by Heather McCargo • November 19, 2015
The torrential downpour that the Northeast experienced at the end of September was a clear reminder that we need to make our landscapes more absorbent of precipitation. Forests and meadows are able to absorb massive amounts of rainfall, slowing the water’s path into streams and rivers, filtering out pollutants, recharging groundwater, cleaning our air and moderating temperatures. Our cities and suburbs are much less absorbent because we replace the native vegetation with roofs, pavement and minimal vegetation and treat rainwater as a waste product to be quickly diverted into culverts and underground pipes. This tightly channeled water, tainted with pollutants, eventually discharges into natural streams with explosive force. Heavy rain can overwhelm municipal systems, mixing rainwater with sewage and causing human waste to flow into streams and coastlines. We can change this, and you, as an individual can start at your downspout.
The water runoff from your roof’s downspout creates an opportunity to plant some of the beautiful natives that are adapted to the fluctuating levels of moisture at a drainpipe. Some of my favorite native plants come from wetlands and thrive in intermittently wet soil. You can create a beautiful garden that will attract butterflies, bees, birds, and amphibians and will also help rainwater to be absorbed back into the earth instead of being diverted as waste.
Natives for rain gardens from top to bottom: Golden groundsel, Joe-Pye weed, Winterberry, Blue lobelia, Cardinal-flower, Witch-hazel, Swamp milkweed
The following are tips for preparing a site for a small downspout rain garden. This is a great project for mild days in the late fall or early spring, though it can be done any time that the soil is not frozen.
- At the outlet of your home’s downspout, create a water catchment area by digging a hole 18” wide by 12” deep and fill it with 1-2” stones. This will dissipate the force of water as it is funneled down the spout and allow for an easy entry into the soil. The area surrounding the gravel hole can be slightly depressed by 1” to expand the area that holds the water. Once this area is planted the gravel will not be visible. Large rain gardens are sometime more deeply recessed, but in a small space a more subtle depression looks better and still can handle a lot of runoff.
- If lawn is growing at the site of your water catchment area, you must remove the sod and roots from the area to be planted. You can dig it out with a fork, or you can use a much easier and highly effective technique called sheet mulching. With sheet mulching you kill the grass by smothering it with layers of newspaper or cardboard covered with 2” of organic mulch. Once you have removed the sod, a good natural compost or bark mulch mixture will restore the microbial community to the soil and provide all the nutrients needed by resilient native species. If the soil is heavily compacted (e.g., by heavy construction equipment or foot traffic), then you should loosen the top foot of soil with a digging fork before adding the mulch.
- After you have prepared the water catchment area and the space to be planted, flood it with a hose or watch it during a heavy rain to see if you want to adjust the grade to better pool the water. This will also give you a chance to see where it spills over; if the bed is near a sidewalk or driveway, you may want to edge it with a lip of brick, stone, or wood. This looks nice and helps hold the moisture and mulch in place.
- Determine the amount of sun that your site receives by watching it throughout the day. This will help you choose the correct species to plant. There are wonderful natives for full sun, part shade, and full shade. Once you have determined the amount of sunlight at your site, choose the species from the lists below that appeal to you. I have included the height and width of the mature plants. Measure the size of area you are planting to determine how many plants you will need. You should plant densely—it is the plant stems and foliage and their root system that are going to absorb and hold the extra water. Site the woody plants first, then add the perennial species. In general, the taller plants go in the back or mid section of the planting, with lower ones in front.
Suggested Native Plants for Rain Gardens
Shrubs and Perennials for part or full sun
- Common buttonbush Cephalanthus occidentalis 5’ (tall) x 5’ (spacing width)
- Common winterberry Ilex verticillata 4-7′ x 5′
- Rosy meadowsweet Spiraea tomentosa 3′ x 3′
- Pussy willow Salix discolor 10′ x 5′
- Smooth arrowwood Viburnum dentatum 7′ x 5′
- Swamp milkweed Asclepias incarnata 3′ x 2′
- Asters Symphotrichumspacenovae-angliae, S. novi belgii 3′ x 2′
- Marsh-marigold Caltha palustris 18″ x 16″
- Joe-Pye weed Eutrochium maculatum 5′-7′ x 3′
- Boneset Eupatorium perfoliatum 3′ x 2′
- Meadow bottle gentian Gentiana clausa 1′ x 16″
- Blue iris Iris versicolor 2′ x 18″
- Cardinal-flower Lobelia cardinalis 3′ x 18″
- Golden grounsel Packera aurea 1′ x 1′
- Three-lobed coneflower Rudbeckia triloba 4′ x 2′
- Blue vervain Verbena hastata 5′ x 2′
Shrubs and Perennials for shade
- Coastal sweet-pepperbush Clethra alnifolia 5′ x 4′ (height x spacing width)
- Witch-hazel Hamamelis virginiana 8′ x 3′
- Northern spicebush Lindera benzoin 10′ x 3′
- Highbush blueberry Vaccinium corymbosum 5′ x 3′
- Cinnamon fern Osmundastrum cinnamomeum 3′ x 2′
- Royal fern Osmunda regalis 3′ x 2′
- Cardinal flower Lobelia cardinalis 3′ x 18″
- Blue lobelia Lobelia siphilitica 18″ x 18″
- Golden groundsel Packera aurea 1′ x 1′
For a more extensive list of natives for wet soils, see our Comprehensive Plant List and scroll down to plants for wet soils.
A final note: Many cities, including Portland, are beginning to charge a fee to property owners based on the amount of pavement, roof coverage, and other hard surfaces they have on their property. Homeowners could pay between $6 and $24 a month, depending on their area of impermeable surfaces. Rain gardens, cisterns, rain barrels, and porous paving surfaces will reduce these fees substantially.
For more information on rain gardens, check out:
- Rain Gardens: Sustainable Landscaping for a Beautiful Yard and a Healthy World by Lynn M. Steiner and Robert W. Domm