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Facilities - Bioretention

Bioretention (rain garden)

Bioretention facilities, also called rain gardens, use engineered soils and specific plants to trap and uptake pollutants in stormwater. Often located along roads and in parking lots, bioretention cells also store stormwater, which is meant to drain within 48 hours. 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. 

Routine maintenance throughout the year is necessary to keep your bioretention cells functioning properly. Maintaining plants, soil, and mulch helps ensure optimum infiltration, storage and pollutant removal capabilities. The majority of routine maintenance procedures are typical landscape care activities that can be performed with common tools. 


A well maintained bioretention cell or rain garden

A well-maintained bioretention cell. 

Virtual tour

Click on the image below to get inside of a bioretention cell (rain garden) and learn about common components and maintenance tasks.

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Where to find it:

Bioretention cells are typically found along roads and in parking lots. There can be multiple cells throughout a neighborhood or in a parking lot.

When to maintain it:

Remove sediment and clear curb cuts in late fall and early spring. Remove weeds and water new plants throughout the summer and early fall. Water new plants for the first three summers. Remove weeds and clear litter and debris throughout the year as needed.

What you'll need:

  • rake
  • bucket lined with trash bags
  • gloves
  • safety equipment (cones, high visibility clothing, etc.) if working near traffic

You many also need a shovel, mulch, replacement plants and water for irrigation.

Tools you'll need:

Tools needed to maintain a bioretention cell including rakes, shovel, buckets, gloves, safety vests and cones.


A bioretention cell before maintenance with weeds and sediment.


A bioretention cell after maintenance, free of weeds, sediment and with mulch.

Bioretention cells needs maintenance in these areas:


Person pulling weeds in a bioretention cell.

 Identify what is a weed and what plants are 
meant to be there. Pull weeds early before
they go to seeds.


Shovel digging into a bioretention cell to replace a dead plant.

Replace dead plants from fall to spring and water during the first three summers. Contact your local jurisdiction for a list of approved plants. 


Arborist mulch being raked into a bioretention cell.

Maintain 3 inches of mulch to help with plant survival. Arborist mulch is effective, low cost and may be required by your jurisdiction. 

Trash and debris

Person picking up trash and debris from a bioretention cell.

Clear trash and debris as needed. 

Curb cuts

Curb cut that is part of a bioretention cell.

 Keep curb cuts clear. Click the photo for more information on maintaining curb cuts.


Abioretention overflow.

 Ensure the overflow is clear from debris.


Riprap in a bioretention cell.

Clear accumulated sediment and vegetation. Click on the photo for more info.  

Printable maintenance card

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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.


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