The following Q&A on freezing grounds and bioretention was consolidated from three informational blogs by Peter MacDonagh, FASLA.

Bioretention is widely used to slow, filter, and infiltrate stormwater. But what happens in winter? Does frozen soil prevent infiltration? Do dormant trees and plants stop the water-quality treatment process?

Research from Minnesota, New Hampshire, Connecticut, and Ontario suggests that properly designed and maintained bioretention systems can continue functioning during cold weather. Winter performance depends less on the mere presence of frost than on the condition of the soil, its moisture content at the time it freezes, and the system’s ability to drain.

Editor’s note: This Q&A consolidates three earlier DeepRoot articles, including cold-climate research summaries and soil-biology observations from Peter MacDonagh. It has been reorganized, edited for clarity, and updated with more recent guidance.

Does bioretention still work when the ground freezes?

Yes — with some important qualifications.

Several cold-climate studies have found that bioretention systems can remain hydrologically active during winter. In a three-winter Minnesota study, three of four monitored bioretention cells continued functioning most of the time. The fourth experienced drainage problems during both warm and cold weather, indicating that its limitations were not caused by freezing alone.

Researchers in Connecticut monitored a rain garden designed to manage a 25-millimeter storm. Despite measurable frost, less than one percent of the water entering the system overflowed during the two-year study. Research in New Hampshire and monitoring of a bioswale in King City, Ontario, also documented continuing winter infiltration. At the Ontario site, soil approximately 50 centimeters below the surface remained above freezing even when air temperatures fell to approximately –25°C.

These studies do not mean that every bioretention system will perform equally well throughout the year. They do show that cold weather, by itself, does not make bioretention ineffective.

If the soil freezes, how can water continue moving through it?

The type and location of frost may be more important than its measured depth.

When saturated soil freezes, water-filled pores can become blocked with ice, severely limiting infiltration. Unsaturated soil, however, may retain open pore spaces through which water can continue moving. As the original cold-climate research summary explained, “frost penetration does not necessarily equate to filter media permeability; frozen media may still have significant porosity and permeability.”

That does not mean ice is harmless. Current stormwater guidance cautions that ice can form at the surface or within the soil and, under certain conditions, stop infiltration completely. The practical goal is therefore to help the system drain before prolonged freezing weather arrives rather than allowing its soil to remain saturated.

Does cold weather stop the water-quality benefits of bioretention soil?

No. The processes may slow or change seasonally, but treatment does not simply switch off when temperatures fall.

Bioretention improves water quality through several mechanisms, including sedimentation, physical filtration, adsorption to soil particles, microbial transformation, and plant uptake. Some biological activity decreases during winter, particularly near the surface, but filtration and adsorption can continue wherever runoff is able to enter and move through the soil.

Current Minnesota guidance also notes that dry soil storage can provide water-quality benefits during winter. Even when deep frost limits infiltration, the initial, more soluble portion of snowmelt may still receive treatment through filtration, adsorption, and microbial activity as it passes through unfrozen or partially frozen soil.

What role do trees, roots, and soil microorganisms play during winter?

A tree adds capacity to a bioretention system as it matures. Its canopy intercepts rainfall, while its roots help maintain soil structure and create pathways for water movement. During the growing season, the tree also returns water to the atmosphere through transpiration.

Those benefits change during winter. A dormant deciduous tree provides less canopy interception and transpiration, but its roots do not suddenly become irrelevant. Roots contribute organic matter and support communities of fungi and other soil organisms. Mycorrhizal fungi form relationships with tree roots, helping trees obtain nutrients and water while receiving carbohydrates from the tree.

As Peter MacDonagh observed in the original article, “Mycorrhizae activity certainly slows down in the winter, but they do not go dormant.” Activity is greatest where the soil remains unfrozen, including deeper in the soil profile and beneath insulating snow cover.

Bioretention treatment should not, however, be attributed entirely to actively growing plants or microorganisms. The soil performs much of the filtration and adsorption work, while trees and soil life strengthen and expand the system’s performance over time.

