Storm-runoff gardens can create a beautiful and practical solution By Anne Spafford Fine Gardening - Issue 147
The term “rain garden” is a bit of a misnomer. Rather than a water-logged bog, a rain garden is a slightly sunken garden designed to capture storm-water runoff. Rain gardens can alleviate erosion while trapping fertilizers, pesticides, and organic pollutants, like grass clippings, before they wash down storm drains. From there, pollutants can enter waterbodies and cause algal blooms, reducing oxygen for fish and other animals. Rain gardens to the rescue! Water is trapped and pollutants are removed by plant roots and the garden’s filter bed. Once caught in the filter bed, earthworms and microorganisms ingest and neutralize contaminants.
A rain garden can be as simple or elaborate as you like, and installed in less than a day! If everyone planted a rain garden, polluted runoff from residential neighborhoods would be substantially reduced.
Select Your Site Carefully
If you’ve noticed a part of the yard where rains cause erosion or pooling, you can probably create a rain garden. Find out where the garden will catch the maximal runoff by going outside in the rain and watching how and where water flows. Start by looking at how water comes off your roof or other hard surfaces and moves across your property. Placed in the runoff’s path, a rain garden can stop erosion or catch water slightly uphill from where it pools. But don’t plant a garden in this pool of water—this isn’t a bog planting.
To deal with runoff that accumulates as it runs down a slope or from neighboring properties, you’ll have to work with what you see. Consider the amount of water moving onto your property and how it enters. Water enters my yard, for example, from a neighbor’s property. I looked at the square footage of their roof and which downspouts dumps runoff toward my yard, then designed my rain garden accordingly. You will never be able to catch all the runoff, but any is better than none.
Site your rain garden at least 10 feet from your home’s foundation to avoid seepage. If you have a well or septic system, check local building regulations to see how far your rain garden should be from them. When you’ve found a spot, conduct an infiltration test. Dig a 4-inch-diameter and 1-foot-deep hole where you’d like to build, and fill it with water. If it drains in less than 48 hours, it is a suitable spot.
The Anatomy of a Rain Garden
You don’t need special equipment or professionals to start catching runoff, but rain gardens need the same care as other gardens: watering when dry, routine pruning and deadheading, and topdressing with mulch every year to provide nutrients and to suppress weeds.
Rockery
If runoff flows across your site with speed and force, add a row of stones or other heavy material that will slow the water like a speed bump. Use something that relates to the overall design of your garden.
Depression
A 3- to 6-inch-deep depression with sloped sides, like that of a pie dish, catches and holds water above the filter bed. A 6-inch ponding depth—or gap between the natural soil grade and the top of the filter bed—holds the most water in the smallest space. Heavy clay soil can be difficult to amend deeply, so a wider and shallower 3-inch depth might be a better option. Choose a depth according to your needs.
Plants
Plants suitable for rain gardens thrive in periods of short-term flooding (less than 2 days) as well as in periods of drought, both of which are common in North American weather patterns. Avoid plants that colonize, as they will run rampant, and include woody plants to maximize the garden’s performance year-round.
Filter bed
The filter bed should be at least 6 to 12 inches deep. Its soil must contain coarse particles that let water in and finer particles that hold water for plant use; the addition of compost secures both. Compost creates pore spaces in slow-draining clay soils for improved water infiltration; in sandy soils, it slows infiltration, so plants get a chance to take up water before it wicks away. Compost, furthermore, provides nutrients necessary for healthy plant and microbial growth. The addition of 25 percent compost usually is enough to get the job done.
Berm
If your rain garden is on a slope, use some soil from the initial dig to build a berm on the rain garden’s lower side. A berm holds water that might flow down the slope, out of the planting, defeating the garden’s purpose.
Popular Plants That Fare Well in Rain Gardens
1. Purple coneflower (Echinacea purpurea and cvs., USDA Hardiness Zones 3–9)
2.Daylily (Hemerocallis cvs., Zones 3–10)
3.Iris (Iris spp. and cvs., Zones 3–9)
4.Switchgrass (Panicum virgatum and cvs., Zones 5–9)
5. American beautyberry (Callicarpa americana, Zones 5–9)
6.Sweet pepperbush (Clethra alnifolia, Zones 3–9)
7.Redtwig dogwood (Cornus alba and cvs., Zones 2–8)
8. Deciduous holly (Ilex verticillata and cvs., Zones 5–8)
How big should it be?
The volume of a rain garden should be between 5 and 10 percent of the impervious surface area (rooftops, concrete walkways, driveways) from which the runoff it catches originates. 3 easy calculations tell us that this rain garden must measure at least 40 square feet to catch 5 percent of this home’s roof runoff.
TIP: Make a call before you dig! Call your local utilities locator (811 is the number for the national one-call center) to request a crew to mark where underground utility lines are buried. This is a free service.