Mulch choices for bioretention

Mulch as the surface layer of a bioretention or rain garden facility provides an initial step in stormwater runoff detention and treatment. In addition, mulch can prevent weeds and make maintenance more efficient.

In this project, the Washington State University - Puyallup (WSU-P) will test the effectiveness of mulch types to reduce weeds, provide carbon, and retain moisture in bioretention facilities. WSU will use 16 bioretention cells located at the WSU-P Low Impact Development (LID) test facilities. The three types of mulch to be tested are: bark mulch (fir), shredded bark mulch (cedar), and arborist wood chips.

Study questions

  • Which mulch minimizes the need for costly maintenance for weeding?
  • What is the capacity of each mulch to improve water retention and release carbon and nutrients?
  • While all three of these mulches are commonly available in Western Washington, are there any concerns with regard to maintenance and treatment?

Study findings

This study is on-going until April 2022.

Project tasks and deliverables

Task 1: Project management

Task 2: Quality Assurance Project Plan (QAPP)

Task 3 Bioretention system preparation and instrumentation

Task 4 Quantifying maintenance effort and plant survival

Task 5 Quantifying mulch effects on water quality and water quantity

Task 6 Communication

  • Deliverable 6.1 Draft report
  • Deliverable 6.2 Final report with Excel spreadsheet of final QA reviewed data
  • Deliverable 6.3 Two presentations: SWG 2/9/2022, and a regional conference or workshop
  • Deliverable 6.4 Draft SAM Fact Sheet