Go Green! We hear that so often now, but while our intentions are good, beyond changing out our light bulbs and recycling, many of us are unsure what difference we can make as an individual. Find out about one easy and fun way you CAN make a difference by minimizing your contribution to water pollution in your own home landscaping as Blue Max Materials' Mike Bishop and WBTV Meteorologist Jim Lytle explore Rain Gardens.
Rain gardens are just what the name implies - first and foremost, they are beautiful gardens, but their design allows them to serve as miniature water treatment plants. They capture rain water that runs off of your roof, driveway or patio, filters it, and allows it to soak into the ground instead of running into storm drains or streambeds.
Why do we need rain gardens?
Nature's cycle has rain falling on pervious natural areas like woodlands or meadows where the soil and plants slow down the water, filtering it and allowing it to soak into the ground. However, in our urban areas, rain falls on all kinds of impervious surfaces like rooftops, roads and parking lots where it can't soak into the ground; instead it rushes along collecting pollutants like fertilizer, pesticides, sediment, motor oil, litter and pet and yard waste. An acre of impervious surface results in about 26,000 gallons of runoff from a 1" rain compared to little to no runoff resulting from the same amount of rain in a natural area.
Where does this stormwater go? Not to the water treatment plant as you might expect - instead, it heads directly for streams, rivers and lakes where it pollutes the water (water that is ultimately the source of drinking water for someone downstream) and can cause flooding during heavy rainfalls. To prevent some of this massive runoff in the greater Charlotte area, you'll be seeing more and more rain gardens (or bio retention gardens) designed to capture this water and minimize runoff being built in such places as parking lot islands and sub-divisions.
Rain garden located at The Sanctuary in Lake Wylie.
Bio retention rain garden located at Wilmore Walk (design by Estes Design).
What does this have to do with you as a homeowner? Think about your yard - you may have a concrete (or other impervious) driveway, you certainly have a roof and you may have an impervious patio. Water runs off of all of these surfaces, collecting the fertilizer you put on your grass, the pet waste you didn't get a chance to pick up, the motor oil dripping from the car in your driveway - you get the picture! Some of it soaks into your grassy area, but during heavier rains, your ground can't keep up, so the water rushes along and may enter a stream behind your house or run onto the street and into the stormwater drain where it eventually ends up in another creek or body of water. A rain garden can absorb 30% more water than the same area of lawn, so by building a rain garden in your yard, you can minimize the water runoff originating on your piece of the earth and do your part to cut down on water pollution.
What is a rain garden?
A rain garden is simply a shallow depression in the ground that captures your water runoff and allows it to soak into the ground. Plants and soil work together to filter pollutants through absorbtion or chemical breakdown, delivering cleaner water through the ground to nearby streams. A rain garden is not a pond or a wetland; it is dry most of the time and holds water after a rain. When properly located and designed, water filters into the ground in less than 48 hours. Because water doesn't sit for more than a couple of days, you don't have to worry about the rain garden becoming a breeding ground for mosquitoes!
Where do you put your rain garden?
First, you need to choose a site by observing the water flow in your yard during a rainfall, watching to see where the water is flowing from (e.g., roof, downspout, driveway)and flowing to (e.g., low spot, drain, street). You'd like to position the rain garden somewhere between these two places, for instance in an area where your downspouts will drain into it. Keep in mind these considerations:
- stay at least 10' away from your house foundation
- stay at least 25' away from a septic tank drainfield
- stay at least 25' away from a well head
- avoid underground utility lines
- look for partial to full sun exposure
- make sure your water table is at least 2' below the surface of the soil. (If you hit the water table at less than 2', you may want to consider a wetland garden instead).
You'll want to locate your rain garden on flat or gently sloping ground. If the slope is greater than 12%, look for an alternate location.
How big do you make your rain garden?
Determine the size and lay-out of the garden. There are mathematical formulas available to determine the correct size given the amount of runoff you're dealing with
(see the website for the NC Cooperative Extension Service at http://www.bae.ncsu.edu/topic/raingarden/index.htm),
but a simple rule-of-thumb is to size the garden at 30% of the area of the roof from which it will be collecting water. Remember that any size of rain garden is better than no rain garden!
What do you need to know about soils for rain gardens?
You need to determine the permeability of your soil in the area you select. Rain gardens obviously need well-drained soils to work properly - without amendment, the clay soils found in much of the Charlotte region do not work for rain gardens! There are several tests you can use to determine how well your soil percolates
(see the website for the NC Cooperative Extension Service at http://www.bae.ncsu.edu/topic/raingarden/index.htm);
many of us know without testing that our soil is full of clay and won't allow water to soak in well, so we'll have to amend it for a rain garden to a mixture of sand, soil and compost. Fortunately, there is an easy fix for this - Blue Max Materials sells a blended soil called Bio Max that has been specially formulated for rain gardens in the Charlotte area. You can remove existing soil and replace it with Bio Max to remove the work and worry of trying to achieve just the right blend on your own.
How do you build a rain garden?
Once you've done all the preparation work discussed above, outline the shape of your garden using stakes and string, spray paint or a garden hose, and then it's time to begin digging! You want the garden to be a bowl 4 to 8 inches deep, with shallow sides and a slight depression in the center. The rain garden should be level so water doesn't pool in one low area of the garden. If you're locating it on a gentle slope, excavate more on the uphill side of the slope than the downhill side. You can use the soil from the uphill side to raise the edge of the downhill side so that the garden is level and of equal depth.
In clay subsoils, which account for virtually all of the Charlotte area region, your garden will perform better if you dig an additional 1.5 to 2 feet down and amend the subsoil, too. Put 3 to 6 inches of drainage stone (#67 stone works well) in the bottom of the garden, followed by a landscape fabric to allow water to pass through, but keeping the soil and sediment from getting into the stone. Place Bio Max (or your amended soil) into the hole, followed by a top layer of 2 to 3 inches of double hammered hardwood mulch (this type of mulch will not wash away and decomposes to add nutrients to your garden).
Use some of the excavated soil from your hole to build a small berm around the "back" side of the rain garden - you want the berm to stop the water in the rain garden and retain it there instead of continuing to flow on through your yard. Tamp this ground down well and cover it with sod or mulch to prevent erosion.