How should bioretention be designed for cold climates?

The most important objective is to keep the system draining properly. A bioretention area that remains saturated going into winter is more likely to develop restrictive ice within its pore spaces.

Several practices can improve cold-weather performance:

• Use an appropriate bioretention soil. The soil must provide enough hydraulic conductivity to meet the project’s drainage requirements while also supporting vegetation and water-quality treatment. The correct composition should be based on local specifications and verified through testing—not assumed from a generic sand percentage.
• Test the installed soil. Field testing helps confirm that the soil performs as intended after placement. Testing should occur before the system is accepted so that drainage problems can be corrected.
• Protect the soil during construction. Runoff carrying sediment from disturbed areas should be kept out of the bioretention system until upstream soil is stabilized and surrounding pavement has been cleaned. Construction sediment can clog the surface and reduce infiltration before the system becomes operational.
• Provide reliable drainage and overflow. Where an underdrain is appropriate, it should be accessible for inspection and maintenance. A controllable outlet may allow the system to support infiltration under favorable conditions while providing dependable drawdown when soils are slow to drain.
• Inspect the system before freeze-up. If water is not drawing down properly in late fall, the cause should be investigated. Current guidance recommends taking a malfunctioning infiltration practice offline, when possible, until corrective work can occur.
• Plan for snow, sediment, and deicing salts. Bioretention areas should not automatically become snow-storage areas. Pretreatment, salt-tolerant planting, and a clear snow-management plan are especially important along heavily traveled streets.

A functioning filtration system is preferable to an infiltration system that remains saturated, freezes, and becomes temporarily nonfunctional. The design should respond to actual soil, groundwater, climate, and maintenance conditions rather than pursuing infiltration at all costs.

What does this mean for Silva Cell bioretention projects?

Silva Cell can provide large volumes of lightly compacted soil beneath pavement while maintaining the structural support needed for streets, sidewalks, and plazas. When a Silva Cell installation is designed to receive stormwater, that soil can contribute to runoff filtration, temporary retention, infiltration where site conditions permit, and long-term tree growth.

Cold-climate performance still depends on the complete design. Runoff must be able to enter the soil, distribute through it, and drain appropriately. Pretreatment, inlet elevations, soil selection, underdrains, overflow routes, groundwater separation, construction protection, and maintenance all matter. Silva Cell creates room for soil and roots beneath pavement, but it does not eliminate the need to design for frost, snowmelt, sediment, and road salt.

The takeaway

Cold weather is not a reason to dismiss bioretention. Research shows that well-draining systems can continue to infiltrate and treat runoff during winter, even when frost is present. Successful performance begins with appropriate soil, good drainage, careful construction, and maintenance that keeps water moving through the system in every season.

Sources and further reading

• Davidson, J.D., Lefevre, N.J., and Oberts, G.L. Hydrologic Bioretention Performance and Design Criteria for Cold Climates. Water Environment Research Foundation, 2008.
• Dietz, M.E., and Clausen, J.C. “Saturation to Improve Pollutant Retention in a Rain Garden.” Environmental Science & Technology, 2006.
• Kratky, H., Li, Z., Chen, Y., Wang, C., Li, X., and Yu, T. “Study on Bioretention for Stormwater Management in Cold Climate, Part II: Water Quality.” Journal of Water and Climate Change, 2021.
• Roseen, R.M., Ballestero, T.P., Houle, J.J., Avelleneda, P., Wildey, R., and Briggs, J. “Seasonal Performance Variations for Storm-Water Management Systems in Cold Climate Conditions.” Journal of Environmental Engineering, 2009.
• Toronto and Region Conservation Authority. Performance Evaluation of Permeable Pavement and a Bioretention Swale: Seneca College, King City, Ontario. 2008–2009.
• Minnesota Pollution Control Agency. Minnesota Stormwater Manual: Cold Climate Suitability and Winter Performance of Infiltration Practices.