Biofiltration in Cold Climates: Risks, Realities, and Proven Design Strategies

Biofiltration and bioretention systems are sometimes viewed skeptically in cold climates, where freezing temperatures, snow management, and deicing practices raise concerns about performance and durability. In practice, however, many biofiltration systems operate successfully in cold regions when they are designed with winter conditions in mind. The key is understanding potential cold-climate risks and mitigating them through thoughtful design, operation, and maintenance planning.

Freezing of the Media

One of the most common concerns is freezing of the filter media. But, if weather conditions are cold enough for the media to be fully frozen, it’s unlikely that there will be stormwater runoff. When snow on top of the biofiltration system or within the drainage area begins to thaw, systems designed with adequate drainage tend to recover quickly.

Unconfined biofiltration systems are not damaged by freezing itself. In fact, engineered media with higher initial permeability drains more completely between events, reducing the amount of water available to freeze and allowing the system to thaw more rapidly. Designers should look for systems with documented cold-climate performance and avoid overly fine or moisture-retentive media that prolong saturation during freeze–thaw cycles. If you are designing your biofiltration system to have an internal water storage zone, make sure it is well below the frost line to avoid damage.

Pollutant Removal at Low Temperatures

Biofiltration removes pollutants through two complementary sets of processes that operate on different timescales. During storm events, treatment is dominated by physical filtration and sorption as runoff passes through the media. These processes are largely independent of temperature and continue to function effectively in cold conditions.

Between storm events, additional treatment occurs through biological and geochemical processes within the root zone, including plant uptake, microbial transformation, and longer-term sorption or precipitation reactions. These inter-event processes slow substantially during winter as temperatures drop and vegetation becomes dormant but will resume in the spring as the growing season returns.

Salt and Chlorides

Deicing salts present a unique cold-climate challenge. Chlorides largely pass through biofiltration systems untreated, and high salt concentrations can stress vegetation and microbial communities during winter months. This is a limitation of biofiltration that should be acknowledged upfront.
Mitigation strategies include selecting salt-tolerant plant species, avoiding media that may release nutrients under saline conditions, and encouraging responsible winter maintenance practices that minimize salt application where feasible.

Snow Management Considerations

Plowing or piling snow directly onto biofiltration systems is not recommended. Snow storage can damage vegetation, obstruct intended flow paths, and concentrate sediment and pollutants at the media surface as meltwater drains through. Where snowmelt is directed to biofilters, pretreatment is especially important to manage the high sediment loads often associated with winter sanding operations.
Clear delineation of biofiltration areas and coordination with site snow-removal plans are essential to prevent inadvertent damage.

Site and System Maintenance

Cold-climate maintenance strategies should focus on sediment management. Where traction sand is used, frequent sweeping of contributing drainage areas can significantly reduce sediment loading. End-of-winter inspections and maintenance are strongly recommended to remove accumulated sand and associated pollutants before they alter surface hydraulics or clog the media.
Pretreatment, such as forebays, sediment chambers, or upstream sumps, plays an especially important role in cold climates by capturing traction sand before it reaches the biofilter.

Contingency Planning and Hydraulic Resilience

Cold climates are prone to sudden temperature swings, including rain-on-snow events that can generate rapid runoff. Providing a bypass pathway that remains functional during freezing conditions helps prevent localized flooding during these events.
Similarly, incorporating an underdrain rather than relying solely on infiltration improves system reliability when native soils remain frozen at depth. Filtration-based designs are generally more resilient to winter hydraulics than infiltration-only systems.

Protecting Systems in Winter: The Case for Armored Designs

One often-overlooked risk in cold climates is physical damage. When sites are covered in snow, biofiltration systems can be difficult to identify and may be inadvertently driven over or struck by plows. Vehicles are more likely to slide or drive into stormwater control features like bioretention areas during winter conditions when visibility is poor or when they are blanketed in snow and ice.

Biofiltration systems with precast top slabs, armored surface elements or robust fencing offer a distinct advantage in these environments. By protecting the media and internal structure, these designs reduce the risk of winter damage and preserve long-term functionality. 

The Bottom Line

Cold climates introduce real design considerations for biofiltration, but not insurmountable ones. With appropriate media selection, drainage, pretreatment, maintenance planning, and physical protection, biofiltration systems can perform effectively and reliably year-round, even in regions with harsh winters and heavy snow management demands.