Downspout Planters for Residential Stormwater Management

A downspout planter is a container with soil and plants connected to a downspout to intercept stormwater before it becomes runoff. Downspout planters are often decorative. They function like rain barrels by capturing water from the roofs This stored water then serves as irrigation water for plants. Planter boxes provide similar benefits as rain gardens, including attracting pollinators, beautifying, and slowing/reducing stormwater. However, they can be easier to install and maintain in locations mostly covered in pavement, like alongside commercial buildings or underneath downspouts that drain directly into driveways. Like rain barrels and rain gardens, downspout planters are a best management practice homeowners can implement to help manage community stormwater issues.

Downspout planters are usually constructed from metal, wood, or plastic and can vary in size from 50 – 100 gallons or more, depending on the drainage area of the roof. Galvanized steel tubs are common, but some plastic tubs come with a preinstalled drainage hole, which is a big timesaver. Wooden containers are often decorative but tend to be heavier and require more preparation, like waterproofing and sealing. Homeowners can purchase galvanized tubs from many major hardware stores and some require little modifications.

Site preparation is critical before setting up a downspout planter. Most homeowners choose a site next to an existing downspout where the ground is level. Cinderblocks, bricks, or wooden blocks are sometimes used to create a stable platform for the downspout planter to sit on.

All downspout planters have a few key components including a drainage outlet or an overflow system in case there is too much water for the planter to hold. An overflow system is typically constructed using 1 – 1.5 inch diameter PVC pipe. It must be a few inches lower than the tub's rim and capped with an atrium-style grate to keep debris out. PVC pipe inserted into the box must have holes drilled into it so water can enter the overflow tube if needed. If the tub does not have an outlet, this PVC overflow will also need to direct surplus drainage water outside the planter. Like a rain barrel overflow, you should ensure any excess water will not cause additional runoff problems.

Below is an example of a downspout planter that includes a drainage outlet. Since there is an outlet, the PVC pipe overflow does not need to direct excess water outside the planter. However, if you do not have drainage, you must ensure the PVC pipe also directs excess water outside the tub.

Photo: Jodi Sulpizio, Penn State

Insert the PVC drainage pipe into a corner of the planter, and it is now ready to be filled with stones, soil, and plants. The first step would be to lay out some larger stones on the bottom of the box to help hold the PVC drainage pipe in place and create a small water reservoir. Next, you will fill the box about a third to halfway with pea gravel and place a layer of filter fabric on top of the gravel. This fabric will help to seal off the bottom layer from the soil and keep the overflow pipe from getting clogged. Lastly, fill the planter box with a mix of potting soil and a little sand to help improve infiltration. The box can now be planted, and Penn State Extension recommends choosing a mix of plants native to your area. Watering immediately after planting is encouraged.

The last thing to do is to connect the downspout to the planter box. This step can be done in various ways using either a flexible hose and a downspout diverter or a downspout extension, but no matter how you do it, it is a good practice to add some gravel or decorative stone on top of the soil at the inflow site. The gravel/stone will help slow the incoming water and reduce any soil erosion problems.

As with most other stormwater green infrastructure, regular maintenance will be required for the downspout planter. Regularly monitor and clear out any debris accumulating at the inflow or around the overflow cap. Also, check all connections and drainage areas regularly. Ensure everything is functioning correctly without leaks or blockages. As with any garden, regular maintenance is required, so trim any weeds and prune plants as needed. Lastly, during the wintertime, ensure that all drainage areas are open and reduce any standing water in the planter so there is minimal risk of freezing and cracking.

For more information about downspout planter construction, visit "How to build a downspout planter."

Resources:

Philly Watersheds (archived website) - How to build a downspout planter

Philadelphia Water Department - Maintenance Guide: Metal Downspout Planter (pdf)

The Watershed Institute - Downspout planters