native rain garden. Here are some considerations:
• Your rain garden should be at least 10 feet from your home or other structure to avoid wet foundations or attraction to pests.
• However, it should be an extension of the water flow pathway from a roof downspout for easier diversion to the garden.
• It’s best placed in full or partial sun.
• Keep it away from your septic system and drain field, rights of way, and underground utilities or service lines.
• Do not put it in an area of your property which tends to collect water during rains; this is an indication of slow infiltration. Remember, you’re not building a pond.
• And, do not excavate under a large tree as its root system and general health would be adversely affected by the digging.

Second, once you think you’ve found the perfect location, determine what kind of soil you have (sandy, loamy, clay) and how fast it drains. Dig a hole 12” deep and six inches wide, and fill it three times with water to saturate the soil. Fill it a fourth time to see how long it takes to drain. You’ll get rapid percolation of sandy soil within an hour; loamy soil may take up to eight hours, and clay sometimes not even within 24 hours. If the last, plan on preparing and adding this recommended soil mix: 50-60% sand, 20-30% topsoil, and 20-30% compost. If part of your property is on a slope, you can dig out a “scallop” for a rain garden and then prepare it in a similar manner.
Now you’re ready to design your garden. Use a hose or soccer field spray paint to create the shape, observing contours and slope of your property. If your yard slopes down from the public sidewalk, you might think about putting a D-shape rain garden immediately adjacent to the sidewalk to capture its run-off. You must kill or remove the existing lawn before excavating. If you have hired someone, have the excavator move the soil to another location on your property to create a berm, knoll, or other contour. Most rain gardens resemble a saucer, about six to eight inches deep, somewhat flat, with a deeper depression in the middle to encourage water to sink into the soil. Loosen the soil to a depth of about two feet and add the soil mix noted above. Grade the surface so the water will spread out evenly. And to reduce wash-out, create an attractive berm of soil and rocks at the end opposite the rainwater’s entrance.
Diverting rainwater from your roof’s downspout to your rain garden can be accomplished in one of several ways. You can install an attractive rock-lined swale or “dry creek bed” leading directly from beneath the downspout to the garden. You can install underground piping. Or, you can place a handsome wine barrel under the downspout with an overflow hose traveling underground or beneath rocks to your rain garden. Water remaining in the barrel can be dipped into with sprinkling can for hand-watering elsewhere. Now, after all this preparation, you can begin the exciting job of selecting plants for your garden—and you must think about it in the same ways you would for any other perennial garden. What are the cultural requirements of the plants—exposure, tolerance to occasional standing water or drought, and type of soil? Size of plants at maturity? Color, shape, texture, form? Pruning requirements? Interesting winter foliage? Berries or messy fruit? Garden “thugs”, i.e. invasive?
My strong recommendation is to use as many native Northern California plants as possible, or compatible Mediterranean plants with similar cultural requirements. Here are a few suggestions: goldenrod, several buckwheats, purple coneflower, various salvias, native ornamental grasses, phacelia, milkweed, island bush snapdragon, blanket flower, butterfly bush, pitcher sage, seaside daisy, yarrow, checkerbloom, verbenas, catmint, ornamental and culinary oreganos, rosemary, lavender, santolina, California poppies and other native wildflowers, and bulbs, to name just a few.
While your rain garden may be magical, it is not magic. After you’ve planted it, add two to three inches of mulch; shredded bark is best, as chipped bark tends to float. Or, locally produced products such as “Mallard Mulch” or “Mango Mulch” can be used as well. You will also have to do the occasional weeding and pruning, and in a particularly dry year, you must provide supplemental water. To help establish the roots, water regularly after installation. If you’ve planted just in time for the typical-year rainy season, the rain will do the job for you.
Your garden will be beautiful, colorful, and natural-looking many months of the year and will attract birds, bees, butterflies, and beneficial insects as well. You will also receive kudos from your city water department for reducing the size of your water-guzzling lawn and for keeping as much rainwater on your own property as possible, rather than have it overload the storm drain system and transport pollutants to the water supply. And your neighbors will admire your new unique and lovely landscaping feature—don’t be surprised if others want to know what a rain garden is and how to make one!
For more plant ideas, call the Sonoma County Master Gardener help desk at 707-565-2608 for native and habitat gardening plant lists, or check out the plant data base on this website www.sonomamastergardeners.org. For additional information on rain gardens, I have found the following sites to be exceptionally helpful: