Saturday, February 4, 2012

Rain Garden = Riparian Zone

I don't have a river bordering my property. So there is no way for me to establish, by definition, a "riparian zone." Though if there WERE a river bordering my property, you can be durn sure I'd have a riparian zone, as I prefer not to let my property wash away in a storm. I'm funny like that. Must get it from my parents. :)

A large rain garden at our public library.
 That being said, most homeowners have storm water issues. It ends up where we don't want it. It comes off of roads and our roofs carrying various chemicals and other mystery substances that we may not want anywhere near our children, our pets or ourselves. I talked a bit about those surface pollutants in March of 2010. I'll just mention that 50% of the contamination of our surface water is the result of homeowners. So I know that water flowing to my yard from those around me is likely contaminated, so even if I practice good stewardship myself (no pesticides, no herbicides), my property may contribute to the storm water problem if I have no installed remediation in place with which to clean the water.

Repair of the riparian zone along the French Broad River.
The first step in dealing with storm water is to slow it down. The faster it moves, the greater the potential for erosion and the less likely it is to be absorbed by your soil. Why would you want your soil to absorb storm water? First, absorption reduces the possibility of flooding. Secondly, soils capable of absorbing storm water are typically a mixture of wet and dry soils that "facilitate a variety of biological and chemical reactions. These reactions reduce the availability of some nutrients and decrease the toxicity of some contaminants (Ohio State)." In other words, soil absorption will help to decontaminate the storm water. The last reason to support soil absorption is so your plants get a chance to get a drink!

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.

Begin your site selection by calling 811 a few days before you plan to demolish your lawn and let them mark any areas that could be...problematic. A good site should already be a bit low (if possible) and at least 10 feet from your home's foundation, any fence lines or septic systems. Plan on extending a downspout to the location--trench and bury pipe as necessary to reach your new rain garden.

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.
Once you have dug out the depression and amended the soil, I advise a fill test. This will show you how  your amendments have improved the retention/drainage rate and where your "overflow" spot is. Make sure your overflow doesn't dump water onto any of the previously mentioned bad spots (particularly foundation/septic field). If it does, dig a little deeper at the edge of the spot you want the water to exit and create a gentle swale to facilitate water migration. Reinforce where the water was previously leaving the rain garden with some clay or small boulders to inhibit the flow.

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

2 comments:

Ellen Honeycutt said...

An excellent post with great details!

Lydia Sorensen said...
This comment has been removed by a blog administrator.

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