Summer is almost here and with it, some of our favorite weather. Rising temperatures drive us to the water, where we enjoy a range of outdoor activities, including watching thunderstorms! While afternoon storms are often accompanied by welcome cooler air, they also bring sudden downpours and large volumes of water that come too fast for the ground to soak up.
What happens to all that water? In many cases, gutters empty along driveways or across short stretches of lawn where the water eventually finds its way into the closest storm drain. Heavy rain can drop water so fast that it doesn’t have time to soak into the ground before flowing into the nearest storm drain — or nearest creek.
Rainwater quickly flowing across the ground’s surface, or along paths and roads, often carries sediment, nutrients from lawns and crops, soaps from car washing, waste from livestock and pets, oil and litter from streets, and many more contaminants. All of that ends up in the closest waterway, and eventually the Chesapeake Bay itself.
From the far-reaching headwaters of Pennsylvania and New York to the Chesapeake, stormwater is a major source of pollution affecting the entire watershed. However, it also offers residents of the watershed great opportunities to get involved and make a difference, starting in their own backyards.
Rain gardens are effective tools to reduce stormwater runoff. They come in all shapes and sizes, depending on the amount of water expected to enter them. A strategically placed rain garden acts as a bowl to temporarily collect and store stormwater runoff as it slowly drains into the underlying soil.
As with all plantings on your property, it’s best to use species native to the Bay region. They not only provide habitat and food for native species as well as other ecosystem services, but typically require less maintenance than nonnatives. Because they are adapted to the climate and soil, they generally require little or no water, fertilizer or pesticides. They are also more likely to attract valuable native pollinators, from bees and birds to butterflies and beetles.
Native plants can also decrease the amount of lawn that must be mowed, saving time and, if gas-powered mowers and trimmers are involved, reducing air pollution. If you think your property may benefit by installing a rain garden, you may want to consult a professional, or at least consider these recommendations:
- Place the rain garden a minimum of 10 feet away from any building foundation or retaining wall. This will prevent water from finding its way into your basement or undermining retaining walls.
- Rain gardens should be placed in areas with a 2–10% slope to allow the adequate collection of water within the rain garden.
- Avoid placing a rain garden at the lowest point on the property. Allow an adequate area for water to overflow should it become full.
- Conduct a percolation test of the soil. Water needs to be able to infiltrate into the ground. If your soil is mostly clay, this location may not be suitable for a rain garden. Water should be absorbed into the ground within 24 hours of a rain event.
- In many cases, the top 18–24 inches of earth should be excavated and replaced with a bioretention soil mix — roughly 65% concrete sand, 20% topsoil, 15% compost/leaf mulch. It should be no more than 10% clay. Existing soil can be amended to the above specifications.
- The excavated earth should be used for the construction of the berm around the perimeter of the garden — which must be compact and level to provide an even overflow.
- If a downspout is piped to the rain garden, the pipe should have at least a 2% slope down and away from the house. Any pipe used more than 10 feet from the house foundation should be perforated. Ideally, the piping should be rigid to prevent it from being crushed or otherwise damaged.
- If the design incorporates a downspout extension, the extension pipe may be buried and should end at the upslope edge of the rain garden (instead of the bottom). If the downspout drains directly into the rain garden, there should be river rock or similar hard material at the outfall to prevent strong gushes of rainwater from washing away mulch and soil.
- Apply and maintain 2–3 inches of shredded hardwood mulch over the soil in your rain garden.
- Place water-tolerant plants toward the center of a rain garden. These plants will be inundated with rainwater for a period of time after a rain.
Small steps to diffuse and slow rainwater before it can enter a storm drain can have a significant impact on the water quality of local streams and rivers. Imagine the benefits if everyone redirected the water flow from just one of their downspouts.
Jamie Alberti is director of the Alliance for the Chesapeake Bay’s Green Infrastructure Program.