There are a lot of reasons you may want to install a rain garden. Maybe you are interested in surrounding yourself with beautiful native plants, or looking to attract butterflies, birds, and bees. The most common reason for wanting a rain garden is to capture and infiltrate stormwater. No matter what your reason, the design and installation of a rain garden should never prevent you from moving forward with one!

Cardinal flower in the rain garden at St. Catherine Laboure Catholic Church in Wheaton, MD,

Throughout the Chesapeake Bay watershed, there are a number of grant and rebate programs that encourage property owners to use rain gardens to mitigate their stormwater. These programs offer differing cost assistance services and typically allow for installation completed by either a contractor or as a DIY project. Once you have decided to install a rain garden, it is best to familiarize yourself with the programs available in your area as some programs have specific requirements.

Whether you are installing the garden yourself or hiring a contractor, the below steps will help ensure that your rain garden is safe, functional, and beautiful!

Garden Placement and Perc Test

  1. Identify a garden location – A rain garden should intercept water, so look for locations with easy access to stormwater. Is there a downspout that can be easily directed to the location? Is there surface runoff from a driveway, parking pad, or walkway that could be captured? Be sure to consider where water will go if the garden overflows.
  2. Identify potential hazards – You will want to stay at least 10 feet from foundations and retention walls to make sure those structures don’t carry the weight of your garden. Placing gardens under trees can cause damage to the roots; while gardens under utility lines pose a serious safety concern as well.
  3. Conduct a percolation test – A perc test is a way of determining how long it takes water to soak into your soil. You can learn how to conduct a perc test here. It is also important to remember to have your utility lines marked before you dig a percolation test well. A typical passing perc rate for a standard residential rain garden is 0.5”/hour or greater, but can go as low as 0.33”/hour. Water should drain within 36 hours of filling. Standing water at 72 hours encourages mosquito breeding. Check your local stormwater management incentive program for required specifications.

Garden Design and Plants

  1. Determine the size of your garden – This is based on the amount of water that will be directed into the garden and the space you have available on your property. You can find in-depth information on calculating your garden’s storage capacity here. If you want a simplified version, you can assume your rain garden will have 18” of bioretention mix and 6” of ponding depth. This will allow you to calculate the area of the garden as 1/10th the contributing drainage area. For example, a 100sf garden will be able to handle the water coming off a 1,000 sq ft roof in a typical rain event.
  2. Create a design – Sketching a rough design will be helpful to track some of the decisions you have made such as garden sizing, plant placement, identification of utilities, critical root zones and other obstacles, and identifying which water sources will be directed towards the garden.
  3. Pick your plants – It is always best to utilize native perennials and shrubs. Never plant annuals in a rain garden as they die off at the end of the year and will leave the garden empty. When looking at possible options, be sure to note the lighting conditions in your garden and look for water-tolerant species to place in the bottom of the garden. The Alliance’s Native Plant Center is a great tool for selecting Chesapeake native plants.

Rain Garden excavation. Photo credit: Eric Braker

Garden Installation

  1. Excavation – Excavation depth is determined by the garden size and depth. For example, if you are planning 18” of bioretention mix and 6” of ponding depth, you will need to dig out your garden to a depth of two feet. If you are mixing the bioretention mix yourself, you may want to save about half the soil you excavate to mix with your amendments. Be sure to level out the bottom of the garden to allow for equal distribution of water for greater infiltration.
  2. Build a berm – A berm is a mound of compacted soil that levels out the garden across the top. If you are on a slight slope, place the berm on the downhill side to raise it up level with the opposing side. If your garden is on flat ground, you may want to skip the berm entirely to allow surface runoff to flow into the garden from any direction.
  3. Add bioretention mix – A good bioretention mix is a combination of sand, compost, and soil. It should be a high percentage of sand to ensure the soil is loose and can hold a significant amount of water. EPA recommends 50-60% sand, 20-30% topsoil, and 20-30% compost.
  4. Connect a downspout – If you are directing a downspout to the garden, be sure to slope the attaching pipe down towards the garden to make sure water flows away from the gutter and doesn’t sit in the pipe. This can be done by running a downspout along a fence or burying the pipe in a trench.
  5. Level the surface of the garden – Using a rake, grade the bioretention mix to achieve gently sloping sides and a flat planting area. It is very important that the bioretention mix within the garden is even and level to avoid soil to shifting over time.
  6. Build an inlet – The inlet is a stone structure around the downspout pipes that helps to slow down and disperse water to prevent soil erosion in front of the pipes. If a significant amount of surface runoff is entering the garden, you may consider adding stones to that entry point to prevent erosion on that portion of the garden.
  7. Plant – Install your plants throughout the garden, making sure to place the most water tolerant near the inlet and center of the garden. Do not plant on the berm, as that will destabilize it and result in erosion. Plant spacing and quantity will depend on the size of the plants and how dense you want the final garden. If you are using plants in gallon-size containers, and want 100% plant coverage within the first three years, try to add a one-gallon plant per two square feet.
  8. Mulch – Once the plants are in the ground, spread your mulch evenly across the full garden, including the berm. Avoid piling mulch on top of your perennials as that may prevent them from growing. The best mulch to use is shredded undyed hardwood. Pine mulch or wood chips should not be used as they tend to float and will likely wash away during a rain event.
  9. Water – Once the rain garden has been completed, be sure to give it a thorough soaking to help set the mulch in place and mitigate any transplant shock the plants may be feeling.

If you wish to learn more about how to install a rain garden, check out our recent rain garden installation videos.

Rain garden provided by RiverSmart Homes contractor