Good: Well-maintained bioretention facility

Bioretention facilities are engineered facilities that store and treat stormwater by filtering it through a specified soil profile. Water that enters the bioretention facility ponds in a depression or other basin (e.g., concrete planter) before it infiltrates into the underlying bioretention soil. Excess stormwater overflows to an adjacent drainage system. Treated water is either infiltrated into the underlying native soil or collected by an underdrain and discharged to drywells or surface water. 

Bioretention facilities require annual plant, soil, and mulch layer maintenance to ensure optimum infiltration, storage and pollutant removal capabilities. The majority of routine maintenance procedures are typical landscape care activities. Soil characteristics and depth in these facilities are important to their proper function and must be maintained according to design. Bioretention facilities may or may not have underdrains or liners.

Maintenance sheets


Maintenance is needed if you see these signs

overgrown vegetation Problem Biofiltration Swale
Problem: Noxious weeds have taken over 
this biofiltration swale.

Bare, exposed soil without mulch cover

Pest infestations, including rodent holes or mounds that disturb water dispersion flow paths and damaged vegetation

Large amounts of pet waste

Inlets or outlets clogged with leaves, trash or sediment

Water ponding for more than 48 hours after the end of a storm

Erosion around inlets, outlets and on slopes

Sediment buildup, usually near inlet

Unhealthy or dead vegetation

Blackberries or other problem vegetation

Diseased or dead vegetation

Leaves, trash and other debris

Tips for fixing problems and general maintenance

Remove weeds such as blackberries and English ivy, then check for them on a regular basis and remove new vines.  Contact Clark County Vegetation Management (360) 397-6140 for information.

Prune trees and shrubs and remove dead plant material regularly.

Replace vegetation damaged or removed during maintenance.

Avoid using fertilizers, herbicides, or pesticides in facility.  These chemicals pollute the water and can cause unwanted plant growth.

Mulch areas annually with 2-3 inches of compost in the bottom and wood chips on side slopes and rim.

Maintain side slopes and replant bare areas.

Removing sediment is best done in dry months before winter rains set in.

If needed, seed and water during dry months to re-establish grass.

Identify and control source of sediment.

Pick up leaves before rains begin.

Inspect and remove debris regularly, particularly after storms.  Pay attention to the the inlet and trash rack (surrounding the overflow).

Remove any material clogging drain, outfalls and channels.

Check roads and fencing in the facility.  They must be maintained to allow easy access.

Additional elements to bioretention

Inletbioretention inlet square.jpg

An inlet can be an opening in a curb, slope, or pipe that collects stormwater and routes it to the bioretention facility. Armored with rocks or vegetation it slows water entering the facility and often traps sediment and debris. Regular maintenance is important. Ensure the opening is clear of debris such as leaves, trash or excess sediment. 

bioretention overflow square.jpg


An overflow ensures that a bioretention cell does not spill out to the area it is draining in the event of a large storm.  The overflow is usually fitted with a slotted grate to prevent large debris from entering. The overflow drains to an under-drain discharge pipe that must be clear to function properly. Regular maintenance is important to remove trash, vegetation and sediment buildup.