As communities across the Mid-Atlantic and beyond face more intense and unpredictable rainfall, stormwater management has become one of the most urgent challenges in urban and suburban development. In this Bloom-hosted webinar, “Bioretention and Infiltration: Effective Tools for Stormwater Management,” veteran horticulturalist Ron Alexander explored how green infrastructure, particularly bioretention systems and infiltration soils, can reduce flooding, protect water quality, and build more resilient landscapes. 

This webinar is part of Bloom’s ongoing educational series for landscape architects, accredited for CEUs for ASLA members. Prior topics covered by Ron Alexander in this series include, “Demystifying Lab Analyses for Landscape Architects,” “Nutrient Management Planning Working with Biosolids,” and “Common Misconceptions about Nutrient Movement & Availability in Soil Amendments.”

Please reach out to Bloom to inquire about a virtual lunch and learn for your local landscape architecture firm.

Why Stormwater Demands New Solutions

Alexander opened the session by framing the growing urgency around stormwater. Climate change is increasing storm intensity, while dense development and compacted soils limit natural infiltration, especially in regions like the DMV, where heavy clay soils are common. These pressures strain aging gray infrastructure and contribute to flooding, erosion, and polluted runoff entering waterways.

The presenter emphasized that Bloom’s mission sits at the intersection of these challenges: transforming biosolids into high-performance soil amendments that support healthier soils, cleaner water, and more resilient infrastructure. 

Green Infrastructure in Practice

Alexander’s core message was that stormwater problems are ultimately soil problems, and better soil design is one of the most cost-effective ways to manage water at scale.

The horticulturist explained that traditional gray infrastructure moves water quickly off-site, often worsening downstream flooding and pollution. In contrast, green infrastructure mimics natural systems by slowing, storing, infiltrating, and treating stormwater where it falls.

He highlighted three primary goals of effective stormwater management:

  1. Protecting water quality, by reducing sediment, nutrients, metals, and other pollutants carried by runoff
  2. Protecting built landscapes and structures, reducing long-term damage and maintenance costs
  3. Protecting natural systems and human health, including aquifers, streams, and urban green space

Understanding Bioretention Systems

Bioretention has become one of the most widely adopted green infrastructure techniques over the past decade. Alexander described bioretention systems as engineered soil and planting systems designed to capture runoff, promote infiltration, and remove pollutants.

Key components of a well-designed bioretention system include:

  • A sand-based bioretention media that allows rapid infiltration while supporting plant growth
  • A mulch layer to reduce erosion, protect soil structure, and bind certain pollutants
  • Vegetation, increasingly native species with deep root systems that maintain infiltration and uptake nutrients
  • Storage layers (such as gravel beds) beneath the soil to temporarily hold water
  • Pretreatment zones (often rock or energy-dissipating features) to slow incoming flows and drop out sediment
  • Overflow and underdrain structures, where site conditions require them

Alexander stressed that bioretention media design is a balancing act. Media must infiltrate quickly enough to manage peak flows, but also retain sufficient moisture and nutrients to keep plants healthy over the long term. Poorly designed mixes, especially those with excessive fines or inappropriate clay content, can crust, clog, and fail.

Why Organic Matter Matters

A central theme of the webinar was the critical role of organic matter in both bioretention and infiltration soils. According to Alexander, organic amendments are not “optional extras”—they are fundamental to system performance.

Organic matter:

  • Introduces and sustains biological activity, which supports plant health and nutrient cycling
  • Increases cation exchange capacity (CEC), improving the soil’s ability to bind nutrients and metals
  • Improves soil aggregation, creating pore space that enhances infiltration and reduces compaction
  • Contributes slow-release nutrients, reducing the need for synthetic fertilizers

This is where Bloom products, particularly Bloom Woody Blend, were highlighted as uniquely well-suited for bioretention applications.

Bloom in Bioretention: Performance and Benefits

Alexander reviewed specifications from the DC Department of Transportation, which list compost and approved alternatives for bioretention media. Bloom Woody Blend (a 50/50 blend of Bloom biosolids product and wood products) meets these specifications and is already being used throughout the region.

Key attributes that make Bloom effective for bioretention include:

  • High organic matter content, supporting biology and soil structure
  • Very low water-extractable phosphorus (under 1%), reducing nutrient leaching risk
  • Slow-release nitrogen, with less than 5% readily available
  • Naturally high iron content, which helps bind metals such as zinc, copper, and cadmium
  • A high carbon-to-nitrogen ratio, particularly from the woody fraction, which further limits nutrient mobility

Alexander noted that thousands of bioretention features in the region have successfully used Bloom Woody Blend, with contractors reporting strong infiltration performance and reliable plant establishment.

He also shared real-world examples, including prominent sites in Washington, DC including the University of District of Columbia, Mount Olivet Cemetery, and Payne Elementary School, where Bloom was used both in bioretention media and as a soil amendment to improve turf and planting areas with historically poor soil conditions.

Infiltration Soils at Scale

In the latter part of the webinar, Alexander expanded the discussion to infiltration soils, which are amended planting soils designed to accept, store, and infiltrate stormwater across broader landscape areas.

Rather than relying solely on discrete bioretention basins, infiltration soils can:

  • Reduce runoff before it reaches bioretention features
  • Lower peak flow pressure on stormwater systems
  • Improve plant health and drought resilience
  • Recharge groundwater and reduce erosion

Research cited during the webinar showed that adding stabilized organic matter, including Bloom products, to dense or compacted soils can significantly lower bulk density, increase water-holding capacity, and improve hydraulic conductivity over time.

Alexander emphasized that thicker soil profiles (often 12 inches or more, rather than the typical 6 inches) are increasingly being explored as a way to meaningfully expand stormwater capacity in urban environments.

Final Takeaways

The webinar concluded that climate change is moving faster than stormwater regulations, and green infrastructure, rooted in better soil design, offers a proven path forward.

Bioretention systems and infiltration soils, when properly designed and maintained, can:

  • Reduce flooding and erosion
  • Improve water quality
  • Support healthy, resilient landscapes
  • Provide long-term economic and environmental benefits

Bloom products, backed by research and practical application, can help achieve these outcomes by supplying organic matter and slow release nutrients that enhances infiltration, supports vegetation, and limits nutrient and metal mobility.

Many technical questions were addressed in the Q&A period; listen to the full recording for more details.