Rain gardens, bioswales and other vegetated stormwater features are often described as hydraulic infrastructure: they slow runoff, hold water temporarily and help protect streams from sediment and pollutants. They are also living soil systems. Beneath the plants and mulch, fungi form part of a wider community that includes bacteria, plant roots, earthworms and other soil organisms.
That does not mean a conventional rain garden needs to be inoculated with mushrooms, or that visible fungi prove the system is treating stormwater successfully. Fungi are normally present when suitable soil, organic matter, moisture and plants are present. The practical question is how to support that biological community without compromising the established requirements of stormwater design.
How rain gardens and bioswales manage runoff
A rain garden is generally a shallow, planted depression that receives runoff from roofs, driveways, paths or other hard surfaces. During rainfall, water spreads across the garden, ponds temporarily and then infiltrates into the soil where site conditions permit. More engineered versions are commonly called bioretention areas or bioretention cells. These may include specified soil media, underdrains, overflow structures, inlet protection and drainage layers.
Bioswales are usually linear, vegetated channels. Their slopes, plants, check dams and soil surfaces slow the movement of water and increase its contact with vegetation and soil. Some bioswales mainly convey and filter runoff, while others include engineered media and ponding zones that function similarly to linear bioretention systems.
Depending on their design, these practices can provide several treatment processes:
- Reducing runoff speed and peak flows.
- Temporarily storing water during and shortly after storms.
- Capturing sediment and particle-bound contaminants.
- Filtering water through vegetation and soil media.
- Infiltrating water into underlying soil, where appropriate.
- Retaining pollutants through adsorption and other chemical interactions.
- Supporting plant uptake, evapotranspiration and microbial transformation.
Pollutant removal is therefore the result of interacting physical, chemical and biological processes. Performance depends on runoff quality, media composition, loading, climate, vegetation, groundwater conditions and maintenance. Some organic-rich media can initially release nutrients, particularly phosphorus, so adding organic material is not automatically an improvement.
Where fungi fit into the soil ecosystem
Fungi are a normal part of soils, mulches and plant-root environments. Saprotrophic fungi decompose dead leaves, woody material, roots and other organic matter. Mycorrhizal fungi form associations with plant roots, exchanging plant-derived carbon for nutrients and sometimes improving access to water. Other fungi live around or within plant tissues, while some can cause plant disease under favorable conditions.
Fungal hyphae are fine, branching filaments that grow through soil and organic materials. They can help bind particles and contribute to soil aggregates, structure and moisture relationships. These effects may benefit plant establishment and soil resilience, but they do not have a uniform effect on infiltration. A fungal network or decomposing organic layer may improve cohesion in one medium while increasing water retention or reducing hydraulic conductivity in another.
Research in urban systems supports the presence and ecological importance of fungi. Studies of New York City bioswales found that fungal and bacterial communities differed among bioswales and were associated with plant species. A 2024 study of 27 Iowa bioretention cells also found substantial fungal diversity and detected genes associated with processes such as laccase production and nitrite reduction. Such genes indicate biochemical potential; they do not, by themselves, demonstrate reliable field-scale removal of a particular pollutant.
Woody mulch, decomposition and plant relationships
Woody mulch supplies carbon-rich material for decomposers and creates habitat for fungal hyphae. As it breaks down, it can contribute organic carbon, support soil aggregation and influence nutrient cycling and water-holding properties. Plants also continuously add organic matter through roots, fallen leaves and root exudates, helping maintain a dynamic soil community.
Mulch selection must still follow the facility’s design guidance. Coarse wood chips or other specified materials may be suitable in some rain gardens, while fine or unstable materials may clog inlets, float away or wash downstream. Dense vegetation can provide erosion control and, in some settings, reduce the need for a thick mulch layer.
More organic matter is not necessarily better. Excess compost, fine mulch or decomposed material can release nutrients, consume oxygen during decomposition and fill pore spaces with fine particles. The objective is a balanced medium that supports plant growth and biological activity while retaining the hydraulic performance required by the design.
Can fungal activity be deliberately encouraged?
For an ordinary rain garden or bioswale, the most defensible approach is indirect encouragement rather than routine inoculation. This means:
- Planting species suited to the site’s wet-and-dry cycle, shade, heat and salt exposure.
- Maintaining living roots and minimizing unnecessary soil disturbance and compaction.
- Using clean, appropriately specified organic materials.
- Protecting the soil surface from erosion.
- Maintaining adequate drainage rather than creating permanent saturation.
