Consider A Rain Garden

Coming off a summer drought as we are, you may think it strange to consider a Rain Garden. But a friend asked us for help in creating one. The madman says, “Rain Garden is a misleading term.” I’m not sure if his “Bio-retention Facility” is any better.

Very basically, a Rain Garden is a shallow depression in your yard that is full of plants that can handle a rainstorm and survive a drought. Planned correctly, it can be an attractive addition to your landscape that invites birds and pollinators. But its most important function is to help filter storm water before it enters the watershed.

Clean Water

Before urbanization, there was very little need to filter rainwater. The ground did a good job. We had meadows, wetlands, and forests all designed to soak up and clean our rainwater before it entered waterways. Nature cleaned our water.

But then our cities and developments added large buildings, paved roads, concrete sidewalks, and large parking areas – lots of surfaces that can’t absorb water. Dirty water began entering our waterways unfiltered. Even our lawns with their shallow roots do a poor job of filtering water.

A New Idea

In 1990, a group of storm water specialists in Maryland came up with the idea to mimic Mother Nature’s way of filtering runoff – the Rain Garden. Of course, to make it sound more impressive, they called it a Bio-retention Area. Regardless of its name, this is how it works.

Rainwater enters the garden during a storm. The water collects in the center of the garden where happy plants begin to absorb it through their deep roots. The water slowly begins to filter through the soil, getting cleaned before entering the waterways. This process may take up to 48 hours, but will leave behind no standing water for our friends the breeding mosquitoes. (By the way mosquitoes need 7-12 days to complete a breeding cycle, and by then the water in a Rain Garden should be long gone.)

Helping The Environment

Think you want one? Don’t do it for the beauty – do it for the good it does to your yard and our environment. Installing a Rain Garden will require some work – both mental and physical. The Chicago Botanical Garden has put together a nice 32-page handbook. You may find this helpful.

The first step will be to find an area in your yard suitable for this garden. You may be tempted to put it in that low spot where water always collects, but remember the purpose of this garden is to filter water downward, not to hold it. If you have an area with poor drainage, adding more water will make it worse not better. So, keep looking.

Look For the Water

While you’re in the thinking stage, take a walk around your house after a rainstorm. Make note of where the wet spots are. Remember, storm water will flow off all those surfaces that can’t absorb it – the roof of the house and garage, the patio, the deck, the walkways, the driveway – now is the time to see where it goes. You want to place the garden in the natural flow of the water, otherwise you’ll have to divert the water, and that’s an awful lot of work.

Keep the Rain Garden at least 10 ft. from the foundation of your house. This will prevent water from collecting and seeping into your basement. Also, avoid your septic system. You don’t want your filtered rainwater going there.

Keep It Flat

Don’t put it on a hill. You want the area of the Rain Garden to be flat in order to collect the water. If the grade is too steep, you will have to terrace the area. Although it can be done, it requires an awful lot of work – most likely by someone above the paygrade of a homeowner.

And, finally, find a sunny spot. Most of the plants suitable for a Rain Garden prefer full sun although many do well in partial shade.

Now that you think you know where you want to put the garden, it’s time to check the soil. A simple percolation test will tell you how the area drains. Dig a hole, fill it with water and see how well it empties. The hole should be 18” deep, 12” in diameter, and any water will do. Come back 6 hours later and if the hole is dry, you’re good to go. If you still have standing water, you will have to amend the soil by adding compost to improve the drainage. Or find a new location.

How Big?

The next step is to figure out how big you want your Rain Garden. The madman says any size will do. But here’s a link to a site that will help you determine the best size garden for the amount of runoff you have. The cool thing about this NEMO site is that it can pull up a satellite image of your property. You outline the impervious areas, and the calculations are done for you. Or you can do the actual measurements, punch in the numbers and get the calculation.

Now it’s time to design the Rain Garden – any shape you want is fine. But you can’t have turf in the Rain Garden, so get rid of it. You will have to dig, but before you do, save yourself some grief and call “Call Before You Dig” – 811. You want no surprises. In our area, storms are most commonly 1” or less, so a level Rain Garden should be 6-8” deep.

Construction

If you are digging on a slope, you should place a berm (pile of dirt) on the downhill side to prevent runoff and to keep the top surface level. You can dig by hand or use a machine to help you, but as you go, incorporate 2” of compost at the bottom. Gently slope the bottom of the area up to the edge, creating a basin for your rainwater. Fill for the garden should be 50% sand, 20% topsoil and 30% compost according to the USDA.

Now you’re ready to plant. Keep in mind that plants in the center will often have wet feet, while at the same time, plants along the edge will be dry. Select accordingly and think native. Here’s a link to suggested Rain Garden plants from UConn. This link to the Penn State plant list divides the plants into the three zones of your garden.

Keep It Personal

Rain Gardens are as personal as any other garden, so your choice of plants may be much different than ours, but here are some of the plants we would include. The size of your Rain Garden will dictate the number of plants you’ll need. A good rule of thumb is to use at least three of each variety and don’t pack them in – allow for growth.

For the backbone of a Rain Garden consider some native shrubs like Spicebush, Winterberry, Elderberry, Swamp Azalea or one of the Dogwoods. Grasses are nice for texture – we like Switchgrass, Meadow Foxtail or even Wild Rye.

Don’t Forget The Flowers

Finally, the madman says you have to have flowers, especially Swamp Milkweed. Then add our native favorites like Joe Pyeweed, New England and New York Aster, Black eyed Susan, Blue Flag, Spiderwort, Liatrus, False Blue Indigo, Great Blue Lobelia, and believe it or not, Goldenrod. The madman knows this is considered a weed, but new cultivars are making this plant socially acceptable. Lots of lists also include trees, but we would prefer not to. Do as you like, it’s your Rain Garden.

When selecting plants for the garden, you have the choice of using large potted plants, plugs or seeds. The large plants may give you instant gratification but will cost a lot more. Consider a combination of these three. There are many sources for Rain Garden plants, and one of our favorites is Prairie Moon Nursey which has a nice selection of native plants.

Water Those Plants

Although an established Rain Garden is relatively maintenance free, your new Rain Garden requires a bit of care. Once plants are in the ground, they should be mulched to hold in moisture and keep weeds down. Water well at planting time and give the garden 1” of water weekly for the first couple of years as the plants get established. Then you can hand this task over to Mother Nature. 

With Mother Nature in charge, you can sit back and have a glass of lemonade. Looks like the madman is ready for one himself.