Mulching

Mike Davison, Feb. 2021

A rural forest floor is generally deep in decomposing leaves and wood. This organic matter is alive with insects and fungi working to break this down into nutrients and minerals. These compounds are used by the trees and plants to support growth. The thick layer of natural compost stores water and protects the roots from the weather. The study of forest soil is fascinating and a thriving, healthy soil is essential to tree and plant health.

Carbon sequestration
This decaying process stores a lot of organic matter in the soil which is something we want to encourage. The decaying process releases some carbon dioxide into the air, but pure carbon is left behind in the soil permanently. This carbon migrates deep into the ground to form coal. In this way, forests, even urban forests sequester carbon from the air. Although oceans store most of the Earth’s carbon, soils contain approximately 75% of the carbon pool on land — three times more than the amount stored in living plants and animals.

Mulching is the urban forest way

In an urban forest, we do not like the mess of just leaving branches and leaves to decay slowly on their own, so we clean them up. Instead, we use mulch to create a healthy soil for our trees and plants. To be effective, the mulch needs to be alive with organisms and decaying. It should consist of primarily wood and leaves. Bark can be used in smaller quantities because it decomposes slowly and is not generally rich in nutrients.

We use inexpensive, quality mulch
In Serrano Park, we get our mulch from Harvest, our landscape contractor. They make this mulch from local clippings and branches that they control.
Features of Harvest mulch:

  • Composed of leaf, bark, and wood chips from local tree and brush trimmings.
  • Does not contain grass nor conifer needles.
  • Contains no chemical additives or dyes.
  • It is baked using the natural decomposing process to kill weeds and seeds and create active growth of the beneficial micro-organisms.

Mulching practices
As a general practice, we require the mulch to be 3″ to 6” deep. The dirt and drip lines should not be exposed. Trees in grass areas should be mulched to the drip line so that the roots can get the nutrients they need as the mulch decomposes.

References:

  • “Uniform Voluntary Product Guidelines for Horticultural Mulches, Growing Media and Landscape Soils”, Mulch & Soil Council, 2015
  • “Caring for New Trees”, Massachusetts Urban & Community Forestry Program
  • “Tree Planting Best Management Practices”, 2014. 2nd ed. Champaign, IL: International Society of Arboriculture
  • “New Tree Planting“, 2011. International Society of Arboriculture
  • “Arboriculture, Second Edition, Mulches p350-365,” Richard Harris, 1992
  • “Carbon Sequestering in Soils”, Ecological Society of America
  • Mycorrhizae! Mycro what? – The Garden Professors
  • Best Practices – Mulching

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