- Avoiding routine fungicide use unless a specific plant-disease problem requires treatment.
Plant selection can shape the fungal community because different plants support different root-associated organisms. However, plants should first be selected for stormwater function, regional suitability, structural roots, maintenance needs and tolerance of pollutants or road salt. A particular plant palette cannot guarantee a particular fungal treatment outcome.
Mycorrhizal inoculation
Mycorrhizal inoculation is intended to establish plant–fungus partnerships, not to create a standalone water filter. Experimental biofilter research has reported improved root growth and, under some conditions, modest increases in nutrient and cadmium removal. Results depend on plant compatibility, existing soil fungi, phosphorus levels, moisture, salinity and whether the introduced fungi persist after installation. Native fungal communities may already provide similar functions, so inoculation is not a universal requirement.
Fungal-colonised materials
More deliberate approaches include inoculated wood chips, colonised straw or other lignocellulosic media. A Washington State field-scale study examined fungal and plant amendments in bioretention mesocosms. Fungi decomposed much of the alder mulch over two years, and fungal colonization eventually occurred in control treatments as well as inoculated ones. The study also found that fungal treatments could reduce hydraulic conductivity, while combined plant-and-fungi treatments produced different results.
These findings illustrate both potential and trade-offs. Fungal activity may accelerate decomposition and alter pollutant processing, but it may also increase the need for mulch replacement or affect hydraulic performance. The study showed that all bioretention treatments improved several aspects of water quality; it did not show that inoculation was universally superior to standard bioretention.
Organic pollutants and mycofiltration
Some wood-decay fungi produce enzymes, including laccases and peroxidases, that can transform chemically complex organic molecules. Research has investigated possible treatment of compounds such as polycyclic aromatic hydrocarbons, pesticides, pharmaceuticals and personal-care chemicals. In real stormwater systems, however, the extent, consistency and completeness of these transformations remain uncertain. A detected fungal gene or enzyme is evidence of potential, not proof that a pollutant has been removed or rendered harmless.
This emerging work should be distinguished from healthy fungal ecology in conventional green infrastructure. Mycofiltration deliberately passes contaminated water through fungal-colonised material or a fungal-supported filter. Such a system requires control of hydraulic loading, residence time, oxygen, temperature, salinity, substrate depletion, clogging, pollutant fate and spent-media disposal. Reviews and government assessments have described mycofiltration as promising but have not established it as routine practice for municipal stormwater facilities.
Design and maintenance still come first
Fungal activity should never be used to justify poor drainage or neglected maintenance. Extended ponding may indicate compaction, sediment accumulation, surface sealing, unsuitable media or blocked underdrains and outlets. In many infiltration practices, ponding that lasts more than about 48 hours is a troubleshooting signal, although some designs intentionally include deeper, longer-lasting storage.
Pretreatment is especially important where runoff carries road sand, trash, hydrocarbons or heavy sediment loads. Forebays, gravel diaphragms, vegetated filter strips, sedimentation areas and source-control measures can protect the treatment soil from clogging. Inspections should check inlets, outlets, erosion, sediment, plant health, ponding duration and mulch condition.
Mulch may need replacement when it has eroded or decomposed, but adding fresh material without checking the profile can worsen clogging or nutrient export. Sedges, clover or dense plantings may be appropriate alternatives in some designs, subject to local specifications.
What mushrooms can—and cannot—tell you
Fruiting mushrooms are only the visible reproductive structures of some fungi. They may appear when mulch is decomposing and moisture is favorable. They do not demonstrate effective pollutant removal, successful mycorrhizal colonization or good hydraulic performance. Conversely, their absence does not mean fungi are absent or that the soil ecosystem is inactive.
The sound approach is to treat fungi as an expected and potentially valuable part of green infrastructure, while keeping deliberate fungal treatment applications experimental unless supported by site-specific evidence and engineering controls. Well-designed hydrology, appropriate plants, stable media, sediment management and regular inspection remain the foundation. Fungal ecology can complement those foundations; it does not replace them.
Key points
- Fungi naturally inhabit rain gardens, bioswales, mulch and bioretention soils.
- They contribute to decomposition, soil structure and plant relationships.
- Some fungi may transform certain organic pollutants, but field performance is context-dependent.
- Routine inoculation is not necessary for every stormwater planting.
- Excess organic material, saturation and clogging can undermine both hydraulic and biological function.
- Healthy fungal ecology and dedicated mycofiltration are related but distinct practices.