|A large rain garden at our public library.|
|Repair of the riparian zone along the French Broad River.|
The best method of slowing down the water is forcing it to pass through densely planted areas that contain plants with deeper roots than your average lawn. I discussed some very effective riparian zone/rain garden plants in June of 2011. (For a terrific list tailored to your zip code, check the guides at the Pollinator Partnership.) Today I want to concentrate on how best to establish a rain garden in your yard, because a good one will improve water quality and provide excellent habitat for pollinators.
The next step is to get an idea of the rate at which your soil can drain/absorb water so you'll know how deep to dig. The formal method involves digging a hole a bit larger than a gallon-size can of paint, filling the hole with water (measure how much water you've got in the hole), wait four hours and measure again. Take the amount that has drained [initial inches minus current inches] and multiply that by 6 to get the number of inches that could be absorbed over a 24 hour period. I can just see my mom doing this. Her sandy soil would drain in about 15 minutes. So ideally, she would amend her soil in her rain garden to encourage a bit of retention--digging out some of the sandy stuff and adding in some compost, peat and maybe some vermiculite, since it rapidly absorbs water and releases it slowly. Vermiculite will break down over time, but it will give your plants a boost by keeping some of that moisture a little longer without suffocating the roots.
Those of us with clay soil have to really take the measurement under advisement, however. If our soil will only drain six inches in 24 hours, then we need to dig down six inches. If it won't drain even six inches, then we need to remove the six inches of soil and amend what remains in the depression with fine gravel, compost, perhaps some perlite, and some peat. Since rain gardens will overflow, you will want your perlite as the bottom layer of amendment if you choose to use it. Check the link for more info on that particular soil amendment. Remember that your rain garden should be lower in elevation than the surrounding soil--if necessary, you can remove a foot of existing soil to achieve this (if you know that you will have to add many amendments). For more good info on sizing your rain garden, check the VA Dept of Forestry site (below). For larger projects, I highly recommend the French Broad site, which gives you access to publications such as the one on Urban Stormwater Structures that includes terrific illustrations for managing water flow.
|Winterberry Holly is a terrific rain garden or riparian shrub.|
Now for the fun part! The plant list on the French Broad site is kind enough to indicate not just moisture-loving plants, but the degree of wet they prefer or tolerate. This list is limited to plants that do well in the French Broad River Basin, but along with your custom list from the Pollinator Partnership, you should be able to locate good choices for every part of your rain garden. Get your new plants into the ground, apply mulch to keep the weeds at bay and water those puppies in! Make sure your new riparian zone/rain garden gets an inch a week for the first couple of months if you aren't getting enough rainfall.
Sources: Virginia Dept of Forestry, French Broad River Watershed, Ohio State, Pollinator Partnership