Time To Make Your Rain Garden?
Last week, snow and high winds kept most of us home, snug indoors while nature ran wild outside. Here on Bainbridge Island, we had over six inches of snow, layered with ice, making for treacherous roads.
However, we never lost power, making this the first major storm I can recall in over 30 years when we did not. What a lovely treat to be “stuck” at home for several days, yet be warm and able to do whatever we liked. I read and knit and cleaned closets and had a splendid time, but by the third day, I was itching to go out. However, the garden was far to wet to permit gardening. Drat!
A Soggy Aftermath
As the snow and ice melted, runoff poured in merry little streams down the driveway and through the garden. Heavy rains deepened the streams into gushing waterfalls that tumbled down the hill, spreading at last into a rather pretty lake. That’s where the rain garden will go, when the soil dries out enough to dig.
When winter rains continue in force, 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. The key to reducing runoff is my favorite soil amendment; compost. Compost-enriched rain gardens absorb water like sponges, then let it percolate slowly into the soil. Compost can also make your existing garden beds and borders more absorbent.
Like forest duff, compost can store many times its weight in water. Adding it to the garden improves the texture and biotic quality of our soil. Compost top dressing even prevents weed seeds from germinating. To accomplish all this and increase the permeability of beds and borders, add 2-3 inches of compost every spring and fall.
Why Lawns Get Soggy
Lawns are often almost as impermeable as a concrete sidewalk, which sheds up to 85% of rainfall or irrigation. To change that situation, rake half-an-inch of compost over the lawn every spring and fall. Over time, this increases soil permeability and reduces lawn’s need for summer watering dramatically. (It also eliminates thatch without all the bother of aerating.)
If you still get puddles after a few seasons of adding compost, your property will benefit from a rain garden. Where to put it? A sunny, fairly flat, unused lawn is ideal, as long as the chosen spot is not
1) on or near your septic field,
2) uphill from your home or well,
3) right under a large tree, or
4) above undergrounded utility lines.
Designing A Working Rain Garden
Place your rain garden at least 10 feet from the house or outbuildings to keep foundations dry. Though it may seem counterintuitive, 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.
What should it look like? Some rain gardens are long and skinny, like a mock stream bed, perhaps running along a fence or tumbling between garden beds. Most often, they are given naturalistic shapes, but if your garden is formal in design, your rain garden can be formally shaped as well.
Give It The Drainage Test
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? There are several good sources for determining how much runoff you need to capture. Check your local library for books on rain garden design, or visit online sites (see below for a good one). The idea is figure out the amount of runoff from your roof and driveway, allowing for the slope of the land and more.
Or Just Do It
If you don’t care for math or just want to give the idea a try, a rain garden of pretty much any size will improve a soggy yard. As a baseline, 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 2-4 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 2-3 inches of shredded wood or leaves. Your rain garden should still be significantly lower than the surrounding lawn or borders.
Ready To Plant
Now you can plant with perennials, grasses, and shrubs. Well planted rain gardens can be as attractive as any ornamental bed. 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 Pierce County’s very useful website (www.pierce.wsu.edu), where a complete rain garden manual is available as a free download.