A rain garden is a bowl-shaped garden designed to capture and infiltrate stormwater runoff. They are created to receive water from natural rainfall, downspouts, and/or rain barrel overflow and sheet flow from surrounding areas.

Rain gardens are depressed areas that fill up with water when it rains. They have a specialized porous soil that can also hold water. Water fills the garden, then filters into the soil over a period of time after the rain. Water-loving native plants with deep roots help to soak up the captured stormwater. Rain gardens come in many sizes and varieties, from highly engineered street-side bioretention to large-scale bioretention ponds.

Benefits

Rain gardens are most known for reducing stormwater runoff by helping water infiltrate into the surrounding soil. This means that they can help recharge groundwater resources and mitigate flooding in the area. In addition, rain gardens provide natural habitat for local wildlife, and will beautify your landscape.

DID YOU KNOW?

Rain gardens soak up 30 percent more water than an equivalent patch of lawn (Source: Penn State Extension)

How to Plant a Rain Garden

Installing a rain garden on your property is something that most homeowners can do all by themselves. With the help of some of the resources provided below and some sweat equity, you can make an impact on water quality and beautify your landscape. A rain garden is an investment, but costs little more than traditional landscaping beds. The steps below outline the process of building your own rain garden.

Choosing a location: Choosing a rain garden location is very important and there are many things to consider. The rain garden should be located in a place that can collect as much impervious area (driveway, roof, sidewalks) runoff as possible. The best areas are generally where water naturally drains but doesn’t hold water. It should also be located at least 10’ away from your home. The most effective rain gardens are located on gently sloping ground where they can intercept surface runoff as well..

Because a rain garden can hold a substantial amount of water, it is important to stay 10’ away from foundations and retention walls. Rain gardens should not be installed under the critical root zone of trees. They should also be built so the overflow is directed away from foundations and impervious surfaces.

Call Miss Utility before digging: Once you have settled on a location for your garden, you will need to contact Miss Utility to make sure there is nothing hidden under your proposed location. If no utility lines are found in the project area, you can begin a infiltration test.

Soil infiltration test: Without soil infiltration, your rain garden may turn into a Koi pond. Conducting an infiltration test (also called a percolation or “perc” test) will let you know if the location will allow water to infiltrate into the ground and for how long after a rain event it will hold water. If the infiltration is really low you may not want to locate the rain garden in that particular location. Learn how to conduct a percolation test.

Rain garden filled with water during a rain storm.

Sizing and design: Rain gardens should be designed to be no less than 1/10th the size of the contributing drainage area. Usually, this is a roof, but a rain garden could be designed to capture water off a parking pad or patio. When designing the shape, keep in mind that the garden ill need to be at least 5’ wide to accommodate the central bowl of the garden.

Installation

  1. Excavation: A typical rain garden should include 18” of bioretention mix (soil your plants will grow in) and 6” of ponding depth (6” depth from the overflow to the top of the bioretention mix).
  2. Building a Berm: A berm is a compacted mound of soil built to keep water in the garden. Berms are constructed on the downhill side of the garden to raise it up to level with the uphill side.
  3. Amending the Soil: To ensure the garden drains properly, the soil should be amended with compost and sand. The amendments should be thoroughly mixed and, if done properly, should result in a loose soil that doesn’t compact when balled up in your fist.
  4. Downspouts and Inlets: When connecting a downspout to a rain garden, the pipe should have a gentle slope towards the garden to make sure water flows through it and doesn’t sit in the pipe. The end of the pipe should be surrounded by rocks to slow the water down and evenly disperse it. This is referred to as the inlet.
  5. Mulch And Plants: The center of the garden should be planted with taller, water-tolerant plants while the sides of the garden should be more drought resistant. All plants should be native to the Chesapeake Bay watershed. Mulch should be a shredded, undyed hardwood, available at any hardware store. The search functions on the Native Plant Center website can help you find appropriate plants native to the Chesapeake Bay watershed based on these factors.

Learn More About Rain Garden Install and Watch a How-To Video

How to Properly Maintain a Rain Garden

Rain gardens require about as much maintenance as a standard landscaped bed. In the first year, weeding, watering, and mulching is about the extent of the required maintenance. Newly installed plants should be water frequently and thoroughly, especially in hot summer days. The freshly disturbed soil of a rain garden can be an appealing location for new and aggressive weeds. Be sure to pull any and all weeds as they spring up, especially if you already have invasive species in other areas of your yard.

  • Over the years, the required maintenance for a rain garden will get easier. Each consecutive year the garden will require less water till you are only watering in the hottest and driest conditions. As the plants fill in, they will be able to outcompete most new weeds and spreading seeds from other parts of your yard will have a hard time taking root in the garden. You will also find that the garden needs very little mulch.
  • With a mature garden, keep an eye on the ponding depth. Be sure to remove dead leaves and old mulch to prevent the garden from filling in with debris. You will also want to clean out the spillway stone once a season, especially if you get a lot of leaves in your gutters, as those can build up in the inlet and block the pipe.

Additional Resources