When it rains, where does the water go? Many erroneously assume storm water is captured, treated, and returned to the water supply. In fact, this is not the case in municipal water systems, where the cost and difficulty associated with treating stormwater makes such a prospect untenable. The reality is that the rain that pours from our gutters and downspouts, runs across our lawns and driveways, and down the drain to be discharged into rivers, streams, and oceans. During heavy to moderate rain events, this storm surge can cause damaging erosion, sediment buildup, and bring a number of other harms to freshwater ecosystems, negatively impacting freshwater mussels and other aquatic life.
With stronger storms fueled by a warming climate, when it rains, it really, truly pours and many areas are now struggling with “100-year storm” events more frequently. As an antidote to this problem, and to reduce demand on already strained stormwater management systems, many cities across the country have embraced rain gardens as a solution that not only helps to manage stormwater, but also adds to urban greening efforts and curb appeal. Homeowners can get in on the action too – by adding rain gardens to their landscape.
What are rain gardens?
A rain garden is a garden area meant to capture runoff so that it can infiltrate into the soil. Planted with deeply rooted native plants, rain gardens do double duty, providing habitat and floral resources for pollinators while reducing storm surge in local watersheds. A well designed rain garden not only looks great, it promotes pollinators while protecting all manner of aquatic life. #winning!
What rain gardens are not
Rain gardens are not ponds, and are not mosquito breeding grounds. Rain gardens should be designed to hold water for only a brief period of time after a storm, quickly infiltrating back into the ground. Unlike a pond or wetland, rain gardens should not generally be more than a foot deep at any given point and should not use any sort of liner. The actual depth and size of your rain garden will vary depending upon how much rainfall you need to collect from your roof or other impervious surface. Review the resources at the bottom of this blog post for more information to help you calculate the size and depth of your rain garden.
Wet, wetter, wettest
When planning a rain garden, you’ll need to take into account the usual factors associated with planning a pollinator garden, picking a diversity of plants that bloom from early spring to late summer and include butterfly host plants where possible. An additional consideration is tolerance of soil moisture. Your rain garden will have a damp area in the lowest parts, a medium area, and a drier area along the edge. When evaluating plants, look at their soil moisture requirements and plan accordingly.
While your rain garden should favor plants that can tolerate occasional flooding, this does not mean you are limited to only wetland species or that they are necessarily the best fit for a rain garden. In some areas, where rain may be seasonal or intermittent, drought tolerance may be a greater consideration than tolerance of flooding. Luckily, native plants with deep roots that are useful for driving water back into the earth are often drought tolerant as well—making them an excellent choice for rain gardens.
Get your grass on!
Native clump-forming grasses and sedges are an essential part of rain gardens. They knit together to hold the soil in place and prevent erosion, support neighboring plants, and are well suited for slowing water movement. Grasses should be included throughout the rain garden, but especially concentrated at water entry and exit points.
Small trees and shrubs are OK too
In larger rain gardens or where space allows, shrubs can be an excellent choice. Wet tolerant pussy willow (Salix discolor) provides vital spring blooms. Buttonbush (Cephalanthus occidentalis) can handle standing water and dry conditions. Red twig dogwood (Cornus sericea) and river birch (Betula nigra) provide winter interest and thrive in wet conditions.
Plant densely and allow plants to “fill in the gaps”.
A rain garden should be planted more densely than other types of gardens and plants should be allowed to fill in the gaps. Doing so allows the plants to slow water movement through the garden. Planting densely also allows roots to knit together and hold the soil in place.
Don’t forget the “off season”
Another reason grasses are a great fit for rain gardens is that they retain some mass when dormant which helps to slow water in winter and fall when other perennials may have died back to the ground. Other plants such as golden alexander (Zizia spp.) and golden ragwort (Packera spp.) retain basal leaves when dormant and are also good selections. Golden ragwort has a creeping habit and will quickly form small colonies and fill in gaps between plantings – making it a particularly good choice for rain gardens.
You don’t have to wait for a rainy day – plan your rain garden now and be prepared for April showers bringing May flowers to your newly installed rain garden! We’ve gathered a variety of resources to get you started, and don’t forget to consult our regional plant lists for additional suggestions.
Below are a selection of guides from across the country that we’ve found to be helpful, but is far from an exhaustive list. Consult your state extension service, Master Gardeners, or local water authority for additional information.
NRCS Fact Sheet: Rain Gardens, Bioswales, and Native Plants – includes helpful technical advice regarding design and installation, as well as a plant list.
Oregon Rain Garden Guide – A comprehensive guide with many design ideas suitable for home landscapes.
Univ. of Wisconsin Rain Garden Manual – Offers design, installation, and maintenance advice, as well as several designs suitable to the Upper Midwest and Great Lakes regions.
Penn State Extension Rain Gardens Fact Sheet – A quick guide with plant list and design example.