Bioretention facilities, also commonly called rain gardens, use a mix of plants and engineered soils to capture and treat stormwater before it’s discharged to a conveyance system or allowed to infiltrate into the ground. Most commonly constructed as shallow depressions, bioretention facilities can often be found in parking lots, along roadsides, and near areas with a high concentration of impervious surfaces.
Bioretention systems are typically not designed to provide stormwater detention for larger storms or store stormwater for more than a few days, but when properly designed and maintained, they’re one of the most effective and versatile stormwater management tools available.
Quick Overview
| Also Known As | Rain gardens |
| Common Locations | Parking lots, roadsides, areas with impervious surfaces |
| Primary Function | Capture, treat, and filter stormwater runoff |
| Pollutants Removed | Nitrogen, phosphorus, heavy metals, suspended solids |
| Maintenance Level | Low — mostly standard landscaping tasks |
How Do Bioretention Facilities Work?
During weather events, stormwater flows into a bioretention cell, where it’s slowed by vegetation and thick mulch. Captured water percolates through engineered soil layers while being absorbed by vegetation. As water travels through different soil mediums, the following pollutants are filtered out:
- Nitrogen and phosphorus
- Heavy metals
- Suspended solids
- Other chemical contaminants present in surface runoff
The result is treated stormwater that’s significantly cleaner by the time it re-enters the ground or a conveyance system, which is the whole point of the system.
Handling Heavy Weather Events
Heavy weather events, which routinely result in excess stormwater, can prove challenging for standard bioretention facility design. To address this, bioretention facilities are often constructed with overflows and underdrains connected to storm drainage systems. These enhanced design elements increase drainage rates and help prevent flooding during large storms.
When a heavy weather event occurs:
- Stormwater enters the bioretention facility as it would during a typical weather event
- Water infiltrates the system and enters the underdrain and the connected stormwater drainage system
- Excess stormwater ponds until it reaches the raised overflow drain
- Overflow enters the raised drain and moves through the conveyance system to a stormwater chamber or drainage system
This layered overflow design is what separates a well-engineered bioretention facility from a basic one, and why proper construction matters as much as routine maintenance.
Year-Round Maintenance Tasks
| Season | Task |
| Early Spring | Remove sediment, clean curbs |
| Summer | Remove weeds, water new plants |
| Early Fall | Remove weeds, continue watering new plants |
| Late Fall | Remove sediment, clean curbs |
| Year-Round | Remove litter and debris as needed |
Note: New plants will need to be watered for the first three summers. Three inches of mulch should be maintained at all times to support plant survival. Always identify and remove weeds before they produce seeds.
Maintenance Schedule
Routine maintenance throughout the year is necessary to keep bioretention cells functioning correctly. Upkeep of mulch, plants, and soil ensures optimum filtration, pollution control, and storage capabilities. Most routine maintenance tasks are standard landscaping activities that can be carried out with common tools.
Tools you’ll need:
- Rake
- Gloves
- Bucket lined with trash bags
- High-visibility clothing and traffic cones (if working near traffic)
Having the right tools on hand before you start makes routine maintenance faster and safer — especially for facilities located near roadways or high-traffic areas.
Signs Your Bioretention Facility Needs Attention
Keep an eye out for the following indicators that maintenance is overdue:
- Exposed or bare soil with little to no mulch cover
- Significant pet waste accumulation
- Water ponding for more than two days following a storm
- Dead or unhealthy vegetation
- Pest infestations such as rodent holes or mounds disrupting water flow paths or damaging vegetation
If you’re seeing one or more of these signs, it’s worth scheduling a professional inspection sooner rather than later. Catching issues early is almost always less expensive than addressing them after the fact.
Common Bioretention Maintenance Mistakes
Even well-maintained properties can fall into habits that quietly compromise a bioretention facility’s performance. Here’s what to watch out for:
- Letting mulch depth drop below three inches. Mulch isn’t just aesthetic. It’s critical to plant survival and filtration performance. Letting it thin out is one of the most common and easily preventable maintenance failures.
- Removing plants that look “dead” after installation. New plantings often go through a period of stress before establishing. Pulling them prematurely leads to bare soil, erosion, and reduced filtration capacity.
- Ignoring ponding water. Water that sits for more than two days after a storm is a red flag. It typically signals a clogged underdrain or compacted soil and should be investigated promptly.
- Allowing weeds to go to seed. Once weeds seed, the problem multiplies fast. Removal needs to happen before that point to stay ahead of it.
- Skipping post-storm inspections. Heavy weather events are when bioretention facilities are working hardest and most likely to surface issues. A quick walkthrough after a major storm can catch problems before they escalate.
Most of these mistakes share a common thread: they’re easy to overlook until they aren’t. A consistent maintenance routine is the simplest way to avoid all of them.
Benefits of Bioretention Facilities
Bioretention facilities offer value well beyond basic stormwater control. Here’s a breakdown of the key benefits:
Water Quality
Bioretention is one of the most effective stormwater treatment practices available, thanks to the variety of mechanisms it uses to remove pollution, including biological and microbiological uptake, settling, infiltration, soil absorption, vegetative filtering, evaporation, and transpiration. When designed for infiltration with highly permeable parent soils, bioretention makes use of engineered media that excels at removing solids, organic chemicals, and metals.
Water Quantity
When soils have lower permeability, an underdrain is typically used to achieve some infiltration and rate control, allowing bioretention facilities to be adapted for a wide range of site conditions.
Habitat Improvement
Properly designed bioretention facilities can provide valuable habitat for invertebrates. These benefits increase significantly when multiple bioretention installations exist across a landscape.
Community Livability
Aesthetically, bioretention is a pleasing practice that integrates easily into a variety of landscapes and can incorporate a wide array of vegetation, including trees, shrubs, and perennial plants.
Health Benefits
Studies indicate that green spaces can contribute to both physical and mental health improvements for nearby residents, and may even support reductions in local crime rates.
Economic Savings
Bioretention practices that are correctly designed and maintained can offer cost savings over their entire life cycle and have a positive impact on surrounding property values.
Taken together, these benefits make bioretention one of the few stormwater management solutions that pays dividends beyond compliance, contributing to healthier, more livable, and more valuable properties over the long term.
Work With a Bioretention Expert
Muller, Inc. brings extensive knowledge and specialized technology to the design, construction and maintenance of bioretention facilities across Virginia, Maryland, and Washington, DC. Whether you’re exploring bioretention for a new development or need maintenance support for an existing facility, our team is ready to help you get it right.
