Making a Rain Garden
As the winter rains return, it’s easy to see if you could benefit from a rain garden. Planted with natives and allies that love wet winters and dry summers, rain gardens capture excess water that might flood basements or leave lawns soggy.
Pavement, roofs, and most lawns create runoff that can make yards unusable all winter. Compost-enriched rain gardens absorb water and let it percolate slowly into the soil.
Compost can also make your existing garden more absorbent. Compost can store many times its weight in water and improves the texture and biotic quality of soil. It even prevents weed seeds from germinating. To increase the permeability of beds and borders, add 2 to 3 inches of compost every spring and fall.
Lawns can shed up to 85 percent of rainfall or irrigation. To change that, rake in half an inch of compost every spring and fall. Over time, this increases soil permeability and reduces a lawn’s need for summer watering dramatically.
If you still get puddles, try a rain garden. Sunny, fairly flat, unused lawn is ideal, as long as the chosen spot is not on or near your septic field, uphill from your home or well, right under a large tree, or above underground utility lines. Place your rain garden at least 10 feet from the house or outbuildings to keep foundations dry. Avoid places where water puddles, since you already have a drainage problem there. The idea is to promote water absorption, not persistent puddling that might harbor mosquitoes.
A rain garden could be long and skinny (perhaps along a fence line) or formally or naturalistically shaped. Check the drainage on your chosen spot by digging several test holes about a foot deep. Fill them with water; if it vanishes quickly, terrific. If it lingers all day, find another spot.
How big should a rain garden be? Library books or online sites can help you figure out the amount of runoff from your roof and driveway, allowing for the slope of the land and more.
However, any rain garden will improve a soggy yard. Start with about 100 square feet of rain garden, see how the situation changes, and add more if you need it.
Make your rain garden at least two feet deep, with a flat bottom and sloping sides. With the removed soil, build a low berm behind your rain garden to keep high water in place. For overflow, slip unperforated drainage pipe through the berm into a catchbasin or “streambed” of crushed rock.
Fill the bed with 2 to 4 inches of clean crushed gravel (1-inch or bigger) or 6 inches of coarse woodchips (not bark), topped with 4-6 inches of compost. Top-dress with 2 to 3 inches of shredded wood or leaves, then plant with perennials, grasses and shrubs.
Rain gardens can be as attractive as any ornamental bed. In high season, you won’t even notice that the bottom of the rain garden is lower than the surrounding lawn. To make a handsome planting, combine water iris, Japanese anemones, bog sage, hostas, and daylilies with carexes, feather reed grasses (Calamagrostis), maiden grasses (Miscanthus), and sea oats (Chasmanthium latifolium). For structure, tuck in some spirea, flowering quince, and twiggy dogwoods, with a chokecherry or vine maple for summer shade.
A mature rain garden needs minimal weeding and no watering. Annual mulching with compost, shredded leaves, or ground wood (not bark) will keep soil open and receptive. Check your rain garden after especially heavy rains to be sure it is working properly. If it is frequently overwhelmed, increase the size of your rain garden or make another bed.
For more information, visit the Pierce County Web site (www.pierce.wsu.edu), where a complete rain garden manual is available as a free download.
To register for Ann Lovejoy’s garden design workshops or intentional knitting classes to benefit the Harmony Hill cancer retreat programs, visit the Web site at harmonyhill.org/retreats/lovejoy.html or call 360-898-2363.
©2007 Caliope Publishing Company
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