Is Your Garden Waterlogged? Plant a Rain Garden! – Simcoe County Master Gardener

By Inez Mitchell, Master Gardener, SCMG

What is a Rain Garden?

A rain garden is a shallow, depressed area in the landscape that is planted with deep-rooted native plants and grasses. This shallow depression has loose, deep soil that absorbs and naturally filters the run-off from hard surfaces such as a driveway, roof, downspouts, sidewalk, or streets. Rain gardens fill with water after a storm and then water filters into the surrounding soil, rather than running off to the street or culvert.

Rain gardens also conserve water, reducing the need for irrigation. Rain gardens are a beautiful and colourful way for homeowners, businesses, and municipalities to help ease storm water problems.

By mimicking the natural absorption and pollutant filtering abilities of a forest or a meadow, rain gardens can absorb runoff more efficiently – as much as 30 to 40% more than a standard lawn. By catching rainwater in a rain garden, holding it, and then slowly releasing it into the soil, runoff from a large storm can be slowed and cleaned – quickly and naturally – reducing the quantity of pollutants that often run from our yards untreated straight into our waterways

It is important to note that a rain garden is not a pond. When constructed properly, the water will drain in 48 hours or faster. Mosquitoes won’t find rain gardens to be good breeding areas because they need much more time to lay and hatch eggs.

Rain Garden Design

Put your garden in the right place! Your property has an existing drainage pattern. Look for the direction of runoff and low spots where water collects. If these spots are away from your house, they will be good places for your rain garden. In order to size the rain garden correctly, measure the area your garden will get rainwater from, by multiplying the width by the length of that area. For sandy soil, your rain garden should be 20 – 30% of the drainage area. For clay soil, 60% of the drainage area because clay absorbs water very slowly.

To build your rain garden, use stakes and string or just a hose to show the shape of the garden bed. Think about where rain goes in and overflows out. Dig a depression of 15 to 30 cm at least 3 meters away from the building foundation. Once the garden is dug, give it a trial run by putting a hose into the garden for 30 to 60 minutes to see how well the water soaks in. Once plants are mature, filtration will be much quicker. Let the garden dry out before planting. If the bed does not drain, remove more soil and add compost or sand, ensure the soil is not compacted. Place plants in pots on the bed before planting, to make sure your design works. After planting, the garden will benefit from a layer of mulch – it will reduce weeds and conserve water. The most common cause of failure of a rain garden is compacted soil. Ensure that the soil is loose and protect the garden during construction from foot traffic.

Any size rain garden, even a small one, will contribute to solving water problems and it will enhance your landscape.

Planting

In addition to protecting our waterways, rain gardens are planted with beautiful, hardy low maintenance waterwise plants, which attract birds, butterflies, and beneficial insects, such as mosquito consuming dragonflies. Typical rain garden plants are perennials and grasses, preferably natives because they are more tolerant of the locale climate, soil, and water conditions, they have deep root systems for enhanced water filtration and drought tolerance. It is important to plant a wide variety of species so that the rain garden is functional during varying seasonal conditions. As illustrated below, group plants with similar water needs together. Placing a rain garden in full shade is not recommended, partial shade is best.

Plants of all types and sizes help to manage storm water. Trees and large shrubs deflect rainfall, slowing it down before it reaches the ground, which allows it to better soak in and not run off immediately. Tall grasses act as filters, sucking up water, trapping pollutants and preventing silt from being washed away. Shorter, deeply rooted plants hold soil and direct water into the ground. Contrary to common belief, a rain garden is not meant to be a place for wetland plants.

Some favourite plants for a rain garden are natives, such as purple coneflower (Echinacea purpurea), summersweet (Clethra alnifolia ‘Hummingbird’) and lady ferns (Athyrium felix-femina) for the shady areas.

Suggestions for a sun rain garden are:

Blue flag iris (Iris versicolor), with lavender-blue flowers in the spring. It looks very natural in a wet setting.

Joe Pye weed (Eupatorium maculatum), which are long-lived moisture-loving perennials that will thrive in the sunny rain garden if planted where they get some afternoon shade.

Daylilies (Hemerocallis), which may not be native but can keep your garden in bloom over a long season if you plant early, midseason, and late varieties.

Bee balm (Monarda), which in summer features brilliant red, pink or white flowers that attract hummingbirds and butterflies.

New England aster (Aster novae-angliae), which will provide bright, violet-purple flowers into the fall.

Sneezeweed (Helenium autumnale), which bears sunny yellow flowers in late summer. It is highly adaptable to wet or dry soil.

Blue cardinal flower (Lobelia siphilitica), which has spikes of true blue flowers in late summer.

Butterfly weed (Asclepias tuberosa), which features orange blossoms that provide ample nectar for butterflies. In addition, the plants are an important larval food for monarch butterflies.

When the rain garden is shadier, consider some of the following plants:

Cardinal flower (Lobelia cardinalis), which grows well in sun or partial shade. It has rich red flowers in late summer.

Pink turtlehead (Chelone lyonii), which is a trouble-free perennial that doesn’t mind wet feet.

Purple meadowrue (Thalictrum dasycarpum), which loves a damp spot in partial shade. It can get quite tall and has clouds of purple-tinged white blossoms in the summer.

Foam flower (Tiarella cordifolia), which is a deer-resistant plant with white flowers in spring.

Jacob’s ladder (Polemonium reptans), which is a low-growing, spreading perennial with clusters of light-blue flowers.

Spotted geranium (Geranium maculatum), which has dainty, pinkish-purple flowers that bloom above the mound of lobed leaves in the spring and often again in the fall.

Maintaining a Rain Garden

No garden is completely maintenance free – and rain gardens are no exception. It is important to weed, clean-up and re-mulch the garden regularly, especially after heavy rains.

The first weeks after planting is critical to its success. Regular watering and weeding are a must. Fertilizer should not be necessary. Keep in mind that your original rain garden design can be changed over time, as you see how the plants grow.

Watching your garden in action the first time, observe where the water enters the rain garden, and where it overflows if it is a heavy rainfall. The mulch layer should stay in place. However, if the mulch is washing away you will need to slow that water down at the water inlet by adding more of a barrier such as more rocks. Over the coming seasons keep your plants happy – there may be plants that simply do not thrive or survive, replace them with a different species. Replace the mulch as needed.

First and foremost, take time and enjoy your rain garden! Installing a garden is a big project, so sit back and enjoy it. With all the native plants that you planted, you will soon have beneficial insects and pollinators visiting your garden.