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Growing a Rain Garden

By AliciaMarie Belchak
Photography courtesy West Michigan Environmental Action Council

You know the spot — that place out front you just haven’t landscaped yet, that area out back where the ground gets squishy after rain, that marginal corner that is just beckoning for something more interesting.

You don’t need to be a Master Gardener or landscape designer to plant a rain garden on your property, according to Patricia Pennell, director of Rain Gardens of West Michigan, a project of the West Michigan Environmental Action Council. And it doesn’t have to cost a fortune either — especially if you can use some plants from your existing garden.

Adding a rain garden to the landscape is a lot like cultivating any other perennial garden — with the unique distinction that your rain garden will be a working garden that also needs to function properly. To avoid common pitfalls of installation and sow some seeds of success, follow these pointers.
Garden location and size.

First and foremost, think about the purpose of a rain garden — stormwater management.
“ The goal of a rain garden is to send water into the garden without creating a pond — or mud-pie,” Pennell pointed out. “You don’t want mosquitoes breeding or plants floating in the mud.”
This means finding a place where roof and yard runoff can be diverted — or where it already is naturally flowing. Common locations are “low spots” and areas close to gutter downspouts, driveways and yard perimeters where the garden can mesh with existing landscaping. A great deal of water percolates through the soil in a properly installed rain garden, so stay away from drainfields and foundations — both yours and the neighbors! Moving 10 feet down-slope from foundations should prevent a leaky basement.

Also part of siting a rain garden: flagging underground utilities by calling “MISS DIG” (800-482-7171). Pennell also advises careful planning under large, well-established trees because root damage during excavation could prove detrimental to a tree’s health. Once you’ve settled on a location where water and garden meet, your next task is to correctly size the rain garden. This is often the biggest mistake rain-gardeners make, according to Pennell.

“Not realizing that maybe 40,000 gallons of water a year is going to come off of their roof, they expect this tiny little rain garden to handle all their water,” she said. “Of course it can’t — and they end up with this giant mud pie. It’s not just the area but the depth of the garden that determines how much water it can handle.”

Runoff estimations and volume calculations based on soil types can be used to determine the right area and depth for a rain garden. In a typical homeowner setting, Pennell suggests a 5-by-8-foot area that is as deep as 8 feet. Bigger gardens range from 150 to 400 square feet but may have shallower depths, as little as 2 feet.

Soil prep, soil prep!
A rain garden typically is a shallow, saucer-like depression filled with porous, absorbent soil that can hold rain as it soaks in, eventually recharging the groundwater. Soil preparation — which often means soil replacement — is vital for proper functioning. “In any garden, you should do a great deal of soil preparation,” Pennell noted. “But to do a rain garden really well, one that’s extremely effective, you need to do a lot of soil preparation.”

Rain Gardens of West Michigan recommends digging at least 2 feet deep and removing or replacing all of that soil. Aim for a sandy loam and not a lot of clay, which swells and stops water from percolating through. A usual mix for area projects consists of 50-60 percent sand, 20-30 percent topsoil, and 20-30 percent compost.

“The key is having that nice loose soil … so that when the water flows into the garden, it soaks in,” Pennell said.

Plant selection
In Pennell’s experience, choosing the wrong plant is the No. 2 predicament overly enthusiastic rain gardeners find themselves in.

“With the soil prepared as well as it is, if you pick plants that are very aggressive, they will take advantage of that and they will overtake the entire garden,” she said. “It makes way too much work for you.”

Among these assertive opportunists are Joe Pye weed, cup plants (a prairie plant), and some grasses.
“ If you pick these plants, you better be ready to beat them back into submission,” Pennell laughed.
Pennell recommends plants that “like to have their feet wet from time to time,” as well as prairie flowers and grasses, which have deep-running roots. She favors native perennial species over non-native because they are more adapted to Michigan’s varying climate and a rain garden’s ever-changing soil condition.

For a simple rain garden in a sunny locale, Pennell suggests swamp milkweed, great blue lobelia, mist flower, Virginia blue bells, black-eyed Susans, New England asters, switch grass, turtle heads, cardinal flower, and Culver’s root. For shady areas, try ferns, hostas and astilbes.

Planting and maintenance
After much planning and digging, the time finally comes for the plants to go in the ground. Pennell prefers the ease and quick returns of potted plants. She usually buys quart-size or bigger, which cost $3-$8 per pot. Plan about $3-$5 per square foot if you’re buying all new plants.

Although the garden may seem on the barren-side, stick to recommended spacings on plant labels to avoid overcrowding a year or two down the road — a common mistake Pennell has fallen victim to on occasion.

“The thing to remember is that even though they’re small now, they won’t be babies forever,” she noted. “Especially if the soil makes them happy; if you’ve done your soil prep well, they’re going to grow like gang-busters.”

Do not fertilize your rain garden. The plants don’t need it and will grow too fast and too tall, requiring staking to keep them from toppling over.

To keep the rain garden low maintenance, Pennell recommends mulching it with shredded hardwood mulch, not bark. Bark mulch tends to float when the rain garden gets flooded and can bury plant crowns when the garden drains.

For more assistance with planning and planting a rain garden, download the “how-to guide” from Rain Gardens of West Michigan at GR

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