Drainage Woes, Be Gone: How To Build A Rain Garden
April 7, 2018
If you’re looking for a way to solve drainage problems in your yard, look no further than a rain garden. It’s a good-looking, effective way to manage soggy ground, and also protects our lakes, streams and wetlands from polluting water runoff. Rain gardens also provide food and habitat for pollinators, songbirds, and other wildlife. Need help building one? John Bly, Education Program Manager at Blue Thumb, is here to help.
What types of properties most benefit from a rain garden?
Any property that has both impermeable surfaces (roofs, driveways, sidewalks etc.) and space for a rain garden would benefit from one. Rain falling from the sky is natural; once it hits the ground and turns to runoff, though, it becomes a threat (the #1 threat, according to the EPA) to our lakes, streams and wetlands. We find it is best to capture runoff as close to the source as possible.
Generally speaking, a site needs at least 3 ft (and more preferably 6″ or more) of unpolluted topsoil between the surface and bedrock or the water table. Some areas are brownfields with chemical spills that we don’t want to be pulling into the water table, and other areas might have geologic Karst formations that might not be stable enough to infiltrate large quantities of water.
How exactly does a rain garden work?
A rain garden works by collecting runoff and seeping it into the ground, effectively restoring some of the ecological function the land had before being developed. A rain garden should be shaped like a flat-bottomed bowl, and located to intercept runoff from downspouts or other sources of runoff.
When planted with native plants, whose roots stretch many feet deep, a rain garden will grow more efficiently as its plants push more roots through the soil, shedding others that add organic matter and create micro-channels for water to travel through.
Good To Know
Some cities, counties, watershed districts or other local government units offer cost shares, grants, or fee abatements for property owners to install rain gardens. For now, the easiest way I know for someone to check what is available to them is to type “rain garden grant OR cost share ” with their location into their preferred online search engine.
What is the best location for the rain garden?
A rain garden should be at least as far away from a foundation (of a home, garage or retaining wall) as the foundation is deep. This generally means keeping them 10′ from houses with basements. They are much easier to install when not under a tree, whose roots generally extend as far as its branches. And the work of digging can be cut in half when installing on a slope, by using a “cut and fill” technique.
Counterintuitively, rain gardens generally don’t work the best at low points in a yard, especially in places where water drains slowly; years and years of runoff have already carried fine soil particles to settle in the low spots and clog up pores between soils. It generally works better to intercept the runoff before it reaches a low (or problem spot) in the yard.
The last thing to remember about placing your rain garden is that it needs an overflow point where it can top out in a big storm—planning for that to be somewhere that won’t lead back to your house is a good idea.
Is there anything we need to consider when it comes to the depth and size of the garden?
Proper depth of a rain garden is hugely important and depends on the infiltration rate of your soils. A simple test can help determine this: dig a hole about 9″ across and 9″ deep. Fill it with water, let it drain, and then fill again. Mark the level of water (a popsicle stick stuck into the side of the hole works well) and wait an hour. Measure the distance between the high water mark and its current position. This will provide a rough estimate of how quickly your saturated soil can infiltrate water.
As long as your soil can infiltrate at least 1/2″ per hour, you shouldn’t have a problem with a rain garden between 6 and 12″ deep. If your soil is heavy with clay and infiltrates slower, perhaps 3″ would suit you better. We want the rain garden to drain within 24 hours, for the sake of the plants in it, as well as the mosquitoes you don’t want. I forget where I heard it first, but I like the saying, “if your rain garden still has water in it after 48 hours, you don’t have a rain garden—you have a swamp.”
Sizing of the rain garden is important too, but not as much. If you estimate the amount of impervious surface draining to the rain garden (in square feet) and divide that by the depth of the rain garden (in inches), you’ll get an idea of the size of the rain garden necessary to capture 1″ of rain, which is what we generally do. We recommend side slopes that have a 3:1 horizontal: vertical slope to prevent erosion, so factor in 1-3′ around the rain garden basin. If you come up with a size that’s too big for what your property can handle, know it’s possible and still helpful to have a smaller rain garden, as it will still capture the “first flush” of rain carrying the most pollution.
What kinds of tools and materials will we need for this project?
We find a sod kicker is handy for removing the turf from where we’re going to install a project. Regular spades are good for digging out the basin, and sometimes a flat shovel is helpful for creating a flat bottom. A hard rake helps smooth out the soil, and a pitchfork is good for spreading the double-shredded hardwood mulch we recommend using (you may want a wheelbarrow to move it to the rain garden). Then, a hand trowel makes it easy to push the mulch away and dig a small hole for each plug or potted plant. Once the rain garden is planted, you’ll want a hose or watering can to provide at least 2″ of water a week through the first growing season.
So, to recap, can you walk us through the steps of building one?
After you determine the location and size of your rain garden, order/buy your materials, and ideally wrangle a couple friends over to throw some dirt around. You’ll remove the turf, dig out the rain garden basin (think: where are you going to put that soil? Can you create a berm along the rain garden? Fill up some raised beds? Or re-grade around a settling foundation), turn the soil at the bottom of the garden to de-compact it, shape it, mulch and plant it.
After the garden is built, are there special maintenance requirements?
The first year, supplement whatever rainfall happens with one watering session a week so that the plants receive 2″ of water. Watering long & deep instead of a little every day promotes the deep root growth that will allow your garden to thrive. Weed at least 3 times a year, around Memorial, Independence, and Labor Day to get the cool and warm season weeds before they go to seed. Keeping at least 2″ of double-shredded hardwood mulch will help limit unwanted seeds from germinating. I’m stressing double-shredded hardwood because it’s dense and locks together, so it’s less likely to float away.
Many thanks to John Bly for all of this incredible information about building a rain garden.To read more, be sure to visit Blue Thumb!
Garden Culture has also put a rain garden plant list together to help get you started. Go ahead and take the plunge! Rain gardens are fairly easy to build, and probably the most beautiful, earth-friendly solution out there to excess water in your yard. Happy digging!
Images courtesy of Nelco Landscaping, photos by METRO BLOOMS.