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Grass Swales

A grass drainage swale is an open channel that collects water from hard surfaces and allows it to percolate into the ground, reducing the amount of runoff leaving the road or property. The grass covering the side slopes and swale bottom provides a filtration surface for the water and helps to reduce the flow velocity.

In steeper areas, some swales have stone or concrete ‘check dams’ across the width to help slow the flow rate, promote infiltration, and prevent erosion.

During large storms, swales can direct extra runoff to other stormwater facilities or the storm drain system. Swales are commonly found along roads, parking lots, or between properties of residential lots.

Some grass swales are bioswales and have special soils to increase infiltration.

Image of a grass swale Image of a residential grass swale.
Multi-family property grass swale
Residential grass swale




Maintaining Grass Swales

As the property owner, you are responsible for all maintenance of your grass swale unless a shared maintenance agreement has been made with the DEP Stormwater Facility Maintenance Program.

An un-maintained grass swale may:

  • Cause rainwater to pool on the surface and become a breeding place for insects

  • Stop filtering the rainwater and allow the pollutants to enter our streams

  • Block the flow of water and cause local flooding

Image of the front of the Grass Swale Maintenance Fact Sheet.
Need Help Maintaining
Your Grass Swale?  
Download the Fact Sheet (PDF, 375KB)



Actions You Can Take to Maintain Your Grass Drainage Swale
Monthly Actions Seasonal Actions As Needed Actions

✔ Inspect your swale after storms to make sure that rainwater has drained and there is no erosion.

✔ Mow turfgrass no shorter than 2 1/2 to 3 inches. Remove or compost grass clippings.
✔ Reseed any bare areas and water during the establishment period.
✔ Remove sediment and debris from in and around the swale.  ✔ Manually remove any weeds or invasive plants.
✔ Contact DEP if you continue to have ongoing erosion problems.
  ✔ Remove or compost leaves in the Fall. Leaves may smother the grass and block the flow of water.   
  ✔ Adjust the mower height to avoid scalping the edges of the side slopes  


Actions You Shouldn't Take to Maintain Your Grass Swale
✘ Don't use fertilizer or pesticides in your swale
✘ Don’t over-mow or mow shorter than 2 1/2 to 3 inches.
✘ Don't mow immediately after a rain event.


Image of grass swale before leaf removal. Image of a grass swale after leave removal.
Grass swale before leaf removal
Grass swale after leaf removal


DEP can answer your questions and provide additional guidance about maintaining your stormwater facility. Please email or call the Montgomery County Customer Service Center at 3-1-1.


Grass Swales in the Stormwater Facilities Maintenance Program

The Stormwater Facilities Maintenance Program is responsible for maintaining all public stormwater facilities as well as all private facilities that transferred their maintenance to the County.  

Not sure if your grass swale is part of the Stormwater Facilities Maintenance Program?  Check out online Facilities Map or email


Can I remove my grass swale after installation?

No, you cannot remove any facilities that were part of your building installation - these are permitted structures and DEP maintains a database of these facility locations as part of the Stormwater Facility Maintenance Program. DEP may perform a maintenance inspection of your practice if it is a permitted structure.  Property owners must contact DEP before any major changes can be performed to the structure. 





Resources and Financial Incentives

Have a grass swale on your property? Financial incentives are available for you.

Image of a green roof.


The Water Quality Protection Charge Credit Program

After you have installed your grass swale, you are now eligible to receive a credit off your annual Water Quality Protection Charge (found on your yearly property tax bill). The credit provides an incentive for maintaining your grass swale and other stormwater management practices.

You must apply for the WQPC credit separately – the credit will not be provided to you automatically.

Residential property owners can receive up to 50% off their WQPC depending on the type and size of stormwater management practices on the property.

Non-residential and multi-family properties can receive up to 50-60% off their WQPC depending on the type and size of the stormwater management practices on the property.

Learn more on the WQPC Credit webpage.