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​The Magic of Mulch and Compost to Manage Stormwater

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Mulching and compost turn ordinary urban open space into a stormwater management tool, say Virginia Tech researchers. Simple landscape management practices, like using mulch and incorporating compost, can help repair compaction damage.

Post-development areas that are not covered with impervious surfaces are often compacted during the construction process–-those soils can’t infiltrate a lot of rainwater and some even shed stormwater like pavement.

Compacted soils are bad for plants, too, forcing plant roots to form mainly in the soil’s surface layers. When plant roots penetrate into deeper soil layers, they act as pipes that funnel rainwater deep into the ground.

Compacted soils can be fixed to a degree through good landscape management. A big part of the answer may be in simple solutions like using mulch over bare ground and deeply incorporating compost.

During a simulated rain, compacted soils generated runoff with suspended solids at five times the concentration of mulched soils. Over time, organic mulches, like bark and pine straw, can increase organic matter, which improves plant growth, too.

Looking beyond mulch, the researchers say that rebuilding urban soils (soil profile rebuilding) to deeper depths (30 to 60 cm) by placing compost deep into the soil profile helps to rebuild organic matter and allows plant roots to penetrate. Rebuilding soil also increases rainwater infiltration.

Tips for low-maintenance rain gardens:

  • Plan for salt tolerance in gardens that receive road or parking runoff.
  • Right of way (ROW) gardens will receive litter and silt. Plan for it.
  • Bioretention and gardens on streets should accommodate viewing speed.
  • Civic entrances to the community should be designed to reflect civic pride.
  • Select three to six species (and no more than nine) for any one garden.
  • Use a mix of 60% woody and 40% herbaceous plants.
  • Provide four seasons of interest.
  • Remember to allow maintenance access.
  • Mass single types of plants rather than interplanting to simplify maintenance.
  • Bioretention gardens with underdrains are much drier than those without—adjust species accordingly.

US Composting Council Fact Sheet: Using Compost for Stormwater